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GSM RNO Subject-Troubleshooting of Problems in Stages of Call Setup_R1.0_20130311
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Transcript of GSM RNO Subject-Troubleshooting of Problems in Stages of Call Setup_R1.0_20130311
Troubleshooting of Problems in Stages of Call Setup
R1.0
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LEGAL INFORMATION
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Revision History
Product Version Document Version Serial Number Reason for Revision
R1.0 First published
Author
Date Document Version Prepared
by Reviewed by Approved by
2012-11-18 R1.0 Yang Tao Zheng Hao Zheng Hao
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Intended audience: GSM Network Optimization Engineers
Proposal: Before reading this document, you had better have the following knowledge and skills.
SEQ Knowledge and skills Reference material
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2
3
Follow-up document: After reading this document, you may need the following information.
SEQ Reference material Information
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About This Document
Summary
Chapter Description
1 Signaling Flow and Relevant Counters and KPIs at the Random Access Stage
Describes the Signaling Flow and Relevant Counters and KPIs at the Random Access Stage
2 SDCCH Signaling Flow and Relevant Counters and KPIs
Describes the SDCCH Signaling Flow and Relevant Counters and KPIs
3 Signaling Flow and Relevant Counters and KPIs During the Voice Channel Allocation
Describes the Signaling Flow and Relevant Counters and KPIs During the Voice Channel Allocation.
4 Call Connection Process Describes the Call Connection Process.
5 Signaling Flow and Relevant Counters and KPIs During the Terminating Paging Stage
Describes the Signaling Flow and Relevant Counters and KPIs During the Terminating Paging Stage.
6 Signaling Flow of the Terminating Connection Stage
Describes the Signaling Flow of the Terminating Connection Stage.
7 Features of the V4 Allocation Process Statistics
Describes the Features of the V4 Allocation Process Statistics.
8 Cases Describes some cases.
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TABLE OF CONTENTS
1 Signaling Flow and Relevant Counters and KPIs at the Random Access Stage 1 1.1 Signaling Flow at the Random Access Stage .................................................. 1 1.1.1 Channel Mode Changing Program ................................................................. 3 1.2 Counter List of the Random Access Stage...................................................... 4 1.3 SDCCH Congestion Description, Cause, and Handling Flow ........................... 5 1.3.1 SDCCH Congestion Description ..................................................................... 5 1.3.2 SDCCH Congestion Rate KPI Definition ......................................................... 6 1.3.3 SDCCH Congestion Counters ........................................................................ 6 1.3.4 Main Causes of the SDCCH Congestion ...................................................... 10 1.3.5 SDCCH Congestion Handling Flow .............................................................. 12 1.4 SDCCH Assignment Failure Description and Troubleshooting ....................... 13 1.4.1 Definitions of SDCCH Assignment Success Rate KPIs ................................. 13 1.4.2 Counters Relevant to the SDCCH Assignment Success Rate ........................ 14 1.4.3 Troubleshooting of High SDCCH Assignment Failure Rate ............................ 15
2 SDCCH Signaling Flow and Relevant Counters and KPIs ........................ 21 2.1 SDCCH Signaling Setup Flow ...................................................................... 21 2.2 SDCCH Counter List ................................................................................... 21 2.3 SDCCH Call Drop KPI Definition and Affecting Factors ................................. 22 2.3.1 SDCCH Call Drop Definition......................................................................... 22 2.3.2 Counters Relevant to the SDCCH Call Drop ................................................. 23 2.3.3 Factors Affecting the SDCCH Call Drop........................................................ 25
3 Signaling Flow and Relevant Counters and KPIs During the Voice Channel Allocation 27 3.1 Signaling Flow During the Voice Channel Allocation ..................................... 27 3.2 Relevant Counter List During the TCH Allocation .......................................... 28 3.3 TCH Allocation Success Rate Definition and Failure Cause Description......... 33 3.3.1 TCH Allocation Success Rate KPI Definition ................................................. 33 3.3.2 TCH Allocation Failure Description ............................................................... 34 3.3.3 Main Causes of TCH Allocation Failure ........................................................ 34 3.3.4 Problem Handling Process........................................................................... 36 3.4 TCH Congestion Description, Cause, and Handling Flow .............................. 37 3.4.1 TCH Congestion Description ........................................................................ 37 3.4.2 TCH Congestion Rate KPI Definition ............................................................ 37 3.4.3 Counters Relevant to the TCH Congestion ................................................... 38 3.4.4 Main Causes of the TCH Congestion ........................................................... 47 3.4.5 TCH Congestion Handling Process .............................................................. 48
4 Call Connection Process ........................................................................... 50 4.1 Signaling Flow of the Call Connection Process ............................................. 50 4.2 Counters in the Call Connection Process ...................................................... 51 4.3 Call Drop During the Call Connection Process .............................................. 53 4.3.1 Causes of Call Drops due to Radio Link Fault ............................................... 53
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4.3.2 Causes of Call Drops Due to LAPD Link Failure ........................................... 55
5 Signaling Flow and Relevant Counters and KPIs During the Terminating Paging Stage .............................................................................................................. 56 5.1 Paging Principle .......................................................................................... 56 5.2 Basic Signaling Flow of the Radio Paging ..................................................... 57 5.3 Paging Counters of ZTE BSS ....................................................................... 58 5.4 Paging Success Rate KPI Definition ............................................................. 60 5.5 Factors Affecting the Paging Success Rate .................................................. 61 5.6 Procedure and Method of Low Paging Success Rate Optimization ................ 62
6 Signaling Flow of the Terminating Connection Stage .............................. 66 6.1 Signaling Flow of the Terminating Connection Stage .................................... 66 6.2 Relevant KPIs of the Terminating Connection Stage ..................................... 67
7 Features of the V4 Allocation Process Statistics ...................................... 68 7.1 Change of the Allocation Flow ...................................................................... 68 7.2 Change of Allocation Statistics ..................................................................... 68 7.2.1 Counter Adding ........................................................................................... 68 7.2.2 Counter Deleting ......................................................................................... 68 7.2.3 Modification and KPI Change ....................................................................... 68
8 Cases ......................................................................................................... 70 8.1 Cases of SDCCH Assignment Failure .......................................................... 70 8.1.1 SDCCH Assignment Failure Due to the LAPD Time Delay ............................ 70 8.1.2 High SDCCH Assignment Failure Rate Due to Co-BCCH and Co-BSIC ......... 73 8.1.3 Noise Signal Access .................................................................................... 74 8.1.4 SDCCH Assignment Failure Due to Co-BCCH and Co-BSIC Handover ......... 75 8.1.5 SDCCH Assignment Failure Due to Poor Network Coverage......................... 76 8.1.6 SDCCH Assignment Failure Due to Continuous Location Update Requests ... 77 8.1.7 Improper Setting of the Tx-Integer Parameter ............................................... 79 8.2 SD\TCH Channel Congestion Cases ............................................................ 80 8.2.1 SD congestion due to LAPD Delay Caused by Transmission Fault ................ 80 8.2.2 SD Congestion due to Strong Interference.................................................... 81 8.3 Paging Cases .............................................................................................. 83 8.3.1 No Paging Response due to the SDCCH Congestion.................................... 83 8.3.2 Call Failure due to the MSC Flow Control ..................................................... 83 8.3.3 No Paging Response due to Wrong T3212 Setting ....................................... 84 8.3.4 Low Paging Success Rate due to Location Area Division .............................. 84 8.3.5 GSM Paging Success Rate Optimization of a China Unicom Branch ............. 85 8.4 V4 Cases .................................................................................................... 88
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FIGURES
Figure 1-1 Signaling Flow at the Random Access Flow ......................................................1
Figure 1-2 Channel Mode Changing Program ....................................................................3
Figure 1-3 SDCCH Allocation Flow ...................................................................................5
Figure 1-4 Immediate Assignment Flow ............................................................................7
Figure 1-5 Immediate Assignment Occupation Failure Flow ...............................................8
Figure 1-6 Immediate Assignment Flow ............................................................................9
Figure 1-7 Immediate Assignment Flow .......................................................................... 15
Figure 1-8 LAPD Delay ................................................................................................... 17
Figure 2-1 SDCCH Signaling Flow .................................................................................. 21
Figure 2-2 Immediate Assignment Flow .......................................................................... 24
Figure 3-1 Signaling Flow of TCH Allocation .................................................................... 27
Figure 3-2 Voice Channel Allocation Flow ....................................................................... 34
Figure 3-3 Common Assignment Flow (Internal TC)......................................................... 39
Figure 3-4 Common Assignment Flow (External TC) ....................................................... 39
Figure 3-5 Common Assignment Failure Flow 1 (Internal TC) .......................................... 40
Figure 3-6 Intra-BSC Handover Occupation Failure Flow ................................................. 41
Figure 3-7 Inter-BSC Handover Occupation Failure Flow ................................................. 42
Figure 3-8 Flow of Handling TCH Congestion .................................................................. 49
Figure 4-1 Call Connection Process ................................................................................ 50
Figure 4-2 Call Drop Caused by the Radio Link Fault ....................................................... 52
Figure 5-1 Paging Message Delivery ............................................................................... 56
Figure 5-2 Basic Signaling Flow of the Radio Paging ....................................................... 57
Figure 5-3 Radio Access Process ................................................................................... 58
Figure 5-4 Measurement Point of the BSC Sending the Abis Message to the BTS ............ 60
Figure 6-1 Signaling Flow of the Terminating Connection Stage ....................................... 66
Figure 8-1 Time Stamp Checking .................................................................................... 71
Figure 8-2 Signaling Flow Checking 1 ............................................................................. 71
Figure 8-3 Signaling Flow Checking 2 ............................................................................. 72
Figure 8-4 Signaling Flow Checking 3 ............................................................................. 72
Figure 8-5 Signaling Tracing Data Observing 1................................................................ 74
Figure 8-6 Signaling Tracing Data Observing 2................................................................ 75
Figure 8-7 Signaling Tracing Data Observing 3................................................................ 78
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Figure 8-8 Signaling Tracing Data Observing 4................................................................ 79
Figure 8-9 SD Channel Congestion Report Analysis (Case 1) .......................................... 80
Figure 8-10 No Response From the BTS (Case 1) ........................................................... 81
Figure 8-11 A Large Number of CHANNEL REQUEST Messages (Case 3) ...................... 82
Figure 8-12 Signaling Tracing Data Observing 5 .............................................................. 84
Figure 8-13 Number of Location Update Times................................................................ 85
Figure 8-14 Paging Success Rate ................................................................................... 88
Figure 8-15 TCH 2 Measurement .................................................................................... 92
TABLES
Table 1-1 Counter List of the Random Access Stage .........................................................4
Table 1-2 T3122 ...............................................................................................................5
Table 1-3 SDCCH Congestion Rate KPI Definition ............................................................6
Table 1-4 Definitions of SDCCH Assignment Success Rate KPIs ..................................... 13
Table 1-5 Corresponding Relationship Between the Tx-Integer Parameter and Interval Between Two Channel Request Messages ........................................................................ 16
Table 2-1 SDCCH Counter List ....................................................................................... 21
Table 2-2 SDCCH Call Drop KPI Definition...................................................................... 23
Table 3-1 List of Counters During the TCH Allocation (for V3) .......................................... 28
Table 3-2 TCH Allocation Success Rate (Handover Excluded) KPI Definition ................... 33
Table 3-3 TCH Congestion Rate Definition ...................................................................... 37
Table 4-1 Counters in the Call Connection Process ......................................................... 51
Table 5-1 Paging Success Rate KPI Definition ................................................................ 60
Table 8-1 Cell Basic Measurement Data 1....................................................................... 76
Table 8-2 Cell Basic Measurement Data 2....................................................................... 77
Table 8-3 Site Information .............................................................................................. 79
Table 8-4 SDCCH Congestion Information ...................................................................... 83
Table 8-5 TCH Congestion KPIs ..................................................................................... 89
Table 8-6 TCH Measurement Analysis ............................................................................ 90
Table 8-7 KPIs in the OMC Statistics .............................................................................. 91
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1 Signaling Flow and Relevant Counters and KPIs at the Random Access Stage
1.1 Signaling Flow at the Random Access Stage
Figure 1-1 Signaling Flow at the Random Access Flow
MS BTS BSC MSC
1 Channel Request(RACH)
2 Channel Required
3 Channel Activationg
4 Channel Activationg ACK
5 Immediate Assigment
Command6 Immediate Assigment (AGCH)
7 CM Service Requst(SDCCH)
8 CM Service Requst
Establish Indicationg9 CM Service Requst
10 UA(SDCCH)
The signaling flow of the random access stage is described as follows.
1. Channel requiring
The MS applies for one channel from one BTS through sending one random access
burst on the RACH dynamically.
In the CHANNEL REQUEST message, the setup reason is included. The reason
may be “response paging”, “emergency call”, “mobile originating call”, “short
message service”, or “others”, such as “location update”. What is more, this
message also includes the random parameters. The MS selects five bits as the
random parameters randomly. With these parameters, when two MSs access the
network at the same time, the network can distinguish the MSs.
2. Channel applying
The BTS sends a CHANNEL REQUIRED message to the BSC. Through this
message, the BTS transfers the channel request initiated by the MS to the BSC.
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Actually, the CHANNEL REQUIRED message includes some information of the
channel request and some information added through the BTS. The application
parameters can be acquired from the channel request information and the initial
time advance is added to this message by the BTS.
3. Channel activation
After receiving the CHANNEL REQUIRED message sent from the BTS, the BSC
starts to find and allocate the SDCCH for this call according to a certain condition. At
the same time, the BSC sends one channel activation message to the BTS. The
important point is which BTS should be allocated with this SDCCH and the SDCCH
combination. In this message, the parameters included are DTX control, channel ID
(distinguishment), channel description, maximum power levels of mobile allocation,
MS and BTS, and initial time advance of this access calculated by the BSC.
4. Channel activation confirmation
This is a response to the channel activation message. After the BTS receives the
channel activation message, it starts to send and receive messages on the SACCH.
5. Immediate assignment command
The BSC informs the BTS of the used SDCCH.
6. Immediate assignment
The BTS sub-system informs the MS of the used SDCCH condition through the
AGCH. In fact, this message is an indication about moving from the AGCH to
defined SDCCH sent from the network to the MS. In this message, the included
parameters are paging mode, SDCCH description, associated SACCH,
frequency-hopping, application parameters, initial time advance, and frequency
allocation (frequency-hopping application).
7. CM service request
The MS sends a CM service request to the network, so as to apply one service for
the connection management sub-layer entity, such as circuit switch connection
setup, subsidiary service activation, or SMS sending.
8. CM service request (setup indication)
The BTS confirms the immediate assignment command through returning the setup
indication message. The setup indication message has two functions. First, it points
out that the MS is on the SDCCH from the BTS aspect. Then the BTS sends one
message to the BSC to indicate that the CM service request of the MS is sent on the
described SDCCH. What is more, the BTS will distinguish this connection and add
the received L3 message to this message.
9. CM service request
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This CM service request message is sent to the mobile switch center (MSC).
10. No ID confirmation
When the L2 link is built in the LAPDm protocol, no ID confirmation is L2
confirmation in the normal condition.
1.1.1 Channel Mode Changing Program
If the TCH should be used as the signaling mode due to no SDCCH during the immediate
assignment, or the system requires for the rate change during the data sending process,
it is necessary to send the channel mode changing program to the TCH, so as to meet
the rate requirement, as shown in the following figure.
Figure 1-2 Channel Mode Changing Program
MS BTS BSC MSCAssignment Request
Mode Modify
Mode Modify ACK
Channel Mode Modify
Channel Mode Modify
Channel Mode Modify ACK
Channel Mode Modify ACK
Assignment Complete
The channel mode changing is always initiated by the network. The network sends a
channel mode changing message to the MS. This message includes the channel
description and the new mode adopted by the channel.
After the MS receives the channel mode changing message, it changes the mode of
the indicated channel and sends a channel mode changing confirmation message to
indicate the new channel mode.
If the MS does not support the specified mode, it will keep the original mode and
send one channel mode confirmation message of the corresponding channel mode
information.
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1.2 Counter List of the Random Access Stage
The counters of this stage mainly include the CS measurement, radio access
measurement, SDCCH measurement, and TCH F/H measurement counters.
The counters of the random access stage are shown in the following figure. The V4 and
V3 counters are the same.
