OMF010001 GSM System Information ISSUE2.0

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www.huawei.com

Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.

GSM System Information

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Page2Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.

Contents

1. System Information Overview

2. Parameters Introduction

3. Case Study

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System Information Overview

System information (SYS INFO) contains network parameters sent to

MS through Um interface

network identity parameters

cell selection parameters

system control parameters

network function parameters

System information can be sent through two kinds of logical channels

System information on BCCH

System information on SACCH

By reading system information, MS can access the network, perform cell selection and reselection, fully utilize various services provided by the network, and achieve favorable cooperation with the network. SYS INFO can be divided into two parts:

SYS INFO sent on BCCH, including SYS INFO 1, 2, 2BIS, 2TER, 3, and 4, which are used by MS in idle mode.

SYS INFO sent on SACCH, including system information 5, 5BIS, 5TER, and 6, which are used by MS in dedicated mode. Additionally Huawei BSC supports the system information 7 and 13 to support GPRS.

BCCH system information belongs to common channel information. SACCH system information basically belongs to TRX management information. Common channel and TRX management information should be selected in order to observe all the system information when tracing on the layer 3 messages of radio interface.

Version G2BSC32.10100.03.0520B and later, the system information 1, 2, 2TER, 3, 4, 5, 5TER, and 6 are default system information to be transmitted to cells. System information 2BIS and 5BIS are conditionally transmitted on GSM1800 cells. It will be explained in the following section.

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SYS INFO 1

SYS INFO 1

It contains random access control information (RACH) and cell

frequency allocation table (i.e. CA table). It is transmitted on BCCH.

Content

SYS INFO 1

Cell Channel Description

(cell channel description format)

RACH Control Parameter

(random access channel control parameter)

The maximum number of cell frequencies configured in CA(Cell Allocation) is 64. Due to the restriction of cell channel description format, the frequencies of a cell cannot be configured unlimitedly.

The formats of cell channel description are determined respectively by Cell Channel Description’s second byte Format ID (bit8, bit7, bit4, bit3, bit2).

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SYS INFO 1 - Control Parameter

These control parameters include:

Maximum re-transmitting times (MAX retrans)

Extended transmission timeslots (Tx_integer)

Common access control level (AC)

Call reestablish permitted (RE)

Emergency call permitted (EC)

These parameters will be explained in the following sections.

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SYS INFO 2

SYS INFO 2

It contains access control information (RACH), network color code permitted (NCC

Permitted), and neighbor cell frequency allocation list (BA1 list). It is transmitted on

BCCH. Generally speaking, SYS INFO 2, 2BIS, 2TER contains different parts of the

neighbor cell’s BCCH frequencies. Via reading and decoding BA1 list, MS can

perform cell reselection in idle mode. For a MS of GSM900 PHASE I, it only reads

the neighbor cell BCCH frequencies defined in SYS INFO 2, and ignores those

carried by 2BIS and 2TER.

Content

SYS INFO 2

Neighbor Cell BCCH Frequency Description

NCC Permitted


Neighbor cell description describes all the neighbor cells’ BCCH ARFCNs. Currently, Huawei supports 32 neighbor cells at most. Except the fifth bit (BA_IND) and sixth bit (EXT_IND) of the second byte, the coding format of neighbor cell description is the same as cell channel description.

Extended indication (EXT_IND) is transmitted in system information 2 and 5. It indicates whether there are BCCH frequencies of neighbor cell transmitted in system information 2BIS and 5BIS. It has one bit. If it is set to “0”, this indicates system information 2 and 5 have carried all the BCCH frequencies (the same band as serving cell’s )in the BA table. If it is “1”, it means system information 2 and 5 carries only part of the content of BA table.

BA table indication (BA_IND) is transmitted in system information 2 and 5. It has one bit and it is used to tell the MS to decide whether to use the data of BA1 or BA2 table. In other words, if the current neighbor cell relationship and BA2 table are modified during the conversation of MS, BA_IND in system information 5 will be set to “1”instead of “0”. It instructs MS to re-decode the neighbor cell frequency shown in the system information 5 (maybe also 5BIS and 5TER) to get the latest information.

Network Color Code Permitted (NCC Permitted) is transmitted on system information 2 and 6. It has eight bits.The eight bits are used to show if the particular NCC codes are permitted for the MS to access. If bit N is set to “0” (0<=N<=7), the MS will not measure the BCCH of the cell with NCC=N at all. Thus, the MS will not be handed over to the cell with NCC=N.

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SYS INFO 2BIS

SYS INFO 2BIS

2BIS contains RACH control data and neighbor cell’s BCCH frequencies

(part of BA1 list). It is optional and transmitted on BCCH. As the number

of frequencies in BA1 list carried by SYS INFO 2 is limited, SYS INFO 2BIS

carries the rest of the BCCH frequencies in the BA1 list that are from the

same band as in SYS INFO 2.

Content

SYS INFO 2BIS

Neighbor Cell Description.

RACH Control Parameter (Same as those in SYS INFO 1)

Note 1: Currently Huawei GSM BSS supports up to 32 neighbor cells. And for GSM900 cells all these 32 neighbor cells’ BCCH ARFCNs can be transmitted via System Information 2 by using the format of bitmap. Therefore, BA1 table can be completely delivered to the MS using system information 2. So, it is not necessary use System Information 2BIS.

Note 2: For GSM1800 cell, when the number of configured neighbor cells with the same band is more than 16, it will send system information 2BIS.

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SYS INFO 2TER

SYS INFO 2TER

2TER contains neighbor BCCH frequencies (part of BA1 list). It is

transmitted on BCCH. Only dual band MS can read this information, and

single band 900 or 1800 MS will ignore it. Since this information carries

BCCH frequencies that belong to different band other than those in SYS

INFO 2, it is unnecessary for single band MS to read.

Content

SYS INFO 2TER

Neighbor Cell Description (extended)

Except the fifth bit (BA_IND), sixth, and seventh bit (Multi-band reporting) of the second byte, the coding of other information is the same with cell channel description.

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SYS INFO 3

SYS INFO 3

It contains LAI, cell identity, RACH control parameters and cell selection

related parameters. It is transmitted on BCCH. It is one of the most

important information.

Content

SYS INFO 3

Cell Identity

LAI (location area identity)

Control Channel Description

Cell Option

Cell Selection Parameter


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Cell Identity and Location Area Identity

Cell Global Identity (CGI) consists of Location Area Identity (LAI) and

Cell Identity (CI). And LAI includes Mobile Country Code (MCC),

Mobile Network Code (MNC), and Location Area Code (LAC). SYS

INFO 3, 6, and 4 contain all or part of the information of CGI.

According to the received SYS INFO, MS decodes the CGI, and

decide whether it can register in the network based on the MCC

and MNC indicated by CGI. At the same time, it can judge whether

the current location area is changed, so as to decide whether it is

necessary to start the location updating process.