Table 1-1 Counter List of the Random Access Stage
Counter ID Counter Name
C900060003 Number of SDCCH seizure attempts for assignment
C900060004 Number of SDCCH seizure success for assignment
C900060005 Number of SDCCH seizure failure for assignment
C900060008 Number of SDCCH allocation success for assignment
C900060009 Number of SDCCH allocation failure for assignment
C900060010 Number of signaling TCH/F seizure attempts for assignment
C900060011 Number of signaling TCH/F seizure failure for assignment
C900060014 Number of signaling TCH/F allocation success for assignment
C900060015 Number of signaling TCH/F allocation failure for assignment
C900060017 Number of signaling TCH/F assignment success for assignment
C900060018 Number of voice TCH/F assignment failure for assignment
C900060038 Number of signaling TCH/H seizure attempts for assignment
C900060039 Number of signaling TCH/H seizure failure for assignment
C900060174 Number of signaling TCH/H allocation failure for assignment
C900060235 Number of signaling TCH/H assignment success for assignment
C900060241 Number of SDCCH assignment attempts
C900060242 Number of SDCCH assignment success
C900060243 Number of SDCCH assignments failure
C900060136 Number of MOC access attempts
C900060236 Number of MOC access success
C901250007 Number of SDCCH seizure attempts for assignment
C901110001 Number of invalid access requests
C901110003 Number of MOC access success for processes
C901110017 Number of other causes access attempts
C901110018 Number of successful random access processes for other causes
C901110019 Number of other causes access success
C901110033 Number of wireless accesses due to other causes
C901260002 Number of signaling TCH/F seizure success for assignment
C901260007 Number of signaling TCH/F allocation attempts for assignment
C901260013 Number of signaling TCH/F assignment attempts
C901270002 Number of signaling TCH/H seizure success for assignment
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Counter ID Counter Name
C901270007 Number of signaling TCH/H allocation attempts for assignment
C901270008 Number of signaling TCH/H allocation success for assignment
C901270013 Number of signaling TCH/H assignment attempts
1.3 SDCCH Congestion Description, Cause, and
Handling Flow
1.3.1 SDCCH Congestion Description
Figure 1-3 SDCCH Allocation Flow
MS BTS BSC MSC
Channel Request(RACH)
Channel Required
Channel Activationg
Channel Activationg ACK
Immediate Assigment
Reject
Immediate Assigment
Reject/T3122
When the BSC receives the CHANNEL REQUIRED message sent by the MS through
the BTS, it acquires the SDCCH resources. If the BSC finds no available SDCCH, it will
send an IMMEDIATE ASSIGNMENT REJECT message to the MS to ask it to wait for a
while (T3122) before requiring for the access again and add 1 to the SDCCH congestion
counter.
T3122 defines the minimum time interval of forbidding the next call of the MS temporarily,
so as to avoid the network congestion.
Table 1-2 T3122
Protection Period of Access Attempt (T3122, s)
Value range 0 ~ 255
Unit s
Default value
10
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Protection Period of Access Attempt (T3122, s)
Description
After the network receives the channel request message sent by the MS. If there is no proper channel for the MS, the network will send an IMMEDIATE ASSIGNMENT REJECT message to the MS. The T3122,
that is the waiting indication message unit, is included in the IMMEDIATE ASSIGNMENT REJECT message, so as to avoid the
channel congestion due to continuous channel requests of the MS.
After the MS receives IMMEDIATE ASSIGNMENT REJECT message,
MS cannot start a new call until T3122 expires. This timer is one of the system control parameters, which is sent to MS in the IMMEDIATE ASSIGNMENT REJECT message.
The recommended value for T3122 is 10~15 s, and 15~25s in areas with dense traffic.
1.3.2 SDCCH Congestion Rate KPI Definition
Table 1-3 SDCCH Congestion Rate KPI Definition
KPI SDCCH blocking rate
Definition Number of signaling channel blocking times × 100%/Number of signaling channel call attempts
Counter formula
V2 (C11625 - C11626 + C11697) × 100%/(C11625 + C11696)
V3
V6.0 (C100030005 + C100030011 + C100030039) × 100%/(C100030003 + C100030010 + C100030038)
V6.2
(C900060005 + C900060011 + C900060039) ×100%/(C900060003 +
C900060010 + C900060038)
V4 V6.50.10 (C901250003 + C901260003 + C901270003)/(C901250001 + C901260001 + C901270001)
For the V3 formula definition, the basic measurement counter is adopted; for the V4
formula definition, the common measurement counter is adopted. And the counter
meaning and formula definitions of the two versions are consistent.
1.3.3 SDCCH Congestion Counters
C900060005: number of SDCCH seizure failure for assignment
Description
The SDCCH allocation attempt is activated by the MS channel request
message CHL_REQ. After the BSC receives this message, it attempts to
allocate the channel for the request. If the allocate succeeds but occupation
fails, this counter accumulates; if the request returns that the transceiver is
faulty, this counter does not change. For failures in other cases, this counter
accumulates.
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Measurement point
This counter counts when the BSC requests for channel (due to assignment)
but fails to occupy the channel. The measurement point is A1, as shown in the
following figure.
Figure 1-4 Immediate Assignment Flow
MS BSC
CHL_REQ
CHL_RQD
CHL_ACT
CHL_ACT_ACK
IMM _ASS _CMD
IMM_ASS
SABM
EST_IND
BTS
A1
A2
A3
C900060011: number of signaling TCH/F seizure failure for assignment
Description
This counter counts the number of TCH/F channel occupation failures during
assigning signaling channels.
After the BSC receives the channel request, it attempts to allocate channel for
the request. If occupation fails, this counter accumulates. If the request returns
that the transceiver is faulty, this counter does not change. For failures in other
cases, this counter accumulates.
Measurement point
The BSC requests for channel (due to assignment and the channel is used as
signaling channel) but fails to occupy the channel, or the BSC fails to wait for
internal resource. The measurement point is A1 in the following figure.
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Figure 1-5 Immediate Assignment Occupation Failure Flow
MS
CHL_REQCHL_RQD
BSCBTS
A1IMM_ASS_REJ
C900060039: number of signaling TCH/H seizure failure for assignment
Description
This counter counts the number of TCH/H channel occupation failures during
assigning signaling channels.
After the BSC receives the channel request, it attempts to allocate channel for
the request. If occupation fails, this counter accumulates. If the request returns
that the transceiver is faulty, this counter does not change. For failures in other
cases, this counter accumulates.
Measurement point
The BSC requests for channel (due to assignment and the channel is used as
signaling channel) but fails to occupy the channel, or the BSC fails to wait for
internal resource. The measurement point is A1 in Figure 1-5.
C900060003: number of SDCCH seizure attempts for assignment
Description
The SDCCH channel allocation attempt is activated by the MS channel request
message CHL_REQ. After BSC receives this message, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060003
and C900060004 accumulate simultaneously. If occupation fails and the failure
is due to transceiver fault, C900060003, C900060004, and C900060005 do not
change. For failures in other cases, C900060003 and C900060005
accumulate simultaneously.
Measurement point
This counter counts when the BSC completes requesting for channel (due to
assignment) (C900060004 + C900060005). The measurement point is A1 in
the following figure.
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Figure 1-6 Immediate Assignment Flow
MS BSC
CHL_REQ
CHL_RQD
CHL_ACT
CHL_ACT_ACK
IMM _ASS _CMD
IMM_ASS
SABM
EST_IND
BTS
A1
A2
A3
C900060010: number of signaling TCH/F seizure attempts for assignment
Description
This counter counts the number of TCH/F channel occupation attempts during
assigning signaling channels, including the number of occupation success
(C901260002) and the number of occupation failure (C900060011).
After BSC receives the channel request, it attempts to allocate channel for the
request. If allocation and occupation succeed, C900060010 and C901260002
accumulate simultaneously. If the request returns that the transceiver is faulty,
C900060010, C901260002, and C900060011 do not change. For other cases,
C900060010 and C900060011 accumulate simultaneously.
Measurement point
This counter counts when BSC completes requesting for channel (due to
assignment and the channel is used as signaling channel) (C901260002 +
C900060011). The measurement point is A1 in Figure 1-6.
C900060038: number of signaling TCH/H seizure attempts for assignment
Description:
This counter counts the number of TCH/H channel being attempted to be
occupied during signaling channel assignment, including the number of
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occupation success (C901270002) and the number of occupation failure
(C900060039).
After BSC receives the channel request massage, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060038
and C901270002 accumulate simultaneously. If the request returns that the
transceiver is faulty, C900060038, C901270002, and C900060039 do not
change. For other cases, C900060038 and C900060039 accumulate
simultaneously.
Measurement point
This counter counts when BSC completes requesting for channel (due to
assignment and the channel is used as signaling channel) (C901260002 +
C900060039). The measurement point is A1 in Figure 1-6.
1.3.4 Main Causes of the SDCCH Congestion
From the aspect of singling flow, there are two main types of causes of SD congestion.
Too many CHANNEL REQUIRED messages exceeds network capacity and all the
SDCCHs are occupied.
Too many CHANNEL REQUIRED messages means the cell is busy, while few
SDCCH are configured, which results in frequent occupancy of SDCCH and
overflow.
During the SD congestion troubleshooting, the engineers should check
whether the traffic increases firstly.
Note:
Some operations at the OMCR (such as HLR configuration or LAC re-planning) may lead
to traffic increase in network.
The occupancy period of SDCCH is too long, due to non-in-time ending of signaling
flow
If signaling flow doesn’t end in time, which means channel activation/release
period is too long due to some reason (say, transmission fault), it will lead to
long period of SDCCH occupancy and reduce SDCCH resource, and
eventually result in SDCCH overflow.
Too-long channel activation/release period causes the MS to repeat the
channel request again and again (the number of repetition is decided by
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system parameter MaxRetrans), and to occupy SDCCH repeatedly, which
worsens SDCCH overflow.
The problem of the intelligent network with the USSD service or the CN with
the LOC leads to the suspension of the MS during the SMS or LOC. The MS
occupies the SDCCH for a long time, which leads to the SDCCH congestion.
From the aspect of fault categories, the main causes of SDCCH congestion are
described as follows.
Unreasonable LAC planning
If the LAC boundary is set at high traffic areas or main transportation ways, where
subscribers are in great number and in frequent movement, LAC renewal can be
very frequent, which will form unreasonable calling modes and lower system
capacity as well;
Unreasonable parameter settings
The relevant parameters of C1 and C2 algorithms are set unreasonably. T3101,
T3122, and T3212 are set unreasonably.
Hardware fault
If the LAPD and TRX become faulty or unstable, the BSC cannot activate the
ground resources of this channel during the immediate allocation. Then, the user
makes multiple SD attempts, which leads to the SD congestion.
Problems with adjacent cells
Because of the faulty adjacent cell, the serving cell absorbs some extra traffic and
the congestion happens.
Frequent registration of the illegal users
The engineers should check whether the congestion is caused by the frequent
registration of the users with roaming limit. If the new service is used in this area,
when the users with roaming limit stay in the limit area and the mobiles are powered
on, the MSs will attempt to register in this network but the authentification always
fails, which increases the signaling load.
Abrupt SMS
In some areas, a large number of abrupt SMSs about the mark-six lottery, soccer
gambling, or SMS fraud, exist, which leads to serious SD congestion.
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1.3.5 SDCCH Congestion Handling Flow
1. Observe the performance report to check whether the congestion happens in all the
cells of the site or only in one cell. The SD congestion in all the cells in the site
seldom happens and it is always relevant to the land transmission or parameter
configuration.
2. Observe the performance report to check whether the channel allocation failure
(channel activation expiration or failure) happens during the SD congestion. If the
channel allocation fails, the transmission or the CMB and TRX of the site may be
faulty. It is suggested to check the transmission equipment or the site CMB or TRX
fault.
Note:
Too large LAPD flow will lead to LAPD transmission delay, which causes timer expiration
before channel activation is completed. This kind of timer expiration shall be differentiated
from that caused by transmission fault.
3. Check the radio access measurement, analyze the access reason of Channel
Request which causes SD congestion, count the number of Channel Request
attempts and success times due to different reasons, and compare indicators with
those in normal period.
The access cause of Channel Request falls into four categories.
MOC
MTC
LOC
Other reasons (call re-establishment)
Usually, the number of LOC attempts is the largest. The number of MOC attempts is
equivalent to that of the MTC attempts and they are relevant to the user’s call
behavior. The number of other reasons is basically 0.
Here are explanations for each category of cause.
If there are a big number of attempts due to other reasons, and all end in failure, the
cause can be confirmed to be interference.
When the number of MOC attempts is big, and even exceeds the number of LOC,
the reasons could be:
The first possible reason is that there is MS malicious pager in the network.
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The second possible reason is interference.
Judge the reasons through analyzing number of MOC success.
Although the MS malicious pager leads to a large number of MOC attempts, the
corresponding success times also increase. The interference cannot lead to the
increase of number of success times.
4. If number of LOC increases abnormally, check if there are any changes on network
parameters, such as re-planning of LAC or amendments of the HLR and VLR.
5. Generally speaking, SD congestion caused by LOC won’t bring large number of
assignment failures. If SD congestion is accompanied with large number of
assignment failures, it’s very possible that the cell traffic volume is high or
co-channel interference exists.
Note:
For BSCV2 system, basic measurement includes the number of attempts/success of
MOC, MTC, LOC and other accesses. For the iBSC system, there is a special radio
access measurement, which needs to establish measurement task. In CS basic
measurement, number of MOC/MTC attempts and number of MOC/MTC success are
included, through which we can calculate the number of attempts/success of LOC
access.
6. Make enquiries and find out if there are newly-setup sites, adjustments on
LAC/HLR;
7. Check the performance report of the week when the problem appears, analyze if SD
congestion exists for a long time during busy hours.
If the SD congestion is a long standing issue, and there’s no big fluctuation in the
number of MOC, MTC, LOC attempt and success times, this means the cell is busy
and its traffic volume is high, and expansion is needed.
1.4 SDCCH Assignment Failure Description and
Troubleshooting
1.4.1 Definitions of SDCCH Assignment Success Rate KPIs
Table 1-4 Definitions of SDCCH Assignment Success Rate KPIs
KPI SDCCH Assignment Success Rate
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Definition SDCCH assignment success rate × 100/(Number of SDCCH assignment success + number of SDCCH assignment failure)
Counter formula
V2 C11644 × 100%/( C11644 + C11645)
V3 V6.0 C100420014 ×100%/(C100420014 + C100420015)
V6.2 C900060242 × 100%/(C900060242 + C900060243)
V4 V6.50.10 C901250014 × 100%/(C901250014 + C901250015)
For the V3 formula definition, the basic measurement counter is adopted; for the V4
formula definition, the common measurement counter is adopted. And the counter
meaning and formula definitions of the two versions are consistent.
1.4.2 Counters Relevant to the SDCCH Assignment Success Rate
C900060242: number of SDCCH assignments success
Description
After BSC responds to the channel request to allocate and activate SDCCH
channel successfully, BSC sends the IMM_ASS message to notify MS to use
this channel. After MS receives this message, it sends SABM frame to BTS on
SDCCH. BTS then sends the EST_IND message to BSC after receiving the
SABM frame.
If BSC receives the correct EST_IND message within specified time, it
indicates that the SDCCH channel assignment succeeds, and then this counter
increments.
This counter counts the number of MS successfully accessing SDCCH channel
when BSC sends the IMM_ASS message, as shown in the following figure.
Measurement point
This counter counts when BSC receives the correct EST_IND message or the
assignment completion message. The measurement point is A3.
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Figure 1-7 Immediate Assignment Flow
MS BSC
CHL_REQ
CHL_RQD
CHL_ACT
CHL_ACT_ACK
IMM _ASS _CMD
IMM_ASS
SABM
EST_IND
BTS
A1
A2
A3
C900060243: number of SDCCH assignments failure
Description
After BSC responds to the channel request to allocate and activate SDCCH
channel successfully, BSC sends the IMM_ASS message to notify MS to use
this channel. After MS receives this message, it sends SABM frame to BTS on
SDCCH. BTS then sends the EST_IND message to BSC after receiving the
SABM frame.
If BSC receives the incorrect EST_IND message or T3101 is timeout, it
indicates that the SDCCH channel assignment fails, and then this counter
increments.
Measurement point
This counter counts when BSC receives the incorrect EST_IND message or
when T3101 is timeout. The measurement point is A3, as shown in Figure 1-7.
1.4.3 Troubleshooting of High SDCCH Assignment Failure Rate
Improper setting of the Tx-Integer parameter
The corresponding relationship between the Tx-Integer parameter and interval
between two channel request messages is described as follows.
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Table 1-5 Corresponding Relationship Between the Tx-Integer Parameter and Interval Between Two Channel Request Messages
TxInteger
Time interval (ms) Time interval (ms)
(CCCHs not Combined With SDCCHs)
(CCCHs Combined With SDCCHs)
12 501ms ~ 593 ms (109~129 slot)
267ms ~ 359 ms (58 ~ 78 slot)
13 750ms ~ 865 ms (163~188 slot)
396ms ~ 511 ms (86 ~ 111slot)
14 998ms ~ 1146 ms (217~249 slot)
529ms ~ 676 ms (115 ~ 147 slot)
15 253ms ~ 483 ms (55~105 slot) 189ms ~ 419 ms (41 ~ 91 frames)
Usually, the Tx-Integer parameter is set to 14 by default. When the transmission
link delay is long and the Tx-Integer parameter is set to be too small, the MS will
send the access request for many times.
Usually, the one-way time delay of the signaling transmission on the Abis interface
is about 60 ms ~ 100 ms. For example, with the time delay on the Um interface
being ignored, the time delay of one immediate assignment flow is shown as
follows.
The CHANNEL REQUIRED signaling: UL: 60 ms
The CHANNEL ACTIVATION signaling: DL: 60 ms
The CHANNEL ACTIVATION ACK signaling: UL: 60 ms
The IMMEDIATE ASSIGN signaling: DL: 60 ms
Total: The time delay from the MS sending a CHANNEL REQUEST message to it
receiving a CHANNEL REQUEST message is about 240 ms.
If the delay of the TX link is large and TxInteger is improperly set — for example, it
is set to 15, and the corresponding the CHANNEL REQUEST message expire time
is 300 ms — the CHANNEL REQUEST message expires before the MS receives
the IMM ASSIGN message and then the MS resends the CHANNEL REQUEST
message. Then the MS receives the IMM ASSIGN message corresponding to the
last CHANNEL REQUEST message and finishes the access flow. The IMM
ASSIGN message corresponding to the second CHANNEL REQUEST fails.
High SDCCH assignment failure rate due to LAPD time delay
The possible causes of LAPD time delay are described as follows.
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If LAPD multiplexing 1:4 is used on site, multiple BCCH TRXs may be
multiplexed on one LAPD, and the traffic volume will be heavy and the delay is
large.
The delay is caused by large LAPD flow. For example, improper LAC dividing
will trigger LAPD flow control.
The transmission equipment fault leads to the message loss of the LAPD link
or too long time delay of the LAPD link. This condition and the SDCCH
allocation failure always happen at the same time. The SDCCH allocation
failure counter accumulates only at the time of activation failure or no response
to activation request. No response to the activation request has two specific
conditions. One is message loss on the LAPD link. Then the BTS cannot
receive the channel activation message or the BSC cannot receive the
activation responding message. Another condition is too long time delay on the
LAPD link, which leads to the expiration of the channel activation timer. Both of
the two conditions indicate that the LAPD link transmission is faulty.
The delay of the TX equipment is large, for example, satellite TX is used on the
Abis interface.
The affection of the PS service. Because the PS service is more sensitive to
the network time delay, the LAPD time delay can lead to the resending of the
PS service message. And the message resending increases the traffic, which
leads to longer LAPD time delay.
If the LAPD time delay achieves to a certain degree, the MS will resend the
CHANNEL REQUEST message, which leads to the SDCCH assignment
failure, even the SDCCH allocation failure.
Figure 1-8 LAPD Delay
MS BTS BSC MSC
1 Channel Request(RACH)
2 Channel Required
3 Channel Activationg
4 Channel Activationg ACK
5 Immediate Assigment
Command6 Immediate Assigment (OK)
1 Channel Request (Re-Send)
2 Channel Required
3 Channel Activationg
4 Channel Activationg ACK
5 Immediate Assigment
Command6 Immediate Assigment (Fail)
Lapd
Delay
TxInteger
MS change
to SDCCH
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Co-frequency and co-BSIC interference
Co-BCCH and co-BSIC interference has two conditions:
Two co-BCCH and co-BSIC cells: The CHANNEL REQUEST message sent
by the MS is received by two cells at the same time and the SDCCH
assignment is made. Because the MS accesses only one SDCCH, the SDCCH
assignment of one cell will fail. During the RACH coding, 6-bit color codes are
added. The 6-bit color codes are acquired through Mod 2 of 6-bit BSIC and
6-bit odd and even verification codes. Therefore, co-BCCH and co-BSIC may
lead to wrong decoding of the initial accesses of MSs of other sites. As a result,
the SDCCH assignment failure happens.
The two cells are co-BSIC and the TCH ARFCN of one cell is the same with
the BCCH ARFCN of another cell. Then the handover access request on the
TCH of one cell is received by the other cell as the CHANNEL REQUEST
message and SDCCH assignment is performed, which will surely lead to large
number of SDCCH assignment failures. The engineers check the faulty
signaling and find the RAs are the same, the TAs are consistent, and the FNs
are continuous. The continuous CHANNEL REQUEST messages indicate the
false access caused by the handover access of the co-BCCH cells.
If the two cells are only co-BCCH and they are near to each other, the DL
interference will happen, which leads to the SDCCH assignment failure.
Overshooting
There are two conditions of overshooting.
If the coverage range of the cell is too large, the signals at the marginal area
will be weak and they will be interfered by other cells. Then, the signaling loss
may happen during the random access, which leads to the SD assignment
failure.
If the coverage range of the cell is too large, this cell and one cell far from it are
co-BCCH and co-BSIC.
For the overshooting problem, the most fundamental solution is to adjust the
engineering parameters of the antenna, so as to control the coverage range. What
is more, the TA_allowed parameter can reduce the number of SDCCH assignment
failures due to overshooting effectively. The side effect is that the MS far from the
site cannot access the network. Therefore, the threshold of the TA_allowed
parameter should be set to be a little bit higher than the actual coverage range. For
the cell coverage range calculation, the influence of the transmission distance of the
repeater should be considered.
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For the TA_Allowed parameter adjustment, the risk is that the MS cannot make
reselection after being limited by the TA_Allowed parameter. The engineers make
a test for this problem.
If the MS selects a cell with the strongest power but the location update fails due to
the TA_Allowed parameter, the MS will select one cell with the second strongest
power in the reselection cells (If the CI of this cell is larger than 0, the MS can
access this cell) and it will not stay in the original cell. The time interval of the cell
reselection is decided by the random waiting time and maximum number of
resending times. And it always lasts for several seconds. The specific calculation
method is shown as follows.
Time interval of cell reselection = Random waiting time × Maximum retransmission
times + T3126
Judging from the reselection, during the cell reselection of the MS, there is penalty
strategy for the cells with reselection failure. Therefore, the MS can make
reselection even after it is limited by the TA_Allowed parameter.
Note:
For the TA_Allowed parameter, other vendors have the similar parameters. For
example, the corresponding parameter of Nortel is the RNDACCTIMADV parameter.
This parameter is relevant to the actual coverage range of the cell. If the threshold of the
parameter is setting properly, the pseudo-RACH request can be filtered. The
unnecessary SDCCH assignment should be avoided. Judging from the test, for the cell
with short coverage radius, if this parameter is set to 35, the RACH misjudgment (The
system decodes the noise as the RACH pulse wrongly) will be 30% of the total RACH
requests. If the threshold of the RNDACCTIMADV parameter is set to 2, there is no
RACH misjudgment.
UL noise interference
The BTS RX sensitivity is from –112 dBm to –125 dBm. If random access signals
lower than the BTS RX sensitivity is received, the signals are usually interference.
This type of interference will surely lead to SDCCH assignment failure.
The RACHMin parameter (network parameter) is set for the BTS filtering noise
signals, corresponding to the BTS receiving sensitivity. If the receiving level is lower
than that of the random access signal of the RACHMin parameter, the signal will be
abandoned as the noise signal. The SDCCH assignment success rate can be
enhanced effectively through the RACHMin parameter adjustment.
The interference signals usually have weak RxLev and large TA (larger than the
actual coverage scope). With parameters RACHAccMin and TA_allowed, the
engineers can greatly reduce the impact of interference.
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Note:
The engineers must be cautious when setting the RachMin parameter. If it is too large,
the paging success rate will be impacted.
Frequent location update initiated by the MS due to poor DL quality
Because the receiving sensitivity of the MS is lower than that of the BTS, the BTS
can receive the CHANNEL REQUEST message sent by the MS but the MS cannot
receive the IMM ASSIGN message sent by the BTS, especially when the MS is put
in the drawer or under the pillow. If the MS needs to initiate the location update, it
will send the CHANNEL REQUEST message frequently, with the cause being
location update. But it cannot receive the IMM ASSIGN message, which leads to a
large number of SDCCH assignment failures.
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2 SDCCH Signaling Flow and Relevant Counters and KPIs
After the random access, authentification, distinguishment, and encryption, the MS stays
on the SDCCH and starts to call and set up signaling.
2.1 SDCCH Signaling Setup Flow
Figure 2-1 SDCCH Signaling Flow
MS BTS BSC MSC Setup(SDCCH)
Setup
Call Proceeding
Call Proceeding(SDCCH)
Assignment Request
The call connection signaling flow is described as follows.
First, the MS sends one SETUP message to the network. This message includes
the specific service category required in this call and the bearing capability of the
MS.
After the MSC receives the SETUP message, it makes the call connection
according to the provided message. The MSC sends a CALL PROCEEDING
message to the MS.
After the call control entity of the MS receives the CALL PROCEEDING message, it
enters the mobile originating connection state.
2.2 SDCCH Counter List
This list mainly includes the CS measurement and call drop measurement counters. The
counters in the following table are applicable for V3 and V4.
Table 2-1 SDCCH Counter List
Counter ID Counter Name
C900060053 Number of SDCCH drops
C901070001 Number of call drops due to disconnection with RANA (On
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Counter ID Counter Name
SDCCH)
C901070002 Number of radio link failures (On SDCCH)
C901070003 Number of LAPD link failures (On SDCCH)
C901070004 Number of OMC-R forced releases (On SDCCH)
C901070005 Number of forced releases due to other calls (On SDCCH)
C901070007 Number of other failures (On SDCCH)
C901070008 Number of call drops due to disconnection with RANA (On TCH/F signaling)
C901070009 Number of radio link failures (On TCH/F signaling)
C901070010 Number of LAPD link failures (On TCH/F signaling)
C901070011 Number of OMC-R forced releases (On TCH/F signaling)
C901070012 Number of forced releases due to other calls (On TCH/F signaling)
C901070014 Number of other failures (On TCH/F signaling)
C901070029 Number of call drops due to disconnection with RANA (On TCH/H signaling)
C901070030 Number of radio link failures (On TCH/H signaling)
C901070031 Number of LAPD link failures (OnTCH/H signaling)
C901070032 Number of OMCR forced releases (On TCH/H signaling)
C901070033 Number of forced releases due to other calls (On TCH/H signaling)
C901070035 Number of other failures (On TCH/H signaling)
C901070053 Number of SDCCH link failures
C901070054 Number of TCH/F link failures
C901070055 Number of TCH/H link failures
C901070056 Number of RANA link releases due to other cause on SDCCH
2.3 SDCCH Call Drop KPI Definition and Affecting Factors
2.3.1 SDCCH Call Drop Definition
The SDCCH call drops indicate the call drops that happen when the BSC has allocated
the SDCCH channel to the MS but has not allocated the TCH channel successfully during
the call.
Statistical point: after the BSC receives the correct EST_IND message or after the
assignment completion and before the TCH assignment completion.
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Table 2-2 SDCCH Call Drop KPI Definition
KPI SDCCH Drop Rate
Definition Number of SDCCH call drop × 100%/(Random access call attempts + Number of voice channel call attempts (handover excluded) (signaling))
Counter formula
V2 C11605 × 100%/(C11625 + C11696)
V3
V6.0 C100030053 × 100%/(C100030003 + C100030010 + C100030038)
V6.2 C900060053 ×100%/(C900060003+C900060010 + C900060038)
V4 C901070050 × 100%/(C901250001+C901260001 + C901270001)
For the V3 formula definition, the basic measurement counter is adopted; for the V4
formula definition, the common measurement counter is adopted. And the counter
meaning and formula definitions of the two versions are consistent.
2.3.2 Counters Relevant to the SDCCH Call Drop
C900060053: number of SDCCH drops
Description
This counter counts the number of call drops on SDCCH channel due to radio
link failure, LAPD link break, or handover failure. Call drop occurs after MS
requests for TCH/H channel successfully. The counter increments if call drop is
due to the above causes.
Measurement point
This counter increments when call drop occurs on SDCCH channel due to
radio link failure, LAPD link failure, or handover failure.
C900060003: number of SDCCH seizure attempts for assignment
Description
The SDCCH channel allocation attempt is activated by the MS channel request
message CHL_REQ. After BSC receives this message, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060003
and C900060004 accumulate simultaneously. If occupation fails and the failure
is due to transceiver fault, C900060003, C900060004, and C900060005 do not
change. For failures in other cases, C900060003 and C900060005
accumulate simultaneously.
Measurement point:
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This counter counts when the BSC completes requesting for channel (due to
assignment) (C900060004 + C900060005). The measurement point is A1 in
the following figure.
Figure 2-2 Immediate Assignment Flow
MS BSC
CHL_REQ
CHL_RQD
CHL_ACT
CHL_ACT_ACK
IMM _ASS _CMD
IMM_ASS
SABM
EST_IND
BTS
A1
A2
A3
C900060010: number of signaling TCH/F seizure attempts for assignment
Description
This counter counts the number of TCH/F channel occupation attempts during
assigning signaling channels, including the number of occupation success
(C901260002) and the number of occupation failure (C900060011).After BSC
receives the channel request massage, it attempts to allocate channel for the
request. If allocation and occupation succeed, C901260002 and C900060010
accumulate simultaneously. If the request returns that the transceiver is faulty,
C901260002, C900060010, and C900060011 do not change. For other cases,
C900060010 and C900060011 accumulate simultaneously.
Measurement point
This counter counts when BSC completes requesting for channel (due to
assignment and the channel is used as signaling channel) (C901260002 +
C900060011). The measurement point is A1.
C900060038: number of signaling TCH/H seizure attempts for assignment
Description
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This counter counts the number of TCH/H channel being attempted to be
occupied during signaling channel assignment, including the number of
occupation success (C901270002) and the number of occupation failure
(C900060039).
After BSC receives the channel request massage, it attempts to allocate
channel for the request. If allocation and occupation succeed, C901270002
and C900060038 accumulate simultaneously. If the request returns that the
transceiver is faulty, C901270002, C900060038, and C900060039 do not
change. For other cases, C900060038 and C900060039 accumulate
simultaneously.
Measurement point
This counter counts when BSC completes requesting for channel (due to
assignment and the channel is used as signaling channel) (C901260002 +
C900060039). The measurement point is A1 in Figure 2-2 .
2.3.3 Factors Affecting the SDCCH Call Drop
Radio environment
The SDCCH call drop happens frequently in the area with poor signal coverage.
Interference, such as the internal interference due to improper frequency planning
and other external interference
Configuration of radio parameters
If he minimum access level of the cell is set to be too low, the call drop may
happen in the weak coverage area.
If the radio link fault timer is set to be too low, the call drop may happen due to
expiration in the condition of sudden deterioration. If the timer is set to be too
high, the radio resource utilization rate will decrease.
The power control parameter is improperly set. For example, if the level or
quality power control threshold is improperly set, the power of the MS may
decrease at the time of poor signal and call quality.
The frequency hopping parameter is improperly set. For example, the MAIO is
set improperly, which leads to the co-frequency and neighbor-frequency
interference in the same site and then the call drop.
Hardware fault
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For example, too weak power amplifier output power, large difference between the
transmission power of different carriers, and carrier transmitters, combiner, and
divider faults can lead to the SDCCH call drop.
Antenna and feeder system fault
For example, if the tilts and azimuths of two antennas in the cell are not consistent,
the SWR is large, the antenna is too high, or the downtilt is improper, the coverage
range will be too large, which leads to the overshooting. As a result, the remote
isolated-island effect exists and the call drop happens.
BTS transmission problem
For example, the transmission is interrupted or the transmission is unstable.
BTS hardware malfunction
For example, the E1 cable is unreliable and the CMM/CMB board and backplane
connections are faulty.
User factor
For example, the contact of the MS battery is poor.
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3 Signaling Flow and Relevant Counters and KPIs During the Voice Channel Allocation
3.1 Signaling Flow During the Voice Channel
Allocation
Figure 3-1 Signaling Flow of TCH Allocation
MS BTS BSC MSCAssignment
RequestChannel Activation
Channel Activation ACK
Assignment Command(SDCCH)
SABM(FACCH)Establish Indicationg
UA(FACCH)
Assignment Complete(FACCH)
Assignment Complete
The signaling flow of the voice channel allocation is shown as follows.
After the MSC sends a CALL PROCEEDING message to the MS, it sends an
ASSIGNMENT REQUEST message to the BSC to require the BSC to allocate the
TCH voice channel for this call. This message includes the call priority, DL DTX,
radio channel distinguish, and available interface bandwidth.
After the BSC receives the ASSIGNMENT REQUEST message from the MSC, if
there is any needed resource, it will send a CHANNEL ACTIVATION message to
the BTS to activate the TCH. This message includes the channel, frequency, time
slot, and frequency-hopping.
If the BTS finishes the resource (such as circuit) preparation, it will send a
CHANNEL ACTIVATIONG ACK message to the BSC. If there is no corresponding
ground resource, the BSC will send a RESOURCE FAILURE message to the MSC.
If the system allows queuing, the BSC will send a QUEUING INDICATION message
to the MSC and put the ASSIGNMENT REQUEST message into the queue and
enable T11. If this timer expires, the BSC will send a CLEAR REQUEST message
to the MSC.
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After the BSC receives the CHANNEL ACTIVATIONG ACK message, it will send
an ASSIGNMENT COMMAND message to the MS through the signaling channel.
This message includes channel description and channel mode (full rate/half rate).
After the MS receives the ASSIGNMENT COMMAND message from the network, it
hands over to the allocated channel.Then, the MS initiates the low layer connection
setup and arrange the sending and receiving configuration to this TCH and then
sends a SABM message to the BTS through the FACCH.
After the BTS receives the SABM message, it will send an ESTABLISH
INDICATION message to the BSC and send one UA confirmation frame to the MS
through the FACCH to make the preemptive judgment.
If the MS occupies the allocated channel successfully, it will send an ASSIGNMENT
COMPLETE message to the system through the FACCH.
The FACCH and TCH use the same channel. The only difference is that the ID of the
TCH burst pulse is changed from 0 to 1. This is called as frame stealing.
Note:
If the MS cannot occupy the specific channel because of radio interface failure, radio
interface message failure, or assignment message distinguish failure due to interference
and hardware problem, the MS will send an ASSIGNMENT FAILURE message to the
system on the original channel.
If the MS cannot receive the assignment command from the system or the system cannot
receive the response from the MS due to interference and then the T3107 expires, the
system will release the allocated channel.
3.2 Relevant Counter List During the TCH Allocation
This list mainly includes the CS measurement and TCH measurement counters.
In V4, the TCH2/F measurement counters and TCH2/H measurement counters are
added and the allocation flow is improved. The V4 allocation flow will be described in
Chapter 7.