MCC is allocated globally. It consists of three decimal numbers. For example, China is 460. MNC is allocated by the country. It consists of two decimal numbers. 00 is for China Mobile, 01 is for China Unicom. LAC and CI are planned by each GSM network operator. They consist of two bytes. Please note that the value range of CI is 0x0001-0xFFFE. 0x0000 and 0xFFFF are reserved.

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Control Channel Description

Control channel description includes the following parameters

IMSI Attach-Detach Allowed (ATT)

Common Control Channel Configuration (CCCH_CONF)

Access Granted Blocks Reserved (BS_AG_BLKS_RES)

Paging Channel Multi-frames (BS_PA_MFRAMES)

Periodic Location Update Timer (T3212)

They will be explained in details later.

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Cell Option

Cell option contains the following parameters:

Power control indication (PWRC)

Discontinuous transmission (DTX)

Radio link timeout

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Cell Selection ParameterCell selection parameters indicate how the MS should behave

after the MS is powered on. It includes the following

parameters:

Cell Reselection Hysteresis (CRH)

Maximum power level of control channel (MS_TXPWR_MAX_CCH)

Minimum receiving level of MS permitted to access

(RXLEV_ACCESS_MIN)

Additional reselection parameter indicator (ACS)

Half-rate supported (Support Half-Rate)

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SYS INFO 4SYS INFO 4

It consists LAI, RACH control parameters, cell selection parameters and optional

CBCH channel information. It is transmitted on BCCH. When the cell supports cell

broadcast function, CBCH together with CBCH MA describe the configuration of

CBCH the relevant frequency information.

Content

SYS INFO 4

LAI



CBCH Channel Description (optional)

CBCH Mobile Allocation

SI4 Rest Octets (parameters related to cell reselection)

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CBCH Channel Description and CBCH MA

These two parameters are optional. When system supports

cell broadcast, CBCH Channel Description explains the CBCH

channel configuration situation. When CBCH Channel is in

frequency hopping mode, CBCH MA is also needed.

CBCH channel description includes: channel type and TDMA offset, Timeslot Number (TN), Training Sequence Code (TSC), Hopping channel indication (H), Mobile Allocation Index Offset (MAIO), Hopping Sequence Number (HSN), Absolute Radio Frequency Channel Number (ARFCN). CBCH mobile configuration contains frequency hopping sequent description and cell channel description.

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SI4 Rest Octets

It contains the following parameters:

Cell Reselection Parameter Indicator (PI)

Cell Bar Qualify (CBQ)

Cell Bar Access (CBA)

Cell Reselect Offset (CRO)

Temporary Offset (TO)

Penalty Time (PT)

Cell reselection is conducted on the basis of C2 value. C2 is obtained by the calculation of a series of parameters. If Cell Selection Parameter “ACS” is set as “No”, it means MS will use the relevant parameters in SI4 Reset Octets to calculate C2. If ACS is “Yes”, it means MS should obtain cell reselection related parameters from the residual bytes of system information 7 or 8 to calculate C2.

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SYS INFO 5

SYS INFO 5

It describes the neighbor cell BCCH frequencies(BA2 list). It is mandatory

and is transmitted on SACCH. Different from SYS INFO 2, MS can read the

frequency described in SYS INFO 5 in dedicated mode, report the relevant

information of neighbor cells in measurement reports, and use it as the

basis for handover. Similarly, for 900 MS of PHASE 1, it only recognizes

the neighbor cell frequencies described by SYS INFO 5, and ignores those

carried by SYS INFO 5BIS and 5TER.

Content

SYS INFO 5

Neighbor Cell Description

Corresponding to System Information 2 (System Information 2 describes BA1),System Information 5 describes part of the BA2 table. The BA2 table is used when MS is in the dedicated mode. Generally, the contents of BA1 and BA2 are the same. In special cases, such as they are modified for radio network optimization purpose, their contents can be different.

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SYS INFO 5BIS

SYS INFO 5BIS

5BIS describes neighbor cells’ BCCH frequencies.These frequencies are

part of BA2 list. It is optional and is transmitted on SACCH. Generally, as

the number of frequencies which can be described by SYS INFO 5 is

limited, SYS INFO 5BIS carries the rest BCCH frequencies in the BA2 which

belong to the same band as SYS INFO 5.

Content

SYS INFO 5BIS


Corresponding to System Information 2BIS (2BIS is for BA1), the content of 5BIS is a part of the BA2 table. It is used when MS is in dedicated mode. The neighbor cells’frequencies which belong to the same band as the serving cell will be put into 5BIS to be transmitted to the MS while there is no room in System Information 5.

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SYS INFO 5TER

SYS INFO 5TER

5TER describes neighbor cells’ BCCH frequencies, these frequencies are

also a part of the BA2 list. It is transmitted on SACCH. Similarly, only dual

band MS can read this information, and single band GSM900 or

GSM1800 MS will ignore it. It carries the BCCH frequencies of neighbor

cells whose band are different from the serving cell’s.

Content

SYS INFO 5TER

Neighbor Cell Description (Extended)

Corresponding to System Information 2TER (2TER is for BA1), the content of the 5TER is a part of the BA2 table. It is used when MS is in the dedicated mode. The neighbor cells’frequencies which belong to different band (900, 1800) than the serving cell, will be put into 5TER for transmitting to the MS.

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SYS INFO 6

SYS INFO 6

It describes LAI, cell identity, and some parameters describing the cell’s

functions. It is transmitted on SACCH. It is also one of the most important

system information.

Content

SYS INFO 6

Cell Identity

LAI

Cell Option

NCC Permitted

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Contents



3. Case Study

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Introduction to SYS INFO Parameters

Huawei GSM System Information Parameters:

Network Identity Parameters

System Control Parameters

Cell Selection Parameters

Network Functional Parameters

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Idle Mode(System Control Para.)

Power on

Access Network(System Control Para.)

Select Network(Network Identity Para. )

SI Mapping in Different Statuses of MS

Cell Selection(Cell Selection Para.)

Paging Mode(System Control Para.)

Dedicated Mode(System Control Para.)(Network Identity Para.)

Cell Reselection Mode(Cell Reselection Para.)

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Network Identity Parameter — CGI

Network identity parameters mainly include cell global identity (CGI)

and base station identity code (BSIC).

CGI=MCC+MNC+LAC+CI

Once MS receives SYS INFO, it decodes the CGI information, and

decides whether it can stay in the cell according to the MCC and MNC

indicated by CGI. At the same time, it judges whether the current

location area is changed, so as to decide whether to execute location

update. During the location update process, MS will report the new

LAI to the network, so that the network can know the LA in which MS

is currently located.