Table 3-1 List of Counters During the TCH Allocation (for V3)
Counter ID Counter Name
C900060019 Number of voice TCH/F seizure attempts for assignment
C900060020 Number of voice TCH/F seizure failure for assignment
C900060023 Number of TCH/F allocation success for assignment (speech version 1)
C900060024 Number of TCH/F allocation failure for assignment (speech version 1)
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Counter ID Counter Name
C900060025 Number of TCH/F allocation success for assignment (speech version 2)
C900060026 Number of TCH/F allocation success for assignment (speech version 3)
C900060027 Number of TCH/F allocation failure for assignment (speech version 3)
C900060028 Number of voice TCH/F assignment success
C900060029 Number of voice TCH/F assignment failure for assignment
C900060042 Number of voice TCH/H seizure attempts for assignment
C900060043 Number of voice TCH/H seizure failure for assignment
C900060125 Average number of available radio channel
C900060126 Average number of unavailable radio channel
C900060127 TCH/H busy time
C900060128 Maximum number of busy TCH/F
C900060129 TCH/F busy time
C900060138 Number of TCH/F allocation failure due to BTS connection failure for assignment (speech version 1)
C900060139 Number of TCH/F allocation failure due to BIU connection failure for assignment (speech version 1)
C900060140 Number of TCH/F allocation failure due to TCU connection failure for assignment (speech version 1)
C900060141 Number of TCH/F allocation failure due to channel activation failure for assignment (speech version 2)
C900060142 Number of TCH/F allocation failure due to BTS connection failure for assignment (speech version 2)
C900060143 Number of TCH/F allocation failure due to BIU connection failure for assignment (speech version 2)
C900060144 Number of TCH/F allocation failure due to TCU connection failure for assignment (speech version 2)
C900060145 Number of TCH/F allocation failure due to BTS connection failure for assignment (speech version 3)
C900060146 Number of TCH/F allocation failure due to BIU connection failure for assignment (speech version 3)
C900060147 Number of TCH/F allocation failure due to TCU connection failure for assignment (speech version 3)
C900060148 Number of data TCH/F allocation failure due to BTS connection failure for assignment
C900060149 Number of data TCH/F allocation failure due to BIU connection failure for assignment
C900060150 Number of data TCH/F allocation failure due to TCU connection failure for assignment
C900060175 Number of TCH/H allocation failure due to channel activation failure for assignment (speech version 1)
C900060176 Number of TCH/H allocation failure due to BTS connection failure for assignment (speech version 1)
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Counter ID Counter Name
C900060177 Number of TCH/H allocation failure due to BIU connection failure for assignment (speech version 1)
C900060178 Number of TCH/H allocation failure due to TCU connection failure for assignment (speech version 1)
C900060183 Number of TCH/H allocation failure due to channel activation failure for assignment (speech version 2)
C900060184 Number of TCH/H allocation failure due to BTS connection failure for assignment (speech version 2)
C900060185 Number of TCH/H allocation failure due to BIU connection failure for assignment (speech version 2)
C900060186 Number of TCH/H allocation failure due to TCU connection failure for assignment (speech version 2)
C900060191 Number of TCH/H allocation failure due to channel activation failure for assignment (speech version 3)
C900060192 Number of TCH/H allocation failure due to BTS connection failure for assignment (speech version 3)
C900060193 Number of TCH/H allocation failure due to BIU connection failure for assignment (speech version 3)
C900060194 Number of TCH/H allocation failure due to TCU connection failure for assignment (speech version 3)
C900060199 Number of voice TCH/H assignment success
C900060200 Number of voice TCH/H assignment failure
C900060201 Number of voice TCH/H handover success
C900060202 Number of data TCH/H allocation failure due to channel activation failure for assignment
C900060203 Number of data TCH/H allocation failure due to BTS connection failure for assignment
C900060204 Number of data TCH/H allocation failure due to BIU connection failure for assignment
C900060205 Number of data TCH/H allocation failure due to TCU connection failure for assignment
C900060217 Number of unavailable defined TCH/H
C900060218 Number of unavailable defined TCH/F
C900060244 Number of voice TCH/F drops due to radio link failure
C900060245 Number of voice TCH/H drops due to radio link failure
C901260021 Number of voice TCH/F seizure success for assignment
C901260026 Number of TCH/F allocation attempts for assignment (speech version 1)
C901260038 Number of TCH/F allocation attempts for assignment (speech version 2)
C901260050 Number of TCH/F allocation attempts for assignment (speech version 3)
C901260062 Number of voice TCH/F assignment attempts
C901260065 Number of voice TCH/F handover attempts
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Counter ID Counter Name
C901260067 Number of voice TCH/F handover failure
C901260068 Number of voice TCH/F handover failure messages do not received by BSC
C901260094 Number of data TCH/F allocation failure for assignment due to TIPB connection failure (speech version 1)
C901260095 Number of data TCH/F allocation failure for assignment due to resource request to iTC failure (speech version 1)
C901260098 Number of data TCH/F allocation failure for assignment due to TIPB connection failure (speech version 2)
C901260099 Number of data TCH/F allocation failure for assignment due to resource request to iTC failure (speech version 2)
C901260102 Number of data TCH/F allocation failure for assignment due to TIPB connection failure (speech version 3)
C901260103 Number of data TCH/F allocation failure for assignment due to resource request to iTC failure (speech version 3)
C901260106 Number of TCH/F allocation failure due to request AIPB resource failure for assignment (speech version 1)
C901260107 Number of TCH/F allocation failure due to request UDP port failure for assignment (speech version 1)
C901260108 Number of TCH/F allocation failure due to AIPB connection failure for assignment (speech version 1)
C901260112 Number of TCH/F allocation failure due to request AIPB resource failure for assignment (speech version 2)
C901260113 Number of TCH/F allocation failure due to request UDP port failure for assignment (speech version 2)
C901260114 Number of TCH/F allocation failure due to AIPB connection failure for assignment (speech version 2)
C901260118 Number of TCH/F allocation failure due to request AIPB resource failure for assignment (speech version 3)
C901260119 Number of TCH/F allocation failure due to request UDP port failure for assignment (speech version 3)
C901260120 Number of TCH/F allocation failure due to AIPB connection failure for assignment (speech version 3)
C901270021 Number of voice TCH/H seizure success for assignment
C901270026 Number of TCH/H allocation attempts for assignment by BSC (speech version 1)
C901270027 Number of TCH/H allocation success for assignment (speech version 1)
C901270038 Number of TCH/H allocation attempts by BSC for assignment (speech version 2)
C901270039 Number of TCH/H allocation success by BSC for assignment (speech version 2)
C901270050 Number of TCH/H allocation attempts for assignment (speech version 3)
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Counter ID Counter Name
C901270051 Number of TCH/H allocation success for assignment (speech version 3)
C901270062 Number of voice TCH/H assignment attempts by BSC
C901270065 Number of voice TCH/H handover attempts
C901270067 Number of voice TCH/H handover failure
C901270068 Number of voice TCH/H handover failure messages do not received by BSC
C901270094 Number of TCH/H allocation failure for assignment due to TIPB connection failure (speech version 1)
C901270095 Number of TCH/H allocation failure for assignment due to resource request to iTC failure (speech version 1)
C901270099 Number of TCH/H allocation failure for assignment due to resource request to iTC failure (speech version 2)
C901270102 Number of TCH/F allocation failure for assignment due to TIPB connection failure (speech version 3)
C901270103 Number of TCH/H allocation failure for assignment due to resource request to iTC failure (speech version 3)
C901270106 Number of TCH/H allocation failure due to request AIPB resource failure for assignment (speech version 1)
C901270107 Number of TCH/H allocation failure due to request UDP port failure for assignment (speech version 1)
C901270108 Number of TCH/H allocation failure due to AIPB connection failure for assignment (speech version 1)
C901270112 Number of TCH/H allocation failure due to request AIPB resource failure for assignment (speech version 2)
C901270113 Number of TCH/H allocation failure due to request UDP port failure for assignment (speech version 2)
C901270114 Number of TCH/H allocation failure due to AIPB connection failure for assignment (speech version 2)
C901270118 Number of TCH/H allocation failure due to request AIPB resource failure for assignment (speech version 3)
C901270119 Number of TCH/H allocation failure due to request UDP port failure for assignment (speech version 3)
C901270120 Number of TCH/H allocation failure due to AIPB connection failure for assignment (speech version 3)
C901270124 Number of TCH/H allocation failure due to request AIPB resource failure for assignment (data)
C901270125 Number of TCH/H allocation failure due to request UDP port failure for assignment (data)
C901270126 Number of TCH/H allocation failure due to AIPB connection failure for assignment (data)
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3.3 TCH Allocation Success Rate Definition and
Failure Cause Description
3.3.1 TCH Allocation Success Rate KPI Definition
Table 3-2 TCH Allocation Success Rate (Handover Excluded) KPI Definition
KPI TCH Allocate Success Rate (%) (Exclude Handover)
Definition Number of TCH allocation success times (handover excluded)/Number of TCH allocation requests (handover excluded)
Counter formula
V2 (1-(C11654+C11610+C11658)/(C11609) × 100%
V3
V6.0
(C100030017 + C100030028 + C100030036 + C100030235 + C100030199 + C100030210) × 100%/(C100030010 + C100030019 + C100030030 + C100030038 + C100030042 + C100030046)
V6.2
(C900060017 + C900060028 + C900060036+C900060235 + C900060199 + C900060210) × 100%/(C900060010 + C900060019 + C900060030 + C900060038 + C900060042 + C900060046)
V4
(C900060017 + C900060028 + C900060036 + C900060235 + C900060199 + C900060210) × 100%/(C900060010 + C900060019 + C900060030 + C900060038 + C900060042 + C900060046)
In V4, for the allocation success rate, part of TCH 2 measurement statistics are not
considered, which is good for the allocation success rate.
ZTE defines three stages for the TCH allocation process: occupation (congestion),
allocation (channel activation), and assignment
TCH occupation (congestion): After receiving an ASSIGN REQUEST message, the
BSC checks the database to confirm whether there is any available channel. If there
is, the occupation is successful. If there is no available channel and the queuing,
directional retry, or force release function does not exist, the TCH congestion
happens. If the queuing, directional retry, or force release function exists, start the
corresponding timer and enable the function, so as to wait the available resources. If
available resources appear within the time range of the timer, the occupation is
successful. If the timer expires, the TCH congestion happens.
TCH allocation (activation): After the channel resources are applied successfully
from the data base, the BSC send a CHANNEL ACTIVATION message to the BTS,
that is the TCH allocation attempt. After the BSC receives a
CHANNELACTIVATIONACK message from the BTS, the allocation is successful. If
the BSC receives a CHANNELACTIVATIONNACK message from the BTS or it
does not receives the CHANNELACTIVATIONACK message within the time range
of the timer, the allocation fails.
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TCH assignment: After the BSC receives the CHANNELACTIVATIONACK
message during the channel allocation, it will send an ASSIGN COMMAND
message on the DL SDCCH and one TCH assignment attempt is counted. After the
BSC receives an ASSIGN COMPLETE message from the BTS, this assignment is
successful. Otherwise, the assignment fails.
3.3.2 TCH Allocation Failure Description
The voice channel allocation flow is shown in the following figure.
Figure 3-2 Voice Channel Allocation Flow
The ASSIGNMENT FAILURE message corresponds to the ASSIGNMENT REQUEST
message, which reflects one TCH allocation failure.For assignment failure 1, in most
cases, the cause is no idle channel; for assignment failure 2, the cause is the channel
allocation failure due to BTS fault; for assignment failure 3, it mainly indicates the channel
assignment failure on the air interface and the causes include the coverage and
interference.
3.3.3 Main Causes of TCH Allocation Failure
The common TCH allocation failure may be caused by the following issues.
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Cell traffic congestion
If the congestion rate of the cell is high, when the MS applies for the voice channel,
the system will find that there is no TCH resource for allocation, which will lead to
the allocation failure
Hardware fault
When the TRX is faulty, the allocation failure rate is always high and the
incoming handover failure rate is also high, for the BSC assigns the channel for
the MS at the time of incoming handover. If the cell allocation failure rate is
higher than 10%, possibility of TRX fault is high. For this kind of cells, in order
to locate the faulty TRX, the engineers can record the Abis-interface signaling
of these cells and find out the specific TRX leading to the allocation failure
through the signaling analysis.
Combiner fault, such as no forward power output
Clock board fault or internal cable fault
Co-frequency or neighbor-frequency interference
Because of the high code error rate caused by interference, MS is unable to set up
L2 link with BTS, which will result in handover failure;
Antenna and feeder system fault
The feeders are corroded or worn down, which leads to high VSWR and affects the
RX performance.
The main and diversity antennas are blocked or the coverage is uneven. When the
antenna with the TCH is blocked is not the same with another antenna with the
BCCH or SDCCH, the MS cannot occupy this TCH.
Improper parameters
If the frequency-hopping is adopted and the HSM or MAIO is set improperly, the
co-frequency or neighbor-frequency interference will be serious in the cell or
between the same cells in the frequency-hopping group, which will lead to the
allocation failure.
If T3107 is set to be too small, the network will release the channel due to T3107
expiration before it receives the assignment completion message.
Transmission fault on the A interface or Abis interface
If the transmission error code rate on the A interface or Abis interface is high, the
signaling exchange cannot be completed normally between the MS and network,
which leads to the allocation failure.
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Influence from the repeater
When the outdoor repeater is adopted, the microwave transmission mode is usually
adopted. Therefore, when the repeater amplifies the UL and DL signals, the
interference signals will also be amplified, which leads to the poor quality
deterioration and call drop. As a result, the TCH allocation failure rate increases
obviously.
3.3.4 Problem Handling Process
It’s recommended to locate problem through checking radio parameters and hardware.
Procedure of handling TCH handover failure problem is described as follows.
1. Check the traffic to confirm whether there is any congestion. If there is, solve the
problem through the capacity expansion and traffic balancing.
2. Check whether the radio parameter setting is reasonable, such as the
frequency-hopping parameter and frequency data. For the improper parameters,
make optimization and adjustment.
3. Check KPIs, just like BER and idle interference band, so as to reduce or eliminate
radio interference.
4. Check the cell hardware, including CDU, RF connection lines between boards, and
change hardware with faults;
5. Check antenna system, including VSWR, direction of antennas in the same cell,
wrong installation or reversed installation of antenna feeders, make necessary
adjustment and changes.
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3.4 TCH Congestion Description, Cause, and
Handling Flow
3.4.1 TCH Congestion Description
After receiving the ASSIGNMENT REQUEST message from the MSC, the BSC will
search for suitable TCHs. If there is no available TCH, the BSC will send an
ASSIGNMENT FAILURE message to the MSC with the cause of no radio resource
available.
3.4.2 TCH Congestion Rate KPI Definition
Table 3-3 TCH Congestion Rate Definition
KPI TCH Congestion Rate
Definition Number of TCH congestion times × 100%/Number of TCH call attempts
Counter formula
V2 (C11612 - C11699) × 100%/(C11611-C11698)
V3 V6.0
(C100030020 + C100030031 + C100030043 + C100030047 + C100030022 +
C100030033 + C100030045 + C100030049) ×
100%/(C100030019 + C100030030+ C100030042 + C100030046 + C100030021 + C100030032 + C100030044 + C100030048)
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KPI TCH Congestion Rate
V6.2
(C900060020 + C900060031 + C900060043 + C900060047 + C900060022 + C900060033+ C900060045 + C900060049) × 100%/(C900060019 + C900060030 + C900060042 + C900060046 + C900060021 + C900060032 + C900060044 + C900060048)
V4
(C900060020 + C900060031 + C900060043+ C900060047 + C900060022 + C900060033 + C900060045 + C900060049) ×100%/(C900060019 + C900060030 + C900060042 + C900060046 + C900060021 + C900060032 + C900060044 + C900060048)
3.4.3 Counters Relevant to the TCH Congestion
C900060019: number of voice TCH/F seizure attempts
Description
This counter counts the number of TCH/F channel being attempted to be
occupied during voice channel assignment, including the number of occupation
success (C901260021) and the number of occupation failure (C900060020).
After BSC receives the channel request massage, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060019
and C901260021 accumulate simultaneously. If occupation fails, C900060019
and C900060020 accumulate simultaneously. If the request returns no channel
available but queuing or forced release is possible, enter the state of waiting
for resource. If the waiting for resource succeeds, C900060019 and
C901260021 accumulate simultaneously. If the waiting for resource fails,
C900060019 and C900060020 accumulate simultaneously. If the request
returns that the transceiver is faulty, C900060019, C901260021, and
C900060020 do not change. For failures in other cases, C900060019 and
C900060020 accumulate simultaneously.
Measurement point
The BSC completes requesting for channel (due to assignment and the
channel is used as voice channel), or BSC receives the internal message of
waiting for resource successfully or waiting for resource failed (C901260021 +
C900060020). The measurement point is B1, as shown in Figure 3-2 and
Figure 3-3.
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Figure 3-3 Common Assignment Flow (Internal TC)
MS
SABM
ASS_COM
ASS_CMD
BSCBTS
CHL_ACT
CHL_ACT_ACK
B1
B2
UA EST_IND
B3
MSC
ASS_REQ
ASS_CMD
Abis,TCU connect
B4ASS_COM
B5 ASS_COM
Figure 3-4 Common Assignment Flow (External TC)
MS BTS BSC
B1
CHL_ACT
ASS_REQ
B2
ASS_CMDASS_CMD
SABM
EST_IND
B3
iTC
CHL_ACT_ACK
UA
ASS_COMASS_COM
ASS_COM
B4
B5
MSC
TCRescr Req
TCRescr Ack
iTC Connect Req
iTC Connect Ack
C900060020: number of voice TCH/F seizure failure
Description
This counter counts the number of TCH/F channel occupation failure during
voice channel assignment.
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After BSC receives the channel request message, it attempts to allocate
channel for the request. If occupation fails, this counter accumulates. If the
request returns no channel available but queuing or forced release is possible,
enter the state of waiting for resource. If the waiting for resource fails, this
counter accumulates. If the request returns that the transceiver is faulty, this
counter does not change. For failures in other cases, this counter
accumulates.Measurement point
The BSC requests for channel (due to assignment and the channel is used as
voice channel) but fails to occupy the channel, or BSC fails to wait for internal
resource. The measurement point is B1, as shown in the following figure.
Figure 3-5 Common Assignment Failure Flow 1 (Internal TC)
C900060021: number of voice TCH/F seizure attempts for handover
Description
This counter counts the number of TCH/F channel being attempted to be
occupied during voice channel handover, including the number of occupation
success (C901260024) and the number of occupation failure (C900060022).
After BSC receives the channel request message, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060021
and C901260024 accumulate simultaneously. If occupation fails, C900060021
and C900060022 accumulate simultaneously. If the request returns no channel
available but queuing or forced release is possible, enter the state of waiting
for resource. If the waiting for resource succeeds, C900060021 and
C901260024 accumulate simultaneously. If the waiting for resource fails,
C900060021 and C900060022 accumulate simultaneously. If the request
returns that the transceiver is faulty, C900060021, C901260024, and
C900060022 do not change. For failures in other cases, C900060021 and
C900060022 accumulate simultaneously.
Measurement point
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The BSC completes requesting for channel (due to handover and the channel
is used as voice channel), or BSC receives the internal message of waiting for
resource successfully or waiting for resource failed (C901260024 +
C900060022).
C900060022: number of voice TCH/F seizure failure for handover
Description
This counter counts the number of TCH/F channel occupation failure during
voice channel handover.
After BSC receives the channel request message, it attempts to allocate
channel for the request. If allocation or occupation fails, this counter
accumulates. If the request returns no channel available but queuing or forced
release is possible, enter the state of waiting for resource. If the waiting for
resource fails, this counter accumulates. If the request returns that the
transceiver is faulty, this counter does not change. For failures in other cases,
this counter accumulates.
Measurement point
The BSC requests for channel (due to handover and the channel is used as
voice channel) but fails to occupy the channel, or BSC fails to wait for internal
resource. The measurement point is C1, as shown in Figure 3-5, and D1, as
shown in Figure 3-6.
Figure 3-6 Intra-BSC Handover Occupation Failure Flow
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Figure 3-7 Inter-BSC Handover Occupation Failure Flow
C900060030: number of data TCH/F seizure attempts for assignment
Description
This counter counts the number of TCH/F channel being attempted to be
occupied during data channel handover, including the number of occupation
success (C901260070) and the number of occupation failure (C900060031).
After BSC receives the channel request message, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060030
and C901260070 accumulate simultaneously. If occupation fails, C900060030
and C900060031 accumulate simultaneously. If the request returns no channel
available but queuing or forced release is possible, enter the state of waiting
for resource. If the waiting for resource succeeds, C900060030 and
C901260070 accumulate simultaneously. If the waiting for resource fails,
C900060030 and C900060031 accumulate simultaneously. If the request
returns that the transceiver is faulty, C900060030, C901260070, and
C900060031 do not change. For failures in other cases, C900060030 and
C900060031 accumulate simultaneously.
Measurement point
The BSC completes requesting for channel (due to assignment and the
channel is used as data channel), or BSC receives the internal message of
waiting for resource successfully or waiting for resource failed (C901260070 +
C900060031). The measurement point is B1, as shown in Figure 3-2 and
Figure 3-3.
C900060031: number of data TCH/F seizure failure for assignment
Description
This counter counts the number of TCH/F channel occupation failure during
data channel assignment.
After BSC receives the channel request message, it attempts to allocate
channel for the request. If occupation fails, this counter accumulates. If the
request returns no channel available but queuing or forced release is possible,
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enter the state of waiting for resource. If the waiting for resource fails, this
counter accumulates. If the request returns that the transceiver is faulty, this
counter does not change. For failures in other cases, this counter accumulates.
Measurement point
The BSC requests for channel (due to assignment and the channel is used as
data channel) but fails to occupy the channel, or BSC fails to wait for internal
resource. The measurement point is B1, as shown in Figure 3-4.
C900060032: number of data TCH/F seizure attempts for handover
Description
This counter counts the number of TCH/F channel being attempted to be
occupied during data channel handover, including the number of occupation
success (C901260073) and the number of occupation failure (C900060033).
If allocation and occupation succeed, C900060032 and C901260073
accumulate simultaneously. If occupation fails, C900060032 and C900060033
accumulate simultaneously. If the request returns no channel available but
queuing or forced release is possible, enter the state of waiting for resource. If
the waiting for resource succeeds, C900060032 and C901260073 accumulate
simultaneously. If the waiting for resource fails, C900060032 and C900060033
accumulate simultaneously. If the request returns that the transceiver is faulty,
C900060032, C901260073, and C900060033 do not change. For failures in
other cases, C900060032 and C900060033 accumulate simultaneously.
Measurement point
The BSC completes requesting for channel (due to handover and the channel
is used as data channel), or BSC receives the internal message of waiting for
resource successfully or waiting for resource failed (C901260073 +
C900060033).
C900060033: number of data TCH/F seizure failure for handover
Description
This counter counts the number of TCH/F channel occupation failure during
data channel handover.
After BSC receives the channel request message, it attempts to allocate
channel for the request. If occupation fails, this counter accumulates. If the
request returns no channel available but queuing or forced release is possible,
enter the state of waiting for resource. If the waiting for resource fails, this
counter accumulates. If the request returns that the transceiver is faulty, this
counter does not change. For failures in other cases, this counter accumulates.
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Measurement point
The BSC requests for channel (due to assignment and the channel is used as
data channel) but fails to occupy the channel, or BSC fails to wait for internal
resource. The measurement point is C1 in Figure 3-2 and D1 in Figure 3-3.
C900060042: number of voice TCH/H seizure attempts for assignment
Description
This counter counts the number of TCH/H channel being attempted to be
occupied during voice channel handover, including the number of occupation
success (C901270021) and the number of occupation failure (C900060043)..
Measurement point
The BSC completes requesting for channel (due to assignment and the
channel is used as voice channel), or BSC receives the internal message of
waiting for resource successfully or waiting for resource failed (C901260021 +
C900060043). The measurement point is C1 in Figure 3-2 and D1 in Figure
3-3.
C900060043: number of voice TCH/H seizure failure for assignment
Description
This counter counts the number of TCH/H channel occupation failure during
voice channel assignment.
After BSC receives the channel request message, it attempts to allocate
channel for the request. If occupation fails, this counter accumulates. If the
request returns no channel available but queuing or forced release is possible,
enter the state of waiting for resource. If the waiting for resource fails, this
counter accumulates. If the request returns that the transceiver is faulty, this
counter does not change. For failures in other cases, this counter accumulates.
Measurement point
The BSC requests for channel (due to assignment and the channel is used as
voice channel) but fails to occupy the channel, or BSC fails to wait for internal
resource. The measurement point is B1, as shown in Figure 3-4.
C900060044: number of voice TCH/H seizure attempts for handover
Description
This counter counts the number of TCH/H channel being attempted to be
occupied during voice channel handover, including the number of occupation
success (C901270024) and the number of occupation failure (C900060045).
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After BSC receives the channel request massage, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060044
and C901270024 accumulate simultaneously. If occupation fails, C900060044
and C900060045 accumulate simultaneously. If the request returns no channel
available but queuing or forced release is possible, enter the state of waiting
for resource. If the waiting for resource succeeds, C900060044 and
C901270024 accumulate simultaneously. If the waiting for resource fails,
C900060044 and C900060045 accumulate simultaneously. If the request
returns that the transceiver is faulty, C900060044, C901270024, and
C900060045 do not change. For failures in other cases, C900060044 and
C900060045 accumulate simultaneously.
Measurement point: The BSC completes requesting for channel (due to
handover and the channel is used as voice channel), or BSC receives the
internal message of waiting for resource successfully or waiting for resource
failed (C901260024 + C900060045).
C900060045: number of voice TCH/H seizure failure for handover
Description
This counter counts the number of TCH/H channel occupation failure during
voice channel handover.
After BSC receives the channel request message, it attempts to allocate
channel for the request. If allocation or occupation fails, this counter
accumulates. If the request returns no channel available but queuing or forced
release is possible, enter the state of waiting for resource. If the waiting for
resource fails, this counter accumulates. If the request returns that the
transceiver is faulty, this counter does not change. For failures in other cases,
this counter accumulates.
Measurement point
The BSC requests for channel (due to handover and the channel is used as
voice channel) but fails to occupy the channel, or BSC fails to wait for internal
resource. The measurement point is C1, as shown in Figure 3-2, and D1, as
shown in Figure 3-3.
C900060046: number of data TCH/H seizure attempts for assignment
Description
This counter counts the number of TCH/H channel being attempted to be
occupied during signaling channel assignment, including the number of
occupation success (C901270070) and the number of occupation failure
(C900060047).
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After BSC receives the channel request massage, it attempts to allocate
channel for the request. If allocation and occupation succeed, C900060046
and C901270070 accumulate simultaneously. If occupation fails, C900060046
and C900060047 accumulate simultaneously. If the request returns no channel
available but queuing or forced release is possible, enter the state of waiting
for resource. If the waiting for resource succeeds, C900060046 and
C901270070 accumulate simultaneously. If the waiting for resource fails,
C900060046 and C900060047 accumulate simultaneously. If the request
returns that the transceiver is faulty, C900060046, C901270070, and
C900060047 do not change. For failures in other cases, C900060046 and
C900060047 accumulate simultaneously.
Measurement point
The BSC completes requesting for channel (due to assignment and the
channel is used as data channel), or the BSC receives the internal message of
waiting for resource successfully or waiting for resource failed (C901260070 +
C900060047). The measurement point is B1, as shown in Figure 3-2 and
Figure 3-3.
C900060047: number of data TCH/H seizure failure for assignment
Description
This counter counts the number of TCH/F channel occupation failure during
data channel assignment.
After the BSC receives the channel request message, it attempts to allocate
channel for the request. If occupation fails, this counter accumulates. If the
request returns no channel available but queuing or forced release is possible,
enter the state of waiting for resource. If the waiting for resource fails, this
counter accumulates. If the request returns that the transceiver is faulty, this
counter does not change. For failures in other cases, this counter accumulates.
Measurement point
The BSC requests for channel (due to assignment and the channel is used as
data channel) but fails to occupy the channel, or the BSC fails to wait for
internal resource. The measurement point is B1, as shown in Figure 3-4.
C900060048: number of data TCH/H seizure attempts for handover
Description
This counter counts the number of TCH/H channel being attempted to be
occupied during data channel handover, including the number of occupation
success (C901270073) and the number of occupation failure (C900060049).
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Measurement point
The BSC completes requesting for channel (due to handover and the channel
is used as data channel), or BSC receives the internal message of waiting for
resource successfully or waiting for resource failed (C901260073 +
C900060049).
C900060049: number of data TCH/H seizure failure for handover
Description
This counter counts the number of TCH/H channel occupation failure during
data channel handover.
After the BSC receives the channel request message, it attempts to allocate
channel for the request. If occupation fails, this counter accumulates. If the
request returns no channel available but queuing or forced release is possible,
enter the state of waiting for resource. If the waiting for resource fails, this
counter accumulates. If the request returns that the transceiver is faulty, this
counter does not change. For failures in other cases, this counter accumulates.
Measurement point
The BSC requests for channel (due to assignment and the channel is used as
data channel) but fails to occupy the channel, or the BSC fails to wait for
internal resource. The measurement point is C1 in Figure 3-2 and D1 in Figure
3-3.
3.4.4 Main Causes of the TCH Congestion
Main causes for channel congestion are as follows:
High traffic density, which even exceeds the designed capacity of BTS
Transmission failure
When the transient or high error code happens for the transmission on the Abis
interface, because this fault has not been happened on the BSC, the congestion
happens due to unavailable ground circuit resource at the time of channel activation
of the BSC. After the queuing function is activated, this event is more obvious.
Unstable hardware
For example, the lack of usable resources or channel congestion caused by
unstable equipment performance
Problems with adjacent cells
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Because of the faulty adjacent cell, the serving cell absorbs some extra traffic and
the congestion happens.
Unreasonable parameter setting
The T3107 and T3103 are set to be too large and the queuing parameters are set
unreasonably; the handover threshold and capacity are set improperly; the minimum
access level and BTS power are defined improperly.
Too large coverage, leading to the isolated-island effect
3.4.5 TCH Congestion Handling Process
The Handling steps for TCH congestion are as follows:
1. Check if the problem cell and its adjacent cells operate normally; check the TCH
usability to locate the unstable equipment. If adjacent cells work abnormally, the
problem cell will have to bear their traffic besides its own load.
2. Check the MS mobility to see if the TCH congestion is caused by excess incoming
handovers. It it’s true, optimize the handover parameters (increasing the HO_
Margin parameter) to reduce number of handovers from adjacent cells to the
congested cell, so as to ease the cell from congestion.
3. Check setting of radio parameters. The unreasonable setting of these parameters
(such as delay of cell reselection, handover tolerance limit, and level of outgoing
handover trigger) can result in pingpong location renewal and pingpong handover.
4. Through test of field strength, analyze if coverage is too large and if the
isolated-island effect exists. When the isolated-island effect happens to one cell in
an area, where predefined adjacent cells cannot be detected, the MS will constantly
stay with the serving cell; and normal handovers cannot be triggered, in spite of any
changes on signals, and finally call drops will be caused. To avoid this case, two
methods can be adopted. The first one is adjusting the antenna of the isolated cell to
eliminate the effect. However, due to the complexity in electric wave transmission, it
takes several tests to abate the effect, and it is really difficult to completely eliminate
the effect due to high buildings. The second method is defining new adjacent cells
for the isolated cell. The principle for defining related parameters is that
handovers/LAC renewal from the isolated cell to normal cells has priority over the
reversed ones.
5. Congestion due to high traffic density
Check if the BTS capacity configuration reaches the max. If not, expand it with
enough TRXs.
The general flow for handling TCH congestion is shown in the following figure.
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Figure 3-8 Flow of Handling TCH Congestion
The TCH congestion of one
cell is too high.
Check the channel
availability rate of the
cell
Check whether the
coverage is too large
and the isolated-island
effect exists.
Reduce the coverage
and eliminate the
isolated-island effect.
Set the parameters
properly.
Optimize the
handover parameter
and reduce the
handovers.
Check whether the
neighbor cell is faulty.
Check whether the fault
is caused by excessive
handovers.
Check the radio
parameter setting.
Yes
Troubleshoot the
neighbor cell fault.
Troubleshoot the
hardware fault.
Low
Yes
Improper
Yes
The congestion is caused by
the high traffic density.
Check whether the BTS
has the maximum
configuration.
End
Plan enough TRXs for the
expansion.
Reduce the BTS
power and increase
the downtilt, so as to
reduce the
congestion.
Yes
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4 Call Connection Process
4.1 Signaling Flow of the Call Connection Process
After the TCH allocation, the call connection process starts. The signaling process is
described as follows.
Figure 4-1 Call Connection Process
MS BTS BSC MSC GMSCRF Channel Release
RF Channel Release
ACKAlerting
ACM
Alerting (SDCCH)
ANSWER
Connect
Connect (SDCCH)
Connect ACK (FACCH)
Connect ACK
Measurement
Report (SACCH) Measurement Report
IAI
When the MS informs the network that it has occupied the TCH and it is
unnecessary to build the SDCCH for occupation for this call. The SDCCH is
released through the channel release program.
The MSC receives an ADDRESS COMPLETE message from the terminating end
and sends an ALERTING message to the MS. At this time, the originating user can
hear the alerting, which means that the terminating user is in alerting.
After the terminating user answers the call, the terminating end will send an
ANSWER message to the originating MSC. At this time, the link between the
originating end and terminating end is connected and the MS send a CONNECT
message to the MS. After the MS receives a CONNECT message, it sends one
connection confirmation message. All the local alerting indications are stopped and
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the billing starts. At this time, the call is set up and the two ends enter the
conversation stage.
After the main signaling channel is built, the MS sends the measurement report
about the voice quality twice per second.
4.2 Counters in the Call Connection Process
Table 4-1 Counters in the Call Connection Process
Counter ID Counter Name
C900060244 Number of voice TCH/F drops due to radio link failure
C900060245 Number of voice TCH/H drops due to radio link failure
C901070017 Number of voice TCH/F drops due to LAPD link failure
C901070038 Number of voice TCH/H drops due to LAPD link failure
C900060054 Number of TCH/F link failures
C900060055 Number of TCH/H link failures
C900060244 (C900060245)
Description
This counter counts the number of voice TCH/F drops due to radio link failure.
After MS applies for TCH/F voice channel, call drops. If this problem is caused
by radio link failure, then this counter increments.
Measurement point
On TCH/F voice channel, when BSC receives a CONNECTION FAILURE
INDICATION message at Abis interface, this counter counts. The
measurement point is A, as shown in the following figure.
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Figure 4-2 Call Drop Caused by the Radio Link Fault
C901070017 (C901070018)
Description
This counter counts the number of call drop due to LAPD link failures (on
TCH/F voice channel). The counter increments if the phenomenon that the call
drop after MS has obtained TCH/F voice channel is caused by LAPD link
failures.
Measurement point
The counter increments when BSC receives a DLREL_IND message from
LAPD on TCH/F voice channel.
C900060054 (C900060055)
Description
This counter counts the number of call drops on TCH/F channel due to radio
link failure, LAPD link break, or handover failure. Call drop occurs after MS
requests for TCH/H channel successfully. The counter increments if call drop is
due to the above causes.