As a global cellular mobile communication system, GSM conducts strict coding for each GSM network in every country, and even every location area, BTS, and cell, so as to ensure that each cell corresponds to a unique number all over the world. The adoption of this coding scheme can achieve the following objectives:

1. Ensure that MS can correctly identify the current network, so that MS can accurately select the network expected by subscribers and operators.

2. Ensure that the network can know the real-time position of MS, so that the network can connect various service requests to the MS.

3. Ensure that the MS can report correct neighbor cells’ information to the network during conversation, so that network can perform handover when necessary to keep continuous conversation for the mobile subscribers.

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Network Identity Parameter — BSIC

BSIC=NCC+BCC

In GSM system, each BTS is allocated with a color code, which is

called BSIC. MS can identify two cells with the same BCCH by the help

of BSIC. In network planning, effort should be made to make sure

that BCCH of neighbor cells are different from the serving cell’s BCCH

to reduce the interference. Practically it is still possible that a same

BCCH is re-used in the surrounding cells. For cells using the same

BCCH in a relevant near distance, their BSIC must be different so that

MS can identify two neighbor cells with same BCCH.

BSIC is transmitted on Synchronous Channel (SCH) of each cell. Its functions are as below:

1. If MS have read SCH, it is considered as being synchronous with that cell. However, to correctly read the information on the downlink common signaling channel, MS must get the TSC (Training Sequent Code) that is adopted by the common signaling channel. According to GSM specification, TS (Training Sequent) has eight fixed formats, which are represented by TSC ranged 0~7 respectively. TSC number adopted by common signaling channel of each cell is just the BCC of the cell. So one of the functions of BSIC is to inform MS of the TSC adopted by the common signaling channel of the cell.

2. Since BSIC attends the coding process of information bits in random access burst, it can be used to prevent the BTS from accepting a RACH transmitted from MS in a neighbor cell as the access signal from the MS of the serving cell.

3. When MS is in dedicated mode, it must measure the BCCH level of the neighbor cells and report it to BTS according to BA2 that is sent on SACCH, including their respective BSIC. In special circumstance, when there are two or more cells using the same BCCH in the neighbor cells, BSC can use BSIC to distinguish these cells and avoid wrong handover or even handover failure.

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4. MS must measure the BCCH signals of neighbor cells in dedicated mode, and report the results to the network. Since MS sends measurement report which contain the contents of a maximum of 6 neighbor cells each time, it is necessary to control MS to report only the cells which have neighbor relationships with the serving cell. The NCC is used for the above purpose. Network operators can use parameter “ NCC Permitted” to control MS to report the neighbor cells with NCC permitted in the serving cell only.

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Mobile Country Code（MCC）

Definition

MCC consists of 3 decimal numbers. It indicates the home country

of the mobile subscriber.

Format

MCC is composed of 3 decimal numbers. The coding range is

decimal 000-999.

MCC is used in international mobile subscriber identity (IMSI) and location area identity (LAI).

1. LAI. It is periodically transmitted in system information of each cell. MCC indicates the home country of GSM PLMN. MS uses the received information as the important basis for network selection.

2. IMSI of MS. MS’s IMSI also contains MCC. It shows the resident country of the mobile subscriber. When MS logs on the network or applies for a certain service, it must report its IMSI to the network (When TMSI is unavailable.). The network uses the MCC in IMSI to judge whether this subscriber is an internationalroaming subscriber.

As the unique country identity standard, MCCs are allocated and managed by the International Telecommunication Union (ITU). ITU Recommendation E.212 (blue book) stipulated the MCC number for every country. The MCC of China is 460 (decimal). Due to the special meaning of MCC, modification of it is prohibited once it has been set in the network.

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Mobile Network Code（MNC）

Definition

MNC is used to uniquely identify a specific GSM PLMN network in

a certain country (decided by MCC).

Format

MNC is composed of two decimal numbers. The coding range is

decimal 00-99.

MNC is used in international mobile subscriber identity (IMSI) and location area identity (LAI).

LAI. It is periodically transmitted in system information of each cell. Here, MNC indicates the network number of GSM PLMN. MS uses the received information as an important basis for network selection.

IMSI also contains MNC. It shows the home GSM PLMN network of the subscriber. When MS logs on the network or applies for a certain service, it must report IMSI to the network (When TMSI is unavailable.). The network judges whether this subscriber is a roaming subscriber according to the MNC in IMSI, and uses it as one of the important parameters for addressing to subscriber HLR.

If a country has more than one GSM PLMN, different networks must have different MNC. MNC is allocated by relevant telecommunication management department of the country. One operator can have one or more MNC (which regards to the scale provided by the service, usually one operator has one MNC.). Different operators can share the same MNC. Currently, China have two GSM networks, which are operated by China Mobile and China Unicom. Their MNC are 00 and 01 respectively. Due to the special meaning of MNC, modification is prohibited once it has been set in the network.

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Location Area Code（LAC）

Definition

To locate the location of MS, the whole area covered by each GSM

PLMN is divided into different location areas. LAC is used to

identify different location areas.

Format

LAI contains LAC, which is composed of two bytes. LAC adopts

hexadecimal coding. The available range is from 0001H to FFFEH.

The code 0000H and FFFFH cannot be used (please refer to

specification GSM0303, 0408, and 1111). One location area can

contain one or more cells.

When MS is powered on or LAC of current cell is found to be different from its originally stored contents, MS will inform network of the current location area via location update, and the network uses the LAI for paging. Generally the allocation and coding of LAC is set at the early stage of network construction, and seldom modified during the operation.

The size of location area (LA) is one of key factors in the system. If the LA coverage is too small, the chances for MS to update location increase, and this will increase the signaling load in the system. If the LA coverage is too large, when network conducts paging to the MS, the same paging information will be transmitted in a large number of cells, and this will lead to the heavy load on CCCH. The adjustment of LA size has no unified standard. Operating departments can decide whether to adjust the size according to the currently running network. If the CCCH signaling load is heavy because of too big LA coverage,then reduce the size of LA, and vice versa. It is generally

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recommended to set the LA as large as possible. The calculation of LA is related with the paging strategies of different manufacturers. If Huawei equipment is employed, it is recommended to set the TRX number within the range of 300 in one location area. In the early stage of network construction, the traffic is not heavy, so the TRX number in one LA can be larger than this value. It is necessary to monitor the PCH load and the increase of traffic in a long term. If necessary, PCH capacity can be increased by adding one extended BCCH channel.

While making LA planning, try to make use of the geographical distribution and behaviors of mobile subscribers to allocate the LA, so as to achieve the objective of reducing the times of location update at the boundary between location areas. Please note that LAC in cell parameters must be in consistent with that in MSC. Otherwise, call setup failure will occur.

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Cell Identity（CI）

Definition

To uniquely identify each cell in the GSM PLMN, the network

operator needs to allocate one code for each cell, which is the cell

identity (CI). Cell identity, together with LAI, is used for identity of

each cell in the world. (specification 0303).