Measurement point
This counter increments when call drop occurs on TCH/F channel due to radio
link failure, LAPD link failure, or handover failure.
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4.3 Call Drop During the Call Connection Process
In the mobile communication, the call drop indicates the call loss or interruption due to a
certain cause after the TCH allocation. The call drop causes a lot of inconveniences for
the user and it is a hot spot for user’s complaint.
The call drops fall into the following categories.
Call drops due to radio link fault (RF loss call drop)
Call drops due to handover failure
LAPD call drops
In this chapter, only the call drops due to radio link fault and LAPD call drops during the
call connection are described.
4.3.1 Causes of Call Drops due to Radio Link Fault
The radio link fault is divided into UL failure and DL failure.
The DL failure
According to the GSM protocol, one initial value is given to the timer S (T100) in the
MS, which is the value of the radio_link_timeout parameter. This value is
broadcasted on the BCCH. When the MS cannot decode one SACCH message
(four SACCH congestion times) correctly, the S will be reduced by 1.When the MS
decodes one SACCH message correctly, the S will be increased by 2.But the value
of S is no larger than the initial value of the radio_link_timeout parameter. When
the value of S is 0, the MS will give up the radio resource connection and enter the
idle mode. As a result, one call drop will happen.
The UL failure
The parameter for the system monitoring the UL link failure is the link_fail
parameter. When the BTS cannot decode one SACCH message correctly, the
counter in the HDPC (the maximum value is decided by the link_fail parameter.)
will be reduced by 1. When the BTS decodes one SACCH message correctly, the
counter will be increased by 2 (The value of the counter cannot exceed the value
decided by the Link_fail parameter.)When the value of the counter is 0, the BTS will
stop transmitting the DL SACCH and start the rr_t3109 (rr_t3109 > T100).When the
T100 of the MS expires, the MS will return to the idle mode and the call drop
happens. When the rr_t3109 expires, the BTS will release the radio channel. And
the BSC needs to send one CLEAR REQUEST message to the MSC.
Either the UL failure or the DL failure can stop the SACCH transmission. Then the radio
resource release is triggered. If one link failure (link_fail) happens on the TCH, one
RF_LOSSES_TCH will be counted.
The main causes of call drops due to radio link fault are listed as follows.
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The weak coverage area exists and the radio signal is poor.
The interference exists, such as the internal interference due to improper frequency
planning and external interference.
Improper configuration of radio parameters
The minimum access level is set to be too low and the MS makes calls in the
weak coverage area and the call drop happens easily.
The NCC Permitted parameter is improperly set. In some network, the serving
cell and neighbor cells may use different NCCs. It is necessary to add the
NCCs used by the corresponding neighbor cells to the NCC Permitted
parameter. Once the setting is improper, the MS will not detect the neighbor
cells with a certain NCC, which leads to the handover failure. Then the RF loss
and call drop happen.
If the radio link fault timer is set to be too low, the call drop may happen due to
expiration in the condition of sudden deterioration. If the timer is set to be too
high, the radio resource utilization rate will decrease.
The setting of power control parameter is unreasonable, such as the level and
the quality power control threshold. As a result, the MS may have poor signal
and quality and the power may become weaker.
The setting of frequency-hopping parameter is unreasonable, such as the
MAIO configuration. Then the co-frequency and neighbor-frequency
interference exists in the same site.
The incomplete neighbor cell data definition or configuration error leads to the
signal improvement through the handover and then the call drop happens due
to the signal deterioration.
The handover parameter setting is improper and the MS cannot make the
handover in time in the condition of poor quality to improve the radio quality. As
a result, the call drop happens.
The handover parameter setting is improper and the MS cannot make the
handover in time in the condition of poor quality to improve the radio quality. As
a result, the call drop happens. The neighbor cell congestion problem should
be solved.
Hardware fault, such as the too low power amplifier output power, great difference
between the transmission power of different TRXs, and the fault of TRX transmitter,
combiner, and divider.
Antenna and feeder system fault, such as different tilts and azimuths of two
antennas in the cell, large SWR, too high antenna or improper downtilt, can lead to
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too large coverage range and the overshooting. Then the remote isolated-island
effect and then the call drop happen.
User factor
For example, the contact of the battery of the MS is poor.
4.3.2 Causes of Call Drops Due to LAPD Link Failure
BTS transmission problem, such as unstable transmission or transmission
interruption
BTS hardware malfunction, such as unreliable E1 cable and CMM board or back
board connection fault.
BSC hardware problem, such as the LAPD handling board fault.
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5 Signaling Flow and Relevant Counters and KPIs During the Terminating Paging Stage
5.1 Paging Principle
Radio paging is a communication process in which the MSC finds out the MS through
paging. Only when the mobile subscriber has been found out can the MSC carry out next
call connection.
Figure 5-1 Paging Message Delivery
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5.2 Basic Signaling Flow of the Radio Paging
Figure 5-2 Basic Signaling Flow of the Radio Paging
MS BTS BSC MS
7 Channel Activation ACK
8 Immediate Assigment
Command9 Immediate Assigment (AGCH)
1 Paging2 Paging Command
3 Paging Request
4 Channel
Request(RACH) 5 Channel Request
6 Channel Activation
10 SABM Paging
Response(SDCCH)
11 Paging Response(SDCCH)
Establish Indicationg12 Paging Response
13 UA(SDCCH)
As shown in the above figure, the basic paging signaling flow is described.
1. The MSC sends, after getting the current LAC information of the MS from the VLR,
paging messages to all BSCs in the LAC.
2. After receiving the paging message, the BSC will send out the paging command
messages to all the cells within this LAC.
3. After the BTS receives the paging command, it will send out, on the paging
sub-channel of the paging group where the IMSI stays, a PAGING REQUEST
message, which carries the IMSI or TMSI number of the paged subscriber.
4. After receiving the PAGING REQUEST message, the MS will request through the
RACH for the SDCCH allocation. And the BSC will assign this SDCCH to the MS
through an IMMEDIATE ASSIGNMENT message on the AGCH after it confirms the
activation of the needed SDCCHs made by the BTS.
5. And MS will use this SDCCH to send a PAGING RESPONSE message.BSC will
then forward this PAGING RESPONSE message to the MSC and one radio paging
will be completed successfully.
Now, the switch in GSM network usually adopts second paging with a paging interval of 5
seconds. After the MSC acquires the LAC of the MS from the VLR, it will send the paging
messages to all the BSCs in the LAC where the MS stays. If the MSC cannot receive the
PAGING RESPONSE message in five seconds after it sends the paging message, the
MSC will send the paging message again. For the second time, the MSC sends the
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paging messages to all the BSCs in the LAC where the MS stays. If the MSC still cannot
receive the PAGING RESPONSE message in five seconds, this radio paging fails. At the
same time, the MSC will send the record notice of “The number you dialed cannot be
connected for the moment” to the originating user.ZTE switch usually adopts second
paging (It can be set as third paging.).
5.3 Paging Counters of ZTE BSS
C900060001: number of MTC random access attempts
Description
This counter counts the number of channel requests due to the MTC random
access. When the MS sends the CHL_REQ message to the BSC through the
BTS to request for radio channel, if the TA does not exceed the cell range and
the access reason is “MTC”, the counter increments.
Measurement point
The counter counts when the MS requests for channel from the BSC and the
TA does not exceed the cell range. The measurement point is A1, as shown in
the following figure.
Figure 5-3 Radio Access Process
MS BSC
CHL_REQ
CHL_RQD
CHL_ACT
CHL_ACT_ACK
IMM_ASS_CMD
IMM_ASS
SABM
EST_IND
BTS
A1
A2
A3
A4
C901110006: number of MTC access success for process
Description
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This counter counts the number of MTC access success for processes. The
counter increments when the BSC allows the MS to be accessed into the
network due to the MTC attempts and accepts channel request sent by the MS
and allocates and activates channel successfully. Meanwhile, the BSC sends
an IMM_ASS message to MS.
Measurement point
The counter increments when channel is activated successfully. The
measurement point is A3 in Figure 5-3.
C900060002: number of MTC access success
Description
This counter counts the number of the MS successfully accessing the channel
assigned in the immediate assignment message (the access reason is due to
the MTC). The counter increments when the following two conditions are
satisfied: the MS receives the IMM_ASS message from BSC and successfully
accesses the channel. The BSC receives the EST_IND message from the MS.
Measurement point
The counter counts when the BSC receives the EST_IND message. The
measurement point is A4, as shown in Figure 5-3 Radio Access Process.
C900060137 number of wireless access due to paging responses
Description
This counter counts the number of accesses due to paging responses. After
BSC receives the EST_IND message, if the access cause in the layer-3
information in this message is "paging responses”, this counter increments.
Measurement point
The counter counts when the BSC receives the EST_IND message. The
measurement point is A4, as shown in Figure 5-3 Radio Access Process.
C900060152: number of ABIS interface paging command messages
Description
This counter counts the number of paging command messages from
A-interface. These messages are part of Abis interface messages sent from
BSC.
Measurement point
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When the BSC sends an ABIS_INTER_PAGING_CMD message to the BTS,
this counter increments. The measurement point is A in the following figure.
Figure 5-4 Measurement Point of the BSC Sending the Abis Message to the BTS
Formulae of calculating the paging success rate at the BSS side are displayed as follows.
V2: Paging success rate (BSC)=∑C10022/C20064
V3: Paging success rate (BSC)= ∑C900060002 / C900060152
5.4 Paging Success Rate KPI Definition
Table 5-1 Paging Success Rate KPI Definition
KPI Paging Success Rate (%)
Definition Number of paging responses × 100%/Number of paging requests
Counter formula
V2 C10022/C20064 × 100%
V3 C900060002/C900060152
V4 C900060002/C900060152
Remarks
The BSC can provide this KPI indirectly. But the statistical point is different from that of the MSC statistics. The main difference is whether one BSC has only one LAC and whether one LAC belongs to only one BSC for the call on the A interface.
The number of paging requests is defined as the sum of PAGING messages sent out by
all the MSCs in the local area, not including the second paging messages. The
measurement point is the MSC.
The number of paging responses refers to the sum of PAGING RES messages received
by all the MSCs in local area, including the second response messages. The
measurement point is the MSC.
The paging success rate, one important network quality KPI of the GSM network, can
affect the call completion rate and radio system connection rate directly. The sound
paging performance is quite important for being the terminating party successfully of all
the users. Therefore, the paging success rate optimization analysis is quite necessary.
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5.5 Factors Affecting the Paging Success Rate
The PAGING message fails to be sent on the radio channel.
The heavy link load leads to the loss of bottom layer SCCP message.
MSC/VLR and BSC flow control leads to the message discarding.
The heavy load results in longer queuing time of messages and delay in receiving
messages of the MS.
The poor transmission link quality leads to the loss of bottom LAPD message.
The T3212 parameter is set unreasonably.
Too many paging messages results in the loss of message on the radio interface
(group sending of short messages).
Abnormal number of sending paging message times in BSC is caused by the
redundant cell data in MSC.
The MS fails to receive the PAGING message.
Coverage issues, including the coverage blind area, poor general coverage rate, the
user not staying in the coverage area, network coverage loophole, and individual
coverage blind point.
Frequent MS reselections
Frequent location update
The MS cannot monitor the messages on the BCCH when it is in GPRS service.
The paging group is set unreasonably, which leads to long paging time or paging
lost. Sometimes, the paging groups of two adjacent cells are different.
The time of the seconding paging is set improperly, so the second paging does take
effect. Besides, the system load also is increased. When the large difference
between adjacent cells causes frequent reselections, the monitoring time will be
different, tending to cause paging lost.
The relevant messages fail to reach the MSC when the MS is responding to the
PAGING message.
SDCCH congestion
SDCCH assignment failure
UL and DL imbalance, with weak UL
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Poor transmission link leading to the message loss
The special case: when two MSs call the same MS simultaneously, the MSC will
connect to only one caller and reply “no paging response” to another.
The time of sending the MSC paging message is improper. Before the MS ends the
call release, the MSC finishes the release and delivers the new paging, but it is
replied with “no paging response.”
5.6 Procedure and Method of Low Paging Success
Rate Optimization
Step 1: Exclude the abnormal phenomena caused by the system
Check the flow control alarm and see whether flow control alarms exist in the
MSC, VLR, or BSC. Keep the trunk link on the A interface or Abis interface in
good maintenance, pay close attention to signaling load on the A interface or
Abis interface, and add signaling link timely, so as to reduce paging failure
caused by too heavy signaling load.
Check whether there is transient transmission interruption alarm in the BTS.
The unstable link between systems (such as the LAPD link on the Abis
interface, and interface link between different entities at the network side) and
within a system (such as the MEM link between the MSC and VLR and links
between BSC or MSC modules) will cause the message loss, leading to the
low paging success rate. This kind of problems can be discovered by checking
alarms.
Check whether there is redundant data in MSC database. As a result of
continuous network expansion and cutover, the cell data in the MSC may differ
from that in the BSC. Therefore, it is necessary to check the cell data and
delete the redundant data timely. In some areas, network expansion adopts
the "dot-distributed network" (such as inserting some ZTE BTSs in the area
under the MOTO BTS coverage), leading to the possibility that there are many
LACs under one BSC. In this case, this BSC will receive paging messages
from many different LACs, resulting in heavier paging load of the BTS in this
BSC.
Step 2: Check the latest status of the MS.
At present, the VLR probe is the only tool for checking the latest status of the MS.
In test, the status of the MS can be judged through the recording signaling on the
SGSN, MSC, and Abis interfaces.
Step 3: Exclude the influence from the GPRS.
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Check whether the GPRS routing area is set correctly. Set the same routing area for
the same site and the routing area of the cells with frequent reselection should be
the same.
Check whether the routing area update cycle is set reasonably.
Use a mobile phone without GPRS function to test.
Step 4: Analyze the KPIs.
Check the SDCCH congestion condition. The SDCCH congestion rate in the
traffic statistics should be 0 or nearly 0.The condition of “no paging response”
caused by the SDCCH congestion should be eliminated.
Analyze whether the MTC success rate is abnormal.
Analyze whether the number of cell location update times is abnormal.
Analyze the average TA and the maximum TA to judge whether there is
overshooting.
Step 5: Check and optimize the radio parameters.
Check the parameter setting relevant to the paging, access, and immediate
assignment. Check the traffic statistics and alarms to see whether there are
messages about RACH, PCH and SDCCH overload.
Improper configuration of the BS-AG-BLKS-RES parameter and
BS-PA-MFRMS parameter tends to cause the PCH congestion or low paging
speed. The large value of the BS_PA_MFRMS parameter is at the expense of
average time delay on radio channel, that is to say, the larger the value of the
BS_PA_MFRMS is, the longer time delay the paging messages will spend in
the air section. Thus, the average service performance of system will also
deteriorate and there will be a longer time for the MS to wait for being paged.
On the one hand, reducing the value of the BS-PA-MFRMS parameter helps to
shorten the user response time, increasing the overall service capability; on
the other hand, when the BS-PA-MFRMS parameter of ZTE BTS <=3, paging
messages can be sent twice, increasing the number of resending paging
messages, so as to increase the paging response rate of MS, which increases
the paging success rate.
If the parameters, such as the MAX retrans parameter and TX-integer
parameter, are set improperly, channel requests may collide or cannot be
detected.
Check whether T3212 (periodic location updating timer) and IDETTIM
(implicate detach time) are set properly. One of the paging failures may be
caused by the MS entering into coverage hole or MS power-down. If the
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IDETTIM of the switch does not expires, (The MSC will check the ATTACH
users regularly and set the MS that has no contact with system to the implicate
detach status.), the MSC will still send the paging message to this user but the
MS cannot respond. At the BSC side, each BTS is set with a timer T3212 to
make MSs contact the system regularly; only in this way will there be the latest
user information in the VLR.T3212 in the BSC and IDETTIM in MSC should
meet the condition of T3212 < IDETTIM.
Check whether the LAC is divided properly and whether the overlapping area
between LACs is configured properly. Please pay attention to the following
points in the LAC configuration. A LAC should be within the same MSC and
MSC crossing is not permitted. Paging capacity and the number of location
update must be balanced. The most important principle for LAC configuration
is that the maximum paging capacity of BTS should not be exceeded. Once it
is exceeded, the LAC splitting should be taken into account.
Step 6: Analyze the MSC paging strategy.
Check whether the system capacity allows the multi-paging. If it does, analyze the
influence upon the system from multi-paging. The MSC forms paging messages and
can resend the paging message in the condition of receiving no response. The
interval between two paging is a vital parameter. From the radio aspect, the longer
the interval between two paging is, the less the MS is correlated with the radio
environment when it is responding paging message, and the more easily the MS will
respond to paging messages successfully. But too long the interval will make MOC
subscriber wait for a long time that he or she tends to hang up. In optimization,
paging interval needs to be adjusted gradually according to paging success rate and
subscriber hang-up ratio. Prolonging the paging interval properly can enhance the
paging success rate. The disadvantage is that the time of waiting for the record
notice of the originating user becomes longer if the terminating user is out of the
service. Some vendors may adopt global paging in the second paging, that is to say.
paging the MS within the whole MSC. However, devices of some vendors do not
support this function. It is recommended that this function should be enabled in the
switch that has the function. This function helps a lot for enhancing the paging
success rate of the MSC that has two or more LACs.
Step 7: Make the field test.
Field test is the most important step, through which the actual phenomenon can be
captured.
When the problem is easily to be reproduced by common mobile phones rather than
the testing handset, it is necessary to use two SIM cards belonging to the same
paging group to conduct the test. Thus, it can be possible to judge whether common
mobile phones respond correctly to paging messages that have been sent normally.
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Check whether frequent reselections occur. If any, modify the reselection
parameters (such as the CRO parameter, TMO parameter, and PT
parameter).Check whether there is any coverage blind area.
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6 Signaling Flow of the Terminating Connection Stage
After the paging, authentification, identity appraisal, and encryption, the terminating
connection stage starts.
6.1 Signaling Flow of the Terminating Connection
Stage
Figure 6-1 Signaling Flow of the Terminating Connection Stage
MS BTS BSC MSCSetup
Setup(SDCCH)
Call Confirm(SDCCH) Call Confirm
Assignment RequestChannel Activation
Assignment Command(SDCCH)
SABM(FACCH)
Establish Indication
UA(FACCH)
Assignment Complete(FACCH)
Assignment Complete
RF Channel Release
RF Channel Release ACK
Alerting (SDCCH)
Alerting
Connect (SDCCH)
Connect
Connect ACK
Connect ACK (FACCH)
Channel Activation ACK
The call connection signaling flow is described in the above figure.
The MSC sends a SETUP message to the MS and this message includes all the
necessary details of the call (such as the required service type and the originating
number.
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The terminating MS receives the SETUP message. And it sends a CALL
CONFIRMED message to the MSC after the test of call capabilities of all the
compatible equipment is completed. This message indicates that all the necessary
information for the call connection setup has been received. No more information is
needed.
After the MSC receives the CALL CONFIRMED message, it will send a Assignment
Request to the BSC, so as to allocate the TCH for this call. The terminating TCH
assignment is similar to the originating TCH assignment.
After the assignment, the terminating MS sends an ALERTING message to the
network. After the terminating MSC receives this message, it sends an ACM
message to the originating end. The originating end sends the ALERTING message
to the originating user after receiving the ACM message.
After receiving the ALERTING message, the terminating user sends a CONNECT
message to the MSC. Then the MSC sends an ANSWER message to the
originating end and sends a CONNECT message to the terminating end.
Then all the transmission links are connected to the network and the end-to-end
transmission of the user is set up.
6.2 Relevant KPIs of the Terminating Connection Stage
The terminating connection stage is corresponding to the originating call setup stage and
TCH assignment stage. The relevant KPIs are displayed as follows.
SDCCH congestion rate
SDCCH assignment success rate
TCH congestion rate
TCH allocation success rate
KPIs described in the originating signaling flow
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7 Features of the V4 Allocation Process Statistics
7.1 Change of the Allocation Flow
Pre-application: applying for the ITC resources (external TC) > selecting the board >
making pre-application
Allocation: acceptation on the A interface and Abis interface > channel activation >
connection
7.2 Change of Allocation Statistics
7.2.1 Counter Adding
TCH/F measurement 2
TCH/H measurement 2
The counters reflect the success or failure statistics of all the stages of resource
application.
7.2.2 Counter Deleting
The statistics of the allocation related to the service handling flow is adjusted and the
original counters relevant to the allocation are deleted.
7.2.3 Modification and KPI Change
The statistical counters are configured. First, the counter information is acquired from the
configuration table of the OMC. After the MO, the counter information can only be
acquired from the BSC. The content should be consistent to that of the V3 (non-MO).
The KPI formula adjustment principle is that the formula should be applicable for the V3
non-MO and V3 MO.
Suppose the original formula is KPI = A + B
After the MO, the formula is KPI = A + C
A means the counters existing in all the versions.
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B means the counters existing in the non-MO version (not included in the statistics in the
MO version).
C means the newly-added counters in the MO version (not existing in the non-MO
version)
Finally, the KPI formula is KPI = A + B + C
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8 Cases
8.1 Cases of SDCCH Assignment Failure
8.1.1 SDCCH Assignment Failure Due to the LAPD Time Delay
8.1.1.1 LAPD Time Delay Due to Large Paging Traffic
Fault description
On a certain field, the engineers found that the SD assignment success rate of ZTE
BSC3 was low, especially in the busy hours at night, through the performance KPI
analysis. The rate was only about 60%.
Fault analysis
The engineers checked the statistical data and found that the high SD
assignment failure existed in each cell. Therefore, the poor assignment due to
radio parameter of the cell was excluded.
Judging from the statistical data, the congestion rate of SD channel was only
0.02%.
The SD assignment success rates of ZTE BSC 1, BSC 2, and BSC 4 were
higher than 95%, which was normal. Only the BSC 3 was abnormal. Because
the BSC 3 was under the MSC 7, being isolated, the engineers contacted the
China Unicom personnel and found that the SD assignment success rates of
all the BSCs (including the BSC of Siemens) under the MSC 7 were about 60%.
And the paging success rate of MSC 7 was quite low. According to the China
Unicom personnel, there was only one LAC under the MSC 7. Because all the
cells under the LAC were included during the paging, the larger the traffic was,
the larger the paging traffic was.
Solution
The engineers communicated with the engineers from Siemens and asked them to
add one LAC for MSC 7 and change the LAC IDs to the new IDs for the cells of
some BSCs of Siemens. After the modification, the SD assignment success rate of
BSC 3 was normal, higher than 95%.
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8.1.1.2 Satellite Transmission Time Delay
Fault description
All the four sites, TBT-G, TBT-D, GWD-G, and GJR-G, were under BSC 01 but they
belong to different peripheral modules. Judging from the performance KPIs, the SD
assignment failure rate of these sites were above 50%.
Fault analysis
The engineers recorded the signaling on the Abis interface of TBT 1, TBT 4, TBT 5,
TBT 6, GAR, and GWD. Take the TBT 5 signaling as an example to describe the
signaling analysis.
Judging from the time stamp, the average time of successful channel activation
was 0.58 s.
Figure 8-1 Time Stamp Checking
Judging from the signaling below, the engineers checked whether the two
pieces of signaling were the CHANNEL REQUIRED messages sent by the
same MS.
Figure 8-2 Signaling Flow Checking 1
The engineers can calculate the FNs of T1, T2, and T3. The formula is FN =
T1× 26 × 51 + ((T3 - T2)mod 26) × 51 + T3
The difference of FN between two messages is 32454 - 32227 = 227 (frames).
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The engineers traced the whole process of the first channel request and found
it was a complete signaling process of power-off. And they traced the second
channel request and found the immediate assignment failure. The BSC cannot
receive the ESTABLISH INDICATION message and the channel was released
after the T3101 expiration.
Figure 8-3 Signaling Flow Checking 2
Figure 8-4 Signaling Flow Checking 3
The two pieces of signaling had the same access delay. The maximum number
of retransmission times was 4 and TxInteger was 14 (T = 32 and S =217). The
interval between two CHANNEL REQUIRED messages sent by a MS in one
call was a random time slot among 217 ~ 248. That is to say, the shortest time
of the MS sending two CHANNEL REQUIRED message was 1001 ms and the
longest time was 1144 ms.
The time interval of the BSC receiving the two CHANNEL REQUIRED
messages was 1.031 s (1.906 - 0.875). For the BTS and BSC signaling
transmission time delays, suppose the UL and DL signaling delays were
consistent, the time length of total immediate assignment signaling was 1.16 s
(0.58 × 2), similar to 1.031 s.
According to the frame ID calculation, the actual interval of the two messages
was 227 frames (1048). Therefore, the two messages were sent by one MS in
one service call attempt.
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Conclusion
Because these sites were far away from the urban area, the satellite transmission
was adopted. The time delay of the one-way transmission of the satellite
transmission is about 260 ms. Therefore, the transmission time delay of four pieces
of signaling is 1040 ms, which is consistent with the above signaling analysis.
8.1.2 High SDCCH Assignment Failure Rate Due to Co-BCCH and Co-BSIC
8.1.2.1 Interference of the Co-BCCH and Co-BSIC Coverage Overlapping Area
Fault description
On a certain field, the high SDCCH assignment failure problem was not solved. The
SDCCH assignment failure rates of many cells in the whole network were over 25%.
Handling process
The engineers changed all the hardware and the problem was still not solved. Then
the engineers traced the signaling and found that the co-BCCH and co-BSIC signals
of another cell were received at the time of TA = 20, which led to the SDCCH
assignment failure. According to this point, the engineers planed the BSICs of more
than ten cells in the whole network again. After the replanning, the KPIs of all the
cells with BSIC modification became normal.
Fault analysis and conclusion
If one MS stays in the area covered by two co-BCCH and co-BSIC cells, the
SDCCH assignment failure may happen. The triggering condition of this possibility
is that the time slots of the SDCCHs of the two co-BCCH and co-BSIC cells are
synchronized. After the MS and BTS are synchronized, if the MS selects one cell for
access, another cell will be interfered.
Therefore, for the SDCCH assignment failure (the high SDCCH assignment failure
rate due to co-BCCH and co-BSIC cells within a certain multiplexing distance), there
are two solutions.
Reset the CMM of the cell with high failure rate, so as to reset the clock. Then
the SDCCH time slots were misaligned and the influence can be reduced. This
is a temporary solution. For the field, the engineers should modify the
parameter and restore them.
Avoid the co-BCCH and co-BSIC condition, which is the fundamental solution.
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8.1.3 Noise Signal Access
8.1.3.1 The Rxlev Lower Than the BTS Rev Sensitivity
Fault description
The SDCCH assignment failure rate of a certain cell was high but the TCH
assignment success rate was normal.
Fault analysis
The EDGE TRX was adopted in this cell and the Rxlev of the random access can be
reported in the physical context in the CHANNEL REQUEST message. The
engineers observed the signaling tracing data of this cell and found a large number
of CHANNEL REQUEST message with Rxlev being –135 dBm (0 × 87). These
messages led to a lot of SDCCH assignment failures.
Figure 8-5 Signaling Tracing Data Observing 1
The engineers judged that most of these CHANNEL REQUEST messages were
noise interference signals. This problem can be solved through the RACHMin
parameter setting.
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8.1.4 SDCCH Assignment Failure Due to Co-BCCH and Co-BSIC Handover
Note:
The co-BCCH and co-BSIC means that the ARFCN of the target channel of the handover
is the same with the BCCH of the faulty cell and the BSIC of the target cell is the same
with that of the faulty cell.
Fault description
The signaling of a certain faulty cell is shown in the following figure. Judging from
the signaling, the engineers found out continuous CHANNEL REQUEST messages,
with the same RA and TA and continuous frame IDs. The SDCCH assignments
corresponding to these CHANNEL REQUEST message all failed. What is more, in
the basic measurement, the number of other access request attempts was high.
Therefore, the continuous CHANNEL REQUEST messages indicated the false
access caused by the handover access of the co-BCCH cells.
Figure 8-6 Signaling Tracing Data Observing 2
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8.1.5 SDCCH Assignment Failure Due to Poor Network Coverage
Fault description
In a certain cell, the SDCCH assignment failure rate and the TCH assignment failure
rate were high. The out-going handover attempts were frequent and the call drop
rate was high, with complaints. The reset TRX or site cannot be restored.
Fault analysis
Judging from the basic measurement of this cell, the access causes corresponding
to the SDCCH assignment failures were various, including the originating access
and terminating access. The number of samples with UL RQ larger than 3 was large.
The UL quality was poor. Therefore, the UL signals of this cell were affected by the
interference or poor coverage.
Table 8-1 Cell Basic Measurement Data 1
Time
11644 (Number of SDCCH
Assignment Success
Times)
11645 (Number of SDCCH
Assignment Failures
)
116114 (Number of Samples with
UL RQ = 0)
116115 (Number of Samples with
UL RQ = 1)
116116(Number
of Samp
les with UL
RQ = 2)
116117(Number
of Samp
les with UL
RQ = 3)
116118(Number
of Samp
les with UL
RQ = 4)
116119(Number
of Samp
les with UL
RQ = 5)
116120(Number
of Samp
les with UL
RQ = 6)
116121(Number
of Samp
les with UL
RQ = 7)
2007-9-28 16:15
68 11 1580
73 106 100 89 140 95 102
2007-9-28 16:30
65 12 2906
144
192 168 185 185 122 120
2007-9-28 16:45
50 12 2573
123
167 166 132 105 51 58
2007-9-28 17:00
67 9 2559
180
256 226 206 142 81 76
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8.1.6 SDCCH Assignment Failure Due to Continuous Location Update
Requests
Fault description
In some boundary sites and suburban sites of City A, the SDCCH assignment failure
rate was increased abruptly and irregularly, but other KPIs of the cells were normal.
The recorded signaling and basic measurement at the time of high SDCCH
assignment failure rate are shown in the following figure. Judging from the signaling,
one MS initiated the channel request with the access cause being location update
continuously but all the requests failed.
Table 8-2 Cell Basic Measurement Data 2
SITE ID
CELL ID
Time
11636 (Number of
MOC
Access Succes
s Times)
11637 (Number of MTC
Access Succes
s Times)
11638(Number of LOC
Access
Success
Times)
11644
(Number of
SDCCH
Assignment
Success
Times)
11645 (Number
of SDCCHAssignme
nt Failures)
11684 (Number of MOC
Access Attempts)
11685(Number of MTC
Access
Attempts)
11686(Number of LOC
Access
Attempts)
32
3 2007-8-31 4:15
4 0 9 13 192 4 0 202
32
3 2007-8-31 4:30
0 0 10 10 155 0 0 165
32
3 2007-8-31 4:45
0 0 16 16 206 0 0 223
32
3 2007-8-31 5:00
2 0 15 17 172 2 0 188
32
3 2007-8-31 5:15
2 1 12 15 174 2 1 187
32
3 2007-8-31 5:30
7 2 13 22 188 7 1 187
32
3 2007-8-31
4 2 18 24 208 4 2 198
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SITE ID
CELL ID
Time
11636 (Number of
MOC
Access Succes
s Times)
11637 (Number of MTC
Access Succes
s Times)
11638(Number of LOC
Access
Success
Times)
11644
(Number of
SDCCH
Assignment
Success
Times)
11645 (Number
of SDCCHAssignme
nt Failures)
11684 (Number of MOC
Access Attempts)
11685(Number of MTC
Access
Attempts)
11686(Number of LOC
Access
Attempts)
5:45
32
3 2007-8-31 6:00
1 2 17 20 170 1 2 187
32
3 2007-8-31 6:15
14 4 14 32 160 14 4 198
32
3 2007-8-31 6:45
7 6 10 23 196 7 6 209
32
3 2007-8-31 7:00
5 1 15 21 237 5 1 249
Figure 8-7 Signaling Tracing Data Observing 3
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8.1.7 Improper Setting of the Tx-Integer Parameter
Fault description
In a certain cell, the common SDCCH assignment failure rate was about 20% and it
was 30% in the busy hours. But other KPIs (such as the TCH assignment failure
rate and in-coming success rate) were normal.
Table 8-3 Site Information
DATETIME
BSC_NAME BSCID
CELL_ID
SITE_NAME MYHO
UR SD_ASSN_FAI
L_RATE
13-Dec-07
JAYANAGAR-BSC
102 12282 THAYAGRAJNAGAR-2-s
21 30.21
Fault analysis
The engineers traced the signaling of the cell and found that the pair of CHANNEL
REQUEST messages (same TA and channel request cause) always appeared in
this cell. The IMM Assign corresponding to the first CHANNEL REQUEST message
was successful, but the one corresponding to the second CHANNEL REQUEST
message was a failure.
Figure 8-8 Signaling Tracing Data Observing 4
As shown in the above figure, the FN of the first CHANNEL REQUEST message
was 964 and the FN of the second CHANNEL REQUEST message was 1086, with
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the difference being 124 frames, corresponding to the Tx-Integer parameter (12)
set by the OMC. Therefore, the two CHANNEL REQUEST messages are sent by
one MS. Because of the delay of the transmission link, the MS resent the
CHANNEL REQUEST message.
The engineers modified the TX-Integer parameter to 14 and the SDCCH
assignment failure rate of the cell was lower than 10%.
8.2 SD\TCH Channel Congestion Cases
8.2.1 SD congestion due to LAPD Delay Caused by Transmission Fault
The performance report shows that the number of SDCCH allocation failures was high
during SD congestion (SDCCH occupancy failure counter).
Figure 8-9 SD Channel Congestion Report Analysis (Case 1)
Number of
SDCCH
Occupation Attem
pts (for Assignment)
Number of
SDCCH
Occupation
Success
Times (for
Assignment)
Number of
SDCCH
Occupation
Failures (for
Assignment)
Number of
SDCCH
Occupation Attem
pts (for
Handover)
Number of
SDCCH
Occupation
Success
Times (for
Handover)
Number of
SDCCH
Occupation Failures (for Hando
ver)
Number of
SDCCH
Allocation
Attempts (for Assignment)
Number of
SDCCH
Allocation
Success
Times (for
Assignment)
Number of
SDCCH
Allocation
Failures (for
Assignment)
1782 1791 63 0 0 0 1720 1062 658
1455 1441 14 0 0 0 1441 908 533
1542 1524 18 0 0 0 1524 928 596
1759 1648 111 0 0 0 1645 1009 636
1606 1583 23 0 0 0 1588 957 631
1628 1582 46 0 0 0 1582 1004 578
2053 1905 148 0 0 0 1904 1068 863
2409 2215 194 0 0 0 2214 1111 1103
1563 1467 96 0 0 0 1469 758 711
1650 1628 22 0 0 0 1622 858 764
1784 1752 32 0 0 0 1754 856 898
1903 1878 25 0 0 0 1879 947 932
873 852 21 0 0 0 855 381 474
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The signaling flow shows that BTS did not respond to the CHANNEL ACTIVATION
message sent by BSC.