Format

CI is composed of 16 bits, The available range is 0~65535.

Cell Identity (CI) is one part of Cell Global Identity (CGI), transmitted in system information of each cell.

There is generally no restriction for the allocation of CI. Value from 0 to 65535 (decimal) can be obtained. But it should be ensured that one location area cannot have two cells with the same CI.

CI is usually determined in the network design. Except for some special cases, CI value should not be changed during the operation of the system.

Please note that one location area is not permitted to have two or more cells using the same CI. CI on MSC should be the same as that on BSC. Otherwise, MS cannot make calls in this cell.

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Network Color Code（NCC）

Definition

NCC is a part of BSIC. MS uses it to distinguish adjacent BTS that

belong to different GSM PLMN.

Format

NCC is composed of 3 bits, with the range of 0 to 7.

NCC and BCC together form the base station identity code (BSIC), transmitted on synchronous channel of each cell.

In many cases, different GSM PLMNs have the identical coverage in many But their network planning are independent from each other. To ensure that adjacent BTSs have different BSICs, it is generally regulated that adjacent GSM PLMN select different NCC.

Adjacent or close cells with the same BCCH frequency must have different BSIC. Special attention should be paid to the configuration of cells in boundary areas.

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BTS Color Code（BCC）

Definition

BCC is a part of BSIC. For its function, please refer to

above sections.

Format

BCC is composed of 3 bits. The available range is from 0

to 7.

BSIC includes BCC and NCC which is transmitted on SCH. BCC is a part of BSIC, used to identify different cell with the same BCCH in the same GSM system. According to the requirements of GSM specification, TSC of BCCH in each cell should be the same with BCC of the cell. Generally this consistency must be ensured by manufacturers. Adjacent or close cells using the same BCCH must have different BSIC, otherwise, inter cell handover might be unsuccessful.

BCC planning has three solutions. All of them have taken distance principle into consideration to avoid collisions of adjacent cells with the same BCCH and same BSIC.

1. Based on the existing BCC set, select one of the BCC that has been used by other cell, ensure at the same time that BCC selected will not cause BSIC/BCCH collision with adjacent cells. The advantage of this solution is that it can ensure BCC be evenly distributed in the whole network. However, if done manually, this solution is time-consuming and troublesome, we can use automatic distribution tools. 2. When defining BCC, try to assign the value from 0. When causes BSIC/BCCH collision, expand the value range. The advantage is that the number of BCC used is kept to the smallest. So when adding a new BTS, in order to avoid the BSIC/BCCH collision, a new BCC can be selected without modifying the BCC of original cells around.

3. Allocate BCC according to its reuse model. That is to use the same BCC within one cluster. It means that adjacent cells cannot use the same BCCH with the service cell. This solution is frequently used, and also the simplest one.

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System Control Parameters

ATT

CCCH_CONF

BS_AG_BLKS_RES

BS_PAMFRAMES

T3212

Radio Link Timeout


NCC Allowed

MS MAX Retrans

TX-Integer

Common Access Control Class

MBR

ECSC

There are a lot of parameters in GSM system, they are usually transmitted to MS from BTS via Um interface. It aims to maintainfavorable cooperation between MS and BTS. On the other hand, the values of these parameters directly affect the traffic load and signaling flow of each part of the system. Therefore, proper configuration of these parameters is important to the favorable and stable operation of the system. The following will elaborateon the definitions, value ranges, and effects on the system of these system control parameters.

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ATT

Value range: Yes, No

Content：It is used to inform MS whether IMSI attach-detach is

allowed in this cell. If it is set to “Yes”, the network will not

process the connection to the called mobile subscriber when

MS is power-off. Thus network processing time and radio

resources are saved. Otherwise the network will process the

connection even though the MS has been powered off.

Recommendation：Yes

Detach process (IMSI) refers to the process that MS informs the network that it is shifting from working state to non-working state (usually a power-off process), or the SIM card is being taken out from MS. Upon receiving the notice from MS, the network knows that the IMSI subscriber is in non-working state. Therefore, if the MS is called, the call connection will be implemented.

IMSI attach process is opposite to detach process. It is the process that MS informs the network it has entered the service area (usually a power-on process) or SIM card has been inserted into MS. After entering service state again, MS will test whether the current location area (LAI) is the same with the latest LAI recorded in MS. If yes, MS will start IMSI attach process. Otherwise MS will start location update process, upon receiving the location update or IMSI attach process, the network will indicate that this IMSI subscriber is in working state.

Note that ATT configuration of different cells in the same LAI must be the same. It is because IMSI detach process will be started when MS is power-off in the cell with ATT set as yes. The network will record that this subscriber is in non-working state and reject all the called connection requests to this subscriber. When MS is power-on again, if it is in the same LAI as it was power-off (thus the LAI update process will not be started) but in another cell, and ATT of the cell is set as no, then the MS will not start IMSI attach process. In this case, this subscriber can not be called normally until the MS starts the location update process.

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CCCH_CONF

Value range: 1 Non-Compounding CCCH, 1 Compounding CCCH, 2 Non-

Compounding CCCHs, 3 Non-compounding CCCHs, 4 Non-compounding

CCCHs.

Content: It is “Common Control Channel Configuration”. CCCH configuration

determines the capacity of PCH, AGCH and RACH. This parameter can be

automatically configured by the BSC Data Auto Configuration System

according to the TRX channel configuration.

Recommendation: When there is one TRX in the cell, one combined CCCH is

recommended (in a system with few paging messages in location area). For

others, it is configured according to the number of TRX in the cell.

In GSM system, the downlink common control channel mainly includes Access Granted Channel (AGCH) and Paging Channel (PCH). It serves to send the access granted (immediate assignment) and paging messages. CCCH is shared. According to the configuration of traffic channel and traffic model, CCCH can be carried by either one or multiple physical channels. Moreover, CCCH and SDCCH can share one physical channel. The MS needs to know how the CCCH(s) is/are configured, so that it can find and select one to listen to. The CCCH_CONF is just used to tell the MS about this matter.

When CCCH is a physical channel which combined with SDCCH, the capacity of CCCH is the lowest. When CCCH is a physical channel which is not combined with SDCCH, the capacity is higher. For other cases, the more the physical channels are used as CCCH, the higher the CCCH capacity is.

Configuration of CCCH_CONF is specified according to the traffic model. This model is closely related to the cell location and environment. According to experiences, when TRX quantity in the cell is 1 or 2, it is recommended to use a combined CCCH as the common control channel. When TRX quantity in the cell is 3 or 4, it is recommended to use a non-combined CCCH as the common control channel.

Currently CCCH can be configured according to actual traffic load. If the paging load is very heavy, the paging traffic of cell should be distributed via multiple CCCH physical channels other way. Special attention should be paid to PCH in CCCH. Generally PCH capacities of various cells under one LAC must be the same.