Figure 8-10 No Response From the BTS (Case 1)
The engineers found the transmission alarm at the time of the fault.
After the BTS was reset, the problem disappeared.
After the transmission was adjusted, the problem was completely solved.
8.2.2 SD Congestion due to Strong Interference
Fault description
On one night, large amount of SD congestion occurred in one cell, which lasted for a
long time.
Normal condition didn’t return even after the reset of CMM and TRM.
The congestion disappeared after adjustment of the ARFCN and the BCC.
The congestion phenomenon appeared again after the ARFCN and BCC were
changed back.
The congestion finally disappeared 30 minutes after the adjustment of the TA
access threshold.
The engineers observed the signaling and found that the SD congestion was
caused by a large quantity of the abnormal CHANNEL REQUEST messages. All
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the Imm Assign generated from these CHANNEL REQUEST messages ended in
failure.
The abnormal CHANNEL REQUEST messages appeared once every four frames;
all the RAs were 01; the TA diminished from 63 to 0 and then restarted from 63 after
815 frames; the level value remained 63, as shown in the following figure.
Figure 8-11 A Large Number of CHANNEL REQUEST Messages (Case 3)
The normal condition did not return even after the reset of the CMM and TRM, which
indicated that the problem was irrelevant to the BTS hardware and software.
The problem disappeared after the adjustment of the ARFCN and BCC, but
reoccurred when the ARFCN was changed back, which indicated that the problem
was relevant to the ARFCN.
Considering the rule of the CHANNEL REQUEST messages, the engineers
confirmed that there was a kind of interference signals were co-BCCH with the site
and the signals just contained all the training sequence of AB frame. The
interference signals were periodical and it created periodical deviation to the window
with Time slot 0.Just because of this deviation, the TX changed periodically.
Besides, the interference signal just affected Time slot 0.Therefore, the adjustment
on the TA access threshold can only relieve the problem, but cannot solve it
completely.
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There was army garrisoning in the area and the interference signals were probably
sent from the army.
8.3 Paging Cases
8.3.1 No Paging Response due to the SDCCH Congestion
Table 8-4 SDCCH Congestion Information
问
题
描
述
:
某
1
Fault description
No paging response occurred continuously in an office in XX country on XX date.
Fault analysis
After the check, it was found that a micro cell debugging was being conducted and
the signals at this site were too strong, which led to many mobile phones staying
within the site. The serious SDCCH congestion led to no paging response.
8.3.2 Call Failure due to the MSC Flow Control
Fault description
The call complete rate was low (about 60%) in the busy hours in a certain location.
Fault analysis
According to the signaling analysis recorded in the CQT on site, it was found that no
PAGING message was sent by the MSC.
The signaling flow is shown in the following figure.
BSC SITE CELL Time Alias 11603 (SDCCH Attempt Total
Number)
11604 (SDCCH Overflow Total
Number)
80 11 1 2006-3-14 16:30 POF1 271 0
80 11 2 2006-3-14 16:30 POF2 293 0
80 11 3 2006-3-14 16:30 POF3 345 0
80 611 1 2006-3-14 16:30 POF7 100 61
Notes: Site 611 was commissioning and the signal of the site was very strong
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Figure 8-12 Signaling Tracing Data Observing 5
According to the above figure, the MSC sent less PAGING messages than it should
do.
Due to the MSC flow control in busy hours, the flow control of messages happened,
leading to no paging response.
8.3.3 No Paging Response due to Wrong T3212 Setting
Fault description
In XX city, it occurred suddenly that many MSs under a BSC cannot be paged. After
the power on/off operation, the paging became normal.
Fault description
The signaling tracing showed that the MSC did not send the PAGING messages.
After checking, it turned out the location update time in MSC was changed from 2
hours to 1 hour, but that in BSC was set to one hour. Thus, many MSs, before the
periodic location update, had been marked as inactive status at the MSC side.
Therefore, the MSs could not be paged.
8.3.4 Low Paging Success Rate due to Location Area Division
Fault description
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After the original 3 location areas under a BSC of the ISB were divided into 8 ones,
the paging success rate calculated in the MSC decreased by about 5%.
Fault analysis
Due to the location area division, the location update became more frequent.
According to the following figure, the number of location update times doubled,
which indicated serious cross location area. As a result, the paging failure
happened.
Figure 8-13 Number of Location Update Times
8.3.5 GSM Paging Success Rate Optimization of a China Unicom Branch
Fault description
In a certain area, all the BTSs in XX area were ZTE equipment, in which there were
two BSCs and a location area LAC 21088. Under BSC1, there were four isolated
sites in a forest which were set as separate location area, LAC21136 and the
Ericsson switch was used. In this area, the paging success rate of LAC 21088
remained as 92%, being middle-level or below in the whole province. And the
provincial company requires the KPI to be increased to the maximum score of 94%.
Fault analysis
0
10000
20000
30000
40000
50000
60000
70000
03-01-2006 20:00
03-03-2006 20:00
03-05-2006 20:00
03-07-2006 20:00
03-09-2006 20:00
03-11-2006 20:00
03-13-2006 20:00
03-15-2006 20:00
03-17-2006 20:00
03-19-2006 20:00
Times of general location update
Times of periodic location update
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According to MSC SY01 statistics, LAC 21088 uses the TMSI paging and the
second paging mode with an interval of 8 s. In the existing network, the number of
the first paging times in the busy hours was 53653, with 49192 succeeded and 4461
failed; the number of the successful second paging times was 1009. The statistical
analysis indicated the second paging success rate was only 1009/4461 × 100% =
22.61%, being relatively low.
Through a systematical DT in the urban district in this area, it was found that this
area was a typical mountainous area, in which there were coverage holes even in
urban areas due to lack of sites. Now, there were totally 211 sites in this area, with
about 70 sites in urban areas, and many other sites are marginal isolated ones.
Therefore, weak coverage was one of the main reasons resulting in lower paging
success rate.
Fault handling
Adjusting the BS_AG_BLK_RES parameter
Currently, this parameter was set to 2, combined with the CCCH_CONF
parameter (0 indicates the CCCH used a basic physical channel and was not
combined with the SDCCH; there were nine blocks of CCCH messages in a 51
multi-frame), that is to say. Two blocks were reserved for the AGCH in each
BCCH multi-frame. Correspondingly, the number of PCH blocks was 9-2=7;
According to the KPIs in the existing network, the maximum number of SDCCH
requests in busy hours was below 2,500. If it was taken as 3,000, the number
of SDCCH allocation requests in each paging period was (3000/3600) ×
0.2354 = 0.2, less than one user. Therefore, the engineers modified the value
of the BS_AG_BLK_RES parameter from 2 to 1 and then the number o f PCH
blocks may reach 8 accordingly.
Adjusting the BS_PA_MFRAMS parameter
Currently, this parameter was set to 3, that is to say the same paging group
was transmitted every 3 51 multi-frames, hence the following calculation can
be made.
The number of paging blocks in a paging period: 8 × 3 = 24
The maximum number of subscribers in each paging block: 1000/24 = 42
According to statistics at the MSC side, the number of radio paging times in
LAC 21088 was 53653 + 4461 = 58114, thus, in the existing network the
number of subscribers in each paging period was 58114/3600 ×0.2354 = 3.8.
In the existing network, the TMSI paging was adopted. The maximum number
of each paging block in the TMSI paging was 4. As 3.8 was close to 4, it was
suggested to increase the value of the BS_PA_MFRAM parameter from 3 to 5.
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Then, the maximum number of subscribers carried in each paging blocks
would change to 1000/(8*5) = 25. Though paging messages would delay
longer in the air section, users in each paging sub-channel would be
decreased, paging channel would be strengthened in bearing capacity, and
interruptions will also be reduced in probability of happening.
Adjusting the TX_INTEGER parameter
Currently, this parameter was set to 14, indicating that the number of time slots
was 32 and the value of S was 217; in existing network the MAX_RETRAN
parameter was set to 7, hence the following calculation can be made.
The maximum number of time slots for sending messages was 32 – 1 = 31, the
maximum resending interval was 217 + 32 – 1 = 248; and the time consumed
for resending seven times was (31 + 217) × 4.615/1000 × 7 + 31 × 4.615/1000
= 8.154705 s.
At the switch side, the second paging mode was adopted. If the paging period
was 5 × 0.2354 = 1.177S, it could be concluded that the maximum time
consumed in radio paging was 8.154705 + 1.177 = 9.331705s. As the interval
between two paging defined at switch side was 8 s < 9.331705 s, the
TX_INTEGER parameter would be reduced to 12.
The value of 12 indicated that the number of time slots was 20 and the value of
S was 109. Through the recalculation, the maximum time consumed in radio
paging was (19 + 109) × 4.615/1000 × 7 + 19 × 4.615/1000 + 1.177 =
5.399725S < 8s.
But this parameter should not be configured too samll; otherwise, the waiting
time may be too short, increasing both the possibility of collision and the
network load, and then the KPIs would be worsened. Slight modification was
needed to be made according to actual KPIs after the calculation.
Adjusting the RXLEV_ACCESS_MIN parameter
In the existing network, this parameter was set to 10, indicating the
RXLEV_ACCESS_MIN parameter was –100 dBm. If the threshold was
lowered to 8, the RXLEV_ACCESS_MIN parameter was –102 dBm.
Note:
This parameter helps to enlarge the BTS coverage and increase the paging success rate,
but it can affect the KPIs such as the call drop negatively.
Adjusting T3212
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This timer is set to 72 min at the switch side and 10 (60 min) at the radio side.
The engineers modified it to 8 (48 min) at the radio side.
Note:
This parameter should not be configured to be too small; otherwise, the network signaling
flow would be increased and the stand-by time of MS would be shortened.
After adjusting the above parameters on June 23th, the paging success rate in
this area was increased by about 1%, close to 94%, as shown in the following
figureError! No bookmark name given..
Figure 8-14 Paging Success Rate
8.4 V4 Cases
Fault description
In a certain area, during the V4 BSC and the SDR BTS swap, the TCH congestion
happened and the service cannot be made normally.
Version information:
iBSCV6.50.100f
SDR V4.11.10.14P05
Networking condition:
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ABIS: IPOE
A: STM-1
Gb: IP
After the swap, according to the KPI condition, the traffic was not large, but the TCH
congestion rate was quite high.
Table 8-5 TCH Congestion KPIs
Start Time SUBNETWORK
Name
Call Setup
Success Rate
Call Setup TCH
Blocking
TP2-SDCCH Blocking
306024:TCH Total Traffic
Number (erl)
2012-10-19 14:00:00
PSH751_ZXB01 (3)
80.66% 13.80% 5.35% 231
2012-10-19 15:00:00
PSH751_ZXB01 (3)
84.31% 12.68% 2.04% 227
2012-10-19 16:00:00
PSH751_ZXB01 (3)
85.63% 12.60% 0.41% 207
2012-10-19 17:00:00
PSH751_ZXB01 (3)
85.46% 12.58% 0.62% 203
2012-10-19 18:00:00
PSH751_ZXB01 (3)
83.15% 13.31% 2.61% 225
2012-10-19 19:00:00
PSH751_ZXB01 (3)
80.31% 14.51% 4.51% 150
2012-10-19 20:00:00
PSH751_ZXB01 (3)
84.21% 13.29% 1.11% 138
In the field test, the engineers found that it was difficult to complete the call.
Fault analysis
The engineers made the CQT on the field and found that it was difficult to
occupy the channel. However, many idle channels existed according to the
channel occupation dynamic observation from the OMC.
All the swap cells had this problem and there was no alarm.
The RQ and interference were normal.
There was no wrong configuration of the swap data.
The engineers found the following abnormalities in the TCH measurement
analysis.
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Table 8-6 TCH Measurement Analysis
Start Time
Number of
TCH/F Allocat
ion Failure due to Apply Abis
Resource
Failure for
Assignment
(Speech
Version
2)(Times)
Number of
TCH/F Allocat
ion Failure due to Apply Abis
Resource
Failure for
Assignment
(Speech
Version3)
(Times)
Number of
TCH/F Allocation Failure due
to Apply Abis Resource
Failure for Handover
(Speech
Version2) (Time
s)
Number of TCH/F
Allocation Failure due to apply Abis
resource failure for handover (speech
version3)(Times)
Number of
TCH/H Allocat
ion Failure due to Apply Abis
Resource
Failure for
Assignment
(Speech
Version1)
(Times)
Number of
TCH/H Allocat
ion Failure due to Apply Abis
Resource
Failure for
assignment
(Speech
Version
3)(Times)
Number of TCH/
H Allocation Failure due
to Apply Abis Resource
Failure for Handover
(Speech
Version
1)(Times)
Number of TCH/
H Allocation Failure due
to Apply Abis Resource
Failure for Handover
(Speech
Version
3)(Times)
14:00:00
2449 74701 613 11278 0 0 0 0
15:00:00
2598 73667 709 15269 0 0 0 0
16:00:00
2331 51398 706 10612 362 14965 152 3863
17:00:00
2088 48746 442 8670 353 16523 113 3786
18:00:00
2277 52048 653 8840 464 20316 131 4811
19:00:00
1532 35065 795 9933 245 12885 126 4437
20:00:00
1162 35029 762 9314 151 9503 15 2329
The counters in the above tables are the newly added V4 counters (TCH2/F
and TCH2/H). And the engineers found that the problems were mainly the Abis
resource application failure. It was necessary to troubleshoot the configuration
of the acceptance and control.
The field BSC engineers made further troubleshooting and found that the IP
address configuration of the transmission path at the BSC side was wrong.
Then the transmission path matching the IP address cannot be found in the
corresponding office according to the BTS service address at the time of
bandwidth acceptance.
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On the field, the temporary avoiding measure was adopted. After the
bandwidth acceptance switch was turned off temporarily, this problem
disappeared. Then, the engineers modified the wrongly configured IP address
and the problem was solved.
Solution:
After the acceptance control adjustment, the field test became normal and the KPIs
in the OMC statistics were restored, as shown in the following table.
Table 8-7 KPIs in the OMC Statistics
Start Time Sub-Network
Name
Call Set up
Success Rate
Call Setup TCH
Blocking
TP2-SDCCH Blocking
306024:TCH Total Traffic
Number (erl)
2012-10-19 14:00:00
PSH751_ZXB01(3)
80.66% 13.80% 5.35% 231
2012-10-19 15:00:00
PSH751_ZXB01(3)
84.31% 12.68% 2.04% 227
2012-10-19 16:00:00
PSH751_ZXB01(3)
85.63% 12.60% 0.41% 207
2012-10-19 17:00:00
PSH751_ZXB01(3)
85.46% 12.58% 0.62% 203
2012-10-19 18:00:00
PSH751_ZXB01(3)
83.15% 13.31% 2.61% 225
2012-10-19 19:00:00
PSH751_ZXB01(3)
80.31% 14.51% 4.51% 150
2012-10-19 20:00:00
PSH751_ZXB01(3)
84.21% 13.29% 1.11% 138
2012-10-19 21:00:00
PSH751_ZXB01(3)
88.62% 8.30% 2.16% 328
2012-10-19 22:00:00
PSH751_ZXB01(3)
96.59% 0.38% 2.64% 545
2012-10-19 23:00:00
PSH751_ZXB01(3)
98.37% 0.47% 0.75% 404
The TCH 2 measurement is displayed as follows.
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Figure 8-15 TCH 2 Measurement
Start Time
Number of
TCH/F Allocati
on Failure due to Apply Abis
Resource
Failure for
Assignment
(Speech
Version 2)(Time
s)
Number of
TCH/F Allocati
on Failure due to Apply Abis
Resource
Failure for
Assignment
(Speech
Version 3)
(Times)
Number of
TCH/F Alloca
tion Failure due
to Apply Abis
Resource
Failure for
Handover
(Speech
Version 2)
(Times)
Number of
TCH/F Alloca
tion Failure due
to Apply Abis
Resource
Failure for
Handover
(Speech
Version 3)
(Times)
Number of
TCH/H Allocati
on Failure due to Apply Abis
Resource
Failure for
Assignment
(Speech
Version 1)
(Times)
Number of
TCH/H Allocati
on Failure due to Apply Abis
Resource
Failure for
Assignment
(Speech
Version 3)
(Times)
Number of
TCH/H Alloca
tion Failure due
to Apply Abis
Resource
Failure for
Handover
(Speech
Version 1)
(Times)
Number of
TCH/H Alloca
tion Failure due
to Apply Abis
Resource
Failure for
Handover
(Speech
Version 3)
(Times)
14:00:00
2449 74701 613 11278 0 0 0 0
15:00:00
2598 73667 709 15269 0 0 0 0
16:00:00
2331 51398 706 10612 362 14965 152 3863
17:00:00
2088 48746 442 8670 353 16523 113 3786
18:00:00
2277 52048 653 8840 464 20316 131 4811
19:00:00
1532 35065 795 9933 245 12885 126 4437
20:00:00
1162 35029 762 9314 151 9503 15 2329
21:00:00
374 11672 325 5375 51 2954 38 1212
22:00:00
0 0 0 0 0 0 0 0
23:00:00
0 0 0 0 0 0 0 0
Summary:
For V4, when the TCH congestion and occupation are abnormal in the condition of
service channel resource sufficiency, it is necessary to troubleshoot the
measurement of the newly-added counter TCH 2, so as to judge whether the
congestion is caused by the Abis interface resource application failure.
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On the field, the engineers can adopte the method of turning off the acceptance and
control switches for observation and problem avoiding. At the same time, they
should check whether the IP setting is wrong.