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BS_AG_BLKS_RES

Value range: 0~2 (1 combined CCCH), 0~7 (others)

Unit: Block

Content: It is also called Access Granted Blocks Reserved. It is

the number of CCCH channel message blocks that are reserved

in one multi-frame for access granted channels (AGCH).

Recommendation: 2 (non-combined CCCH)

As downlink CCCH includes both AGCH and PCH, it is necessary to set the number of blocks, which are reserved for AGCH among CCCH message blocks. To let MS know such configuration information, the system information of each cell includes a configuration parameter, which is the number of access granted blocks reserved (BS_AG_BLKS_RES). This parameter actually assigns the proportion of AGCH and PCH on CCCH. It affects the time of MS’s response to the paging.

The network operator can adjust this parameter to balance the traffic of AGCH and PCH by referring to the following principles:

1. Principle for BS_AG_BLKS_RES: make this parameter as small as possible without causing overload of AGCH, so as to increase the capability of paging and improve the system performance.

2. Generally it is recommended to select 1 (when CCCH_CONF is 1 combined CCCH), 2 or 3 (when CCCH_CONF is one of other values) for BS_AG_BLKS_RES.

3. During operation, observe the statistics of AGCH overload and adjust BS_AG_BLKS_RES properly.

Note: In Huawei system, when AGCH has been all occupied, if PCH is free, it can be used to send the immediate assignment command. If AGCH blocks reserved is set as 0, the immediate assignment would be sent only when there is free PCH channel. Therefore, a fixed capacity reserved for AGCH is necessary.

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BS_PA_MFRAMES

Value range: 2～9

Unit: Multi-frame period (51 frames)

Content: It is Paging Channel Multi-frames. It defines the

number of multi-frames used as a cycle of paging sub-channels.

Recommendation: 2

This parameter specifies the number of paging sub-channels that are assigned in a cell. In the network, MS only monitors the paging sub-channel it belongs and ignores the content of the others. When this parameter is set larger, there will be more paging sub-channels in the cell and accordingly there will be less MS in each paging sub-channel. Therefore, the bearing capability of PCH will be more (theoretically the capacity of each PCH does not increase, but the buffer that buffers paging message in each BTS is increased, which makes the sending of paging messages more even in the time domain), and the lifetime of MS battery will be longer. The value of this parameter should be as small as possible under the condition that the overload on PCH does not occur. In the operation, the PCH load should be measured regularly and the value of this parameter should be adjusted properly according to the PCH load. In a location area, paging is sent in all the cells. Therefore, all cells in the same location area should have the same or nearly the same PCH capacity (number of paging sub-channels). In the area where the PCH bears a medium or large load, it is suggested to be set as 6 or 7 (6 or 7 multi-frames are used as a cycle of paging). For the area with a small load, it is set as 4 or 5. Besides, it is often set as 2.

39

Note:

1. One CCCH block (four consecutive CCCH timeslots) can bear the information of two IMSI pagings or four TMSI pagings or two AGCH immediate assignments.

2. In idle mode MS camps in a cell. The DSC is initialized to the integer part of 90/N (N is BS_PA_MFARMES, with the value range: 2~9). when MS can successfully decode the message on paging sub-channel, DSC will increase by 1, but it will not exceed initially value. If decoding fails, DSC will decrease by 4. If DSC<=0, the downlink signaling link fails, resulting in cell reselection.

40


T3212

Value range: 0~255

Unit: 6 minutes

Content: It is the Periodic Location Update Timer. It defines the

interval of periodic location update.

Recommendation: 30 (for urban area), 20 (for suburban area)

MS will make location update when detecting the change of location. Besides, MS will make periodic location update controlled by parameter T3212. Once MS read T3212 from system information, it will store it in SIM card. When the time reaches T3212 value, the location update process will be triggered. The shorter the period is, the better the performance is. But it will bring more signaling load for system. On setting of this parameter, the processing capability of MSC and BSC, the flux of A interface, Abis interface and Um interface, the flux of HLR and VLR should be considered. Generally this parameter is set as a larger value for urban area and smaller for suburb, countryside or the place with poor coverage.

Large T3212( 16 hours 20 hours) is recommended for the area with heavy traffic, and small T3212 (3 hours, 2 hours) for the area with normal traffic. For the area where the traffic exceeds the system capacity, it is recommended to set T3212 as 0 (no periodic location update). To set the value of T3212 properly, it’s necessary to conduct long-term measurement on the processing capability and flux of each entity in the system. If any overload occurs, increase the value T3212.

41

Note that this value should be smaller than the period by which the network queries the IMSI attached subscriber. Otherwise, the following situation occurs: When MS has not done any operation in a certain time, and it is not yet the time for periodic location update, the network will set IMSI flag of MS as detached, because its query result shows that MS has not done any operation. Thus, the network will not process the paging of this MS. So, before MS initiates another round of periodic location update, once there is a call to the MS, the network will voice the calling party that the called MS is has been powered off. As usual, the T3212 is set smaller than one third of the MSC’s check time.

When MS reselects a cell in a different location area, it will make a non-periodic location update and reset T3212 in the new cell. If it reselects in the same location area, then the timer value will be remainder of the original one divided by the new T3212.

42


LA 1

LA 2

Definition of Location Update

Inform the system the LA (Location Area) where the MS is to be paged.

43


LA 2

LA 3

Condition of Location Update

Periodical location update

Moving into a new LA

LA 1

44


Procedure of Location Update

BSC

MSC

MS

1 Hour 1 Hour 1 Hour

≥ ++ ++ +

45


Abnormal Occasion

LA 3

LA 2

LA 1

But I am always powered on

The subscriber you dialed has been powered off

46


Reason1 of Abnormal Occasion

BSC

MSC

MS

1 Hour 1 Hour1 Hour

47


Reason2 of Abnormal Occasion

BSC

MSC

MS

1 Hour 1 Hour 1 Hour

≤

48


Application of T3212

Even if T3212 setting is less than the system (MSC) query time, the system

will still sometimes voice “The subscriber you dialed is powered off”.

A B

T3212=4 T3212=8

LAC

“Ping-pong Reselect”

Let us make the following assumption. The system query time is set to 1 hour, T3212 value of cell A is set to 4 (0.4 hour), and T3212 value of the adjacent cell B is set to 8 (0.8 hour), and they are in the same location area. MS reselects B when the periodic location update status in A is 3/4, and the MS periodic location update status in B changes to 3/8. If MS stays in B for some time and the location update status reaches 7/8, then MS reselects A. At this time, it can be seen that in cell A, MS reselection status changes to “7/4”, i.e. 3/4. If MS reselects to B at this time, the status will change to 3/8 instead of 7/8. The above analysis shows that if the above case occurs (the probability is high), though T3212 values of both cells are smaller than the system query time, the MS’s frequent cell reselection leads to the final equivalent time is greater than system query time. Thus the subscriber will be considered as a power-off subscriber within certain time even it is in normal idle mode.

49


Radio Link Timeout

Value range: 4~64, the step size is 4

Unit: SACCH period (480ms)

Content: This parameter is used for MS to decide down-link

disconnection in case of SACCH decoding failures.

Recommendation: 20~56

Once assigned with a dedicated channel, MS will start counter S. From then on, S will decrease by 1 when a SACCH message fails to be decoded, and will increase by 2 when decoded correctly. When S decreases to 0, there will be a radio link failure. This allows either re-establishment or release of the connection. If the value of thisparameter is too small, the radio link will easily get failed which will result in call drops. If it is too large, MS will not release for a long time which will lower the availability of resources (this parameter functions for the downlink).

For area with little traffic (remote area), it is recommended to be between 52~64.

For area with light traffic and large coverage(suburb or countryside), it is recommended to be between 36~48.

For area with heavy traffic (urban), it is recommended to be between 20~32.

For the area with very heavy traffic (area covered by microcell), it is recommended to be between 4~16.

For the cell with obvious coverage hole or the area where the call drops is serious during movement, this parameter can be increased appropriately in order to increase the possibility to resume the conversation.

Note: Radio link timeout is the parameter used to judge the downlink failure. Likewise, the uplink will be monitored at BTS, either based on the uplink SACCH error or based on the receiving level and quality of the uplink.

50


Application of Radio Link Timeout

Impact of radio link timeout

A BP Q

Poor coverage

If cell A and B are adjacent to each other, assume that one MS moves from point P to point Q during a conversation, usually an outgoing cell handover will occur. If the value of parameter “radio link timeout” is too small and the quality of signal at the edge of cells A and B is poor, the radio link will time out before the handover occurs, thus resulting in call drops.

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There are table BA1 and table BA2.

Table BA1 describes BCCH frequencies of the adjacent cells to

be measured when the MS is in idle mode.

Table BA2 describes BCCH frequencies of the adjacent cells to

be measured when the MS is in dedicated mode.

MS keeps on measuring the BCCH signal level of the serving cell and the neighbor cells. In order to know the adjacent cells, neighbor cell description information will be broadcast periodically in system information of each cell. This information lists the BCCH of all neighbor cells. MS extracts the information from system information and use it as basis for neighbor cell measurement.

For GSM network, the neighbor relationship between cells is accomplished when designing the network topology. During the network construction, the neighbor cell relationship must be configured in accordance with the topology design that has been planned. Moreover, after the commission of network, neighbor relationship should be modified according to the data of drive test and traffic measurement. When the network’s architecture is changed (e.g. adding BTSs or changing the network frequency configuration.), the network operator must strictly follow the changed-cell-neighbor-relationship, re-set and verify it. Improper neighbor cell description is usually one of the main reasons of call drop. Besides, since the actual network topology structure is often greatly different from the theoretical calculation result, and network is in ever-changing environment, the network operator must configure the neighbor cell description according to the actual situation.

52


Theoretical Neighbor Cells Actual Neighbor Cells

BCA

D

B

CA

D

Application of Neighbor Cell Description

In theoretical calculated neighbor cell relationship, cell A and cell C are not adjacent cells. Assume that one MS moves from cell A to cell C during the conversation, theoretically, MS needs twice of inter-cell handovers. Assume that the interference in cell D is rather serious, call drop is may occur during this period. But in fact, the coverage of A, B, C, and D is not the case as the theory. A and C have overlapping coverage. If A and C are regarded as adjacent cells here, that is to say, add the BCCH of C and A respectively to the neighbor cell description of A and C, then when MS passes from A to C, only one handover happens. What’s more, call drop could be avoided because of the good quality of cell C.

53


BC

A

A

Application of Neighbor Cell Description

Part of the signals from cell A leaks out and covers some areas far away from this cell. It is overshooting. If MS is in dedicated mode in the shady area and moves from this area towards B and C, since there are no BCCH of cell B and C in cell A’s neighbor cell description, call drop is unavoidable. If the antenna of BTS is located too high, or the transmitting power is too large, overshooting will occur. BTSs built at the early stage of GSM construction usually have this problem, because coverage is the major purpose at that time and the antenna height is very high. The best solution for this phenomenon is to adjust the location and downtilt of the antenna, or to adjust the transmitting power of the BTS to eliminate the BTS’s over-covered area. In real situation, it is hard or even impossible to change the location of antenna. So one more simple and applicable method is to add BCCH of B and C to the neighbor cell description of cell A (no need to add A’s BCCH to B and C). But it must be ensured that there are no cells which are all neighbor cells of cell A and using the same frequency and same BSIC with cell B and C. Generally, this method is not recommended.

54


NCC Allowed

Value range: Check box, including options 0~7.

Content: Network Color Code, sent in system information 2

and 6. It lists NCC that need be measured by MS. If a neighbor

cell’s NCC is permitted, MS will report MRs of it to the network.

Recommendation: Subject to the concrete conditions.

In working status, MS needs to measure adjacent cells BCCH signals and report them to network. But each report can only include a maximum of six adjacent cells. Thus it is necessary to make MS only report the potential target cells for handover, instead of reporting all according to the signal level (usually MS does not report the signals of cells from other GSM PLMN). The above function can be implemented by making MS only measure the cells whose NCC are selected. Parameter “NCC Allowed” lists the NCCs of the cells that the MS needs to measure.

BSIC is transmitted continuously on SCH of each cell and the higher three bits of BSIC are NCC. MS only needs to compare the measured NCC of the adjacent cell with parameter NCC Allowed. If it is allowed, MS will report it to BTS, otherwise it will discard the measurement result.

Note: Improper setting of this parameter will lead to lots of call drops.

55


MS MAX Retrans

Value range: 1, 2, 4, 7

Unit: Time

Content: It is one of the random access control parameters. MS MAX Retrans

is the upper limit of times that MS is allowed to send “Channel Request” in

one immediate assignment procedure.

Recommendation:

Set to 7 for areas with low traffic (suburban or rural area) and the cell radius more

than 3 km.

Set to 4 for areas with ordinary traffic (non-busy area in the city) and the cell

radius is less than 3km.

Set to 2 for micro-cell

Set to 1 for micro-cell with heavy traffic or with obvious congestion.

Set to 4 or 7 for satellite transmission BTS.

After initiating immediate assignment process, MS keeps monitoring messages on BCCH and CCCH group it belongs to. If the network does not send Immediate Assignment or Immediate Assignment Extend message, MS will resend the channel request message at a certain time interval. The larger this parameter is, the higher the call setup success rate is, but also the heavier the load of RACH and SDCCH is.

When MS initiates immediate assignment, it will send the “channel request”message to the network via RACH. As RACH is an ALOHA channel, the network is incapable of controlling the access time of MS. Thus in heavy traffic spot, it is unavoidable that several MS may simultaneously make access request and cause collision which will lead to two results: one is when one request signal level is obviously higher than the others’ access signals, the access request with higher level will be handled; the other is the network can recognize none of them due to mutual-interference. As the traffic is increasing, access request loss due to collision will increase, too. To make sure that the system can correctly receive the access request and increase the access success rate, the network allows MS to send several channel requests before receiving an immediate assignment

56

message so as to achieve a higher access success probability. MS will return to idle mode if it fails to receive an immediate assignment command after the MAX Retrans exceeds. Once MS sends a channel request, it will start timer T3120 and wait on the downlink CCCH. When T3120 times out and RACH resend times are not more than “MAX Retrans.”, MS will resend channel request message (containing one new random reference), and restart T3120 with a random value. When T3120 is times out and “MAX Retrans” is reached, MS will start T3126. If MS still fails to receive a response from the network after T3126 times out, it will give up the access. If MS receives the access rejection, it will stop T3120 and start T3122. Within T3122, no new access attempt will be allowed.

57


TX-Integer

Value range: 3~12, 14, 16, 20, 25, 32, 50

Unit: RACH timeslot (equals to a TDMA frame, 4.615ms)

Content: Used to calculate the number of timeslots in the

interval between multiple channel requests sent by MS.

Recommendation: 20 (If the cell immediate assignment success

rate is low, set the S with a larger value. Select 32 for satellite

transmission, so as to reduce satellite transmission delay).

It is set to reduce the collisions on RACH. It mainly affects the execution efficiency of the immediate assignment process. The value of this parameter is related to CCCH configuration mode, both of them together determine the parameter S (see the next page). The MS sends the first random access burst at a random TS in the set {0, 1, …, MAX(T, 8)--1}. And the TS number between any two adjacent channel request messages is a random value in the set {S, S+1, …, S+T-1}.

Generally, parameter T+S should be as small as possible (in order to shorten the access time of MS), but AGCH and SDCCH must not be overloaded. If AGCH or SDCCH of the cell is overloaded, then parameter T can be changed to make parameter S larger, until AGCH or SDCCH of the cell is not overloaded.

When the RACH collisions is serious, value T should be large. When the number of RACH collisions is small, value T should be as small as possible.

58


Calculation of S

STx-integer

Non-Combined CCCH Combined CCCH

3, 8, 14, 50 55 41

4, 9, 16 76 52

5, 10, 20 109 58

6, 11, 25 163 86

7, 12, 32 217 115

When T becomes larger, the interval range between channel request messages sent by different MSs will increase and RACH collisions will be reduced. When value Sbecomes larger, the interval between channel request messages sent by the same MS will increase, collisions on RACH will be reduced and the availability of AGCH and SDCCH will increase. But the increase of either will prolong the access duration of MS, resulting in deterioration of access performance of the entire network. Generally, value T should be selected to make S as small as possible (in order to shorten MS access time), but AGCH and SDCCH must not be overloaded.

59


Common Access Control Class

Value range: Check box, including level 0 forbidden,lever 1

forbidden,…, till level 9 forbidden

Content: One of the parameters of random access control

information. It is used for load control of ordinary subscribers,

to permit or forbid the network access of some common level

users. “1” stands for forbidden and “0” for permitted.

Recommendation: 0000000000

In some special case, the operator expects to prohibit all or part of the MSs from sending access requests or paging response. For example, emergency status occurs or a serious fault occurs to a GSM PLMN. Therefore, GSM specification 0211 requires to assign an access level for each common GSM subscriber. The common access level is divided into level 0~9, which is stored in the SIM card of MS, and has nothing to do with access priority.

Some cells with extremely heavy traffic may be congested during busy hour, resulting in a large number of RACH collisions, AGCH overload, Abis interface overload, etc. GSM specifications provide a variety of ways for dealing with the overload and congestion, but most of them will lower the availability of equipment resources. The network operator can set the access control parameter (C0-C9) properly to control the traffic inside the cell. For example, when the cell suffers traffic overload or congestion, Ci can be set as 0 to prohibit MS with this access level from accessing this cell (change of Ci will not affect MS in dedicated mode), thus reducing the traffic of the cell. To solve this problem, values of C0-C9 in the cell can be changed periodically. For example, at intervals of five minutes, alternatively allow the access of MS with odd access levels and those with even access levels.

For example, 1000000000 indicates to allow the access of subscribers with the levels other than 0. During installation and commissioning of BTS or during maintenance test for some cells, they all can be set to “1” to prohibit the access of subscribers.

60


Special Access Control Class

Value range: Check box, including level 11 forbidden,level12

forbidden,…, till level 15 forbidden

Content: It is used for load control, permitting or forbidding

the network access of some special level users. “1” stands for

forbidden and “0” for permitted.

Recommendation: 00000

For some special subscribers, GSM specifications have reserved five special access levels 11-15, which usually have higher access priority. A special subscriber can have one or multiple access levels (between 11 and 15) at the same time, which are also stored in the SIM card of the subscriber.

For subscribers with the access levels 0-9, their access right is also applicable to the home PLMN and visit PLMN. For subscribers with the access levels 11-15, their access right is only applicable to the home PLMN. For subscribers with the access levels 12, 13 and 14, their access right is applicable to the area of the country to which the PLMN belongs.

Subscribers with the access level 11-15 have a higher access priority than those with the access levels 0-9.

The access level control parameter consists of 16 bits: C0-C15, which respectively corresponds to 15 access levels in bit mapping mode (C10 is used for permitting emergency call). When a bit is 1, it indicates not to allow MS with the corresponding level to access then cell. Otherwise it indicates to allow the access.

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MBR

Value range: 0~3

Content: Multi-band Reporting. It is used to inform MS to

report the adjacent cells in a controllable way. It is sent in the

System Information 2ter and 5ter.

When the value is “0”, MS will report measurement results of six strongest adjacent cells no matter which band they are in. When it is “X(X<=3)”, MS will report measurement results of X strongest adjacent cells in each band other than the serving band, and the remaining positions are used to report the strongest ones in the serving band. If there are still some position left, MS will use them to report the left strongest cells no matter what band they are. If there are no special requirements on different bands and the traffics in the various bands are basically the same, “0” is recommended. When the traffics on various bands are obviously different from each other and MS is expected to enter a band preferably, “3” is preferred. When traffics on various bands are slightly different from each other, “1” or “2” is recommended.

In the initial stage of dual-band network, the traffic of GSM1800 system is very light, usually dual-band MS are expected to work on this band preferably. Therefore, the priority of GSM1800 cells for HO should be higher than that of GSM900 cells, and “3”is recommended for MBR.

Note: In the single-band system, system messages 2ter and 5ter are not effective. Therefore, the MBR parameter does not exist.

62


Application of MBR

-92dBm -82dBmA B

C -68dBm

D -90dBm

E -78dBm

F -88dBm

G -96dBm

H -84dBm

GSM1800

GSM900

S

S is a GSM900 cell, cells A~H are adjacent to cell S. Of them, A and B are GSM1800 cells and others are GSM900 cells. The above diagram shows the influences of different MBR parameters as follows:

1)When MBR = 0, MS will report six adjacent cells with strongest signals without considering the bands, the report result is: C, E, B, H, F, D.

2)When MBR = 1, the result is: C, B, E, H, F, D.

3)When MBR = 2, the result is: C, E, B, A, H, F.

4)When MBR = 3, the result is: C, E, H, B, A, F (3 GSM1800 cells should be reported. But there are only 2 currently, so 2 GSM1800 cells are reported. For the rest, GSM900 cells will be reported).

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ECSC


Content: Early Classmark Sending Control. It indicates if an MS

in the cell is allowed to use early Classmark sending. See

Protocol 0408.

Recommendation: No.

In GSM network, MS’s service capability, supported band, power capability, encryption capability and so on are described by classmarks. There are three classmarks which are classmark1, classmark2 and classmark3. The network can know the MS’s capability by checking the classmarks of the MS. After receiving the class mark enquiry message, MS will send classmark change message to the network as soon as possible. CM3 (Classmark 3) includes the information about MS power, multiband and/or multislot capability. To perform handover between different bands, the power level must be described correctly. In the process of paging and sending of the BA2 information between different bands, the CM3 message must be known.

64

Note:

1) ECSC is invalid for single-band MS. For dual-band MS, when ECSC is not used, after the MS sends EST IND , MSC will still send the CLASSMARK REQUEST message, and MS will response with the CLASSMARK UPDATE message, and other functions are not affected. For the dual-band MS, when this parameter is set to Yes, the connection time between different MS will be obviously shortened.

2) When the encryption function is enable, The parameter must be set to "Yes".

3) M900/M1800 hybrid cells sharing BCCH are advised to be configured as "yes", and M1800 cells in dual-band network are advised to be configured as “yes".

65


Class Mark Update Flow

Classmark Change

Classmark Enquiry

MS BSS MSC

Classmark Request

Classmark Update

Class mark update flow:For dual-band MS, after the SCCP connection is established and before receiving message "CM Service Accepted" there will be a class mark update flow.CR message reports the information of class mark 2. And in the class mark update flow, MS will report class mark 3 information which is related to the dual-band capabilty.

66


Content of Class Mark 1~3

67



CBQ (Cell Bar Qualify)

CBA (Cell Bar Access)

MS_TXPWR_MAX_CCH

RXLEV_ACCESS_MIN

PI (Cell Reselect Parameters Indication)

CRO (Cell Reselection Offset)

TO (Temporary Offset)

PT (Penalty Time)

CRH ( Cell Reselection Hysteresis)

When MS is powered on, it will try to find a GSM PLMN. MS will select an appropriate cell and read system information. This process is called cell selection. The “appropriate cell” is restricted by many factors. For example, whether this cell belongs to the selected network (under manual network selection mode), whether the cell is barred, the cell selection priority of the cell, whether the access level of MS is prohibited by the cell, whether the quality of radio channel meets the requirement of communication, etc. Among them, the quality of radio channel is one important factor of cell selection. GSM specification stipulates one parameter called path loss principle C1. The appropriate cell must ensure the C1>0. C1 is obtained by the calculation of receiving level and the cell selection parameters.

After MS selects cell, MS will settle in the selected cell. At the same time it begins to measure the signal level of BCCH of neighbor cells. It records six neighbor cells with the strongest signals (refresh at least every 60s), extracts various system information and control information of each neighbor cell. (MS must conduct data block decoding for all the six strongest BCCH of neighbor cells within 5 minutes, including parameters affecting cell reselection. When MS regards a new neighbor BCCH as one of the six strongest BCCHs, it will conduct data block decoding for this new BCCH at least every 30s). Moreover, MS must check one of the six strongest at least every 30s. If BSIC has any changes, it will be regarded as a new BCCH and data decoding will be conducted again. During this process, MS will not stop monitoring PCH. When a certain condition

68

is satisfied, MS will move from current cell to anther cell. This process is called cell reselection. The condition includes many factors, and they are all related to the quality of radio channels. When a neighbor cell’s radio channel quality is better than current cell’s, cell reselection occurs. The channel quality standard for cell reselection is C2. C2 is obtained by calculation of the receiving level and a number of parameters.

69


Cell Selection Process

There are two kinds of cell selection.

Stored list cell selection

Normal cell selection

In fact, the process might be different for different MSs

If the SIM card of MS has not stored any BCCH (usually it is a new SIM card), it will search all the 124 RF channels and 374 more GSM1800 channels for dual-band MS, and measures the receiving signal level of each. The whole process lasts about 3s to 5s. During this period, MS obtains at least 5 measurement samples from each RF channel. Then MS tunes to the carrier with strongest receiving level, and judge whether it is BCCH (by searching for FCCH). If yes, MS tries to decode SCH and makes itself synchronous with that BCCH, then the MS reads system information on it. If MS can correctly read the system information and verify this cell belongs to the selected PLMN and its C1 is larger than 0, and also its cell selection priority is normal, then MS conducts location update. After passing, MS resides in that cell. Otherwise, MS will tune to second strongest BCCH and go on with the same procedure. If after trying the strongest 30 (single-band) or 40 (dual-band) carriers it still can’t find a suitable cell to reside in, the MS will try to access the cells with low cell selection priority. If still unsuccessful, the MS will try the cells of other PLMNswhich are allowed by the SIM card. If failed again, MS will stay at a cell (signal is the strongest, C1 is larger than 0, cell selection priority is not prohibited.) without considering the PLMN and enter the emergency call mode (service bar mode). Meanwhile, the MS keeps on monitoring all RF channels.

70

Note:

1. When MS access level is prohibited by this cell, cell selection algorithm is not affected. If the condition is fulfilled, MS will still try to reside in this cell.

2. MS belongs to the PLMN selected, but is prohibited from access, or C1<0, then MS will obtain the BA table from this cell and search those BCCHsaccording to this BA.

When MS is powered off, it will store some BCCH carrier information. When MS is powered on, it will first search the BCCHs which have been stored. If MS can decode the BCCH data of this cell but cannot reside, it will check the BA table of this cell and try these BCCHs. If it still cannot pass, MS will start the cell selection process without BCCH list.

71


Stored List Cell Selection

Search the BCCH which have been

stored

Can not find it

Normal cell selection

Can find it

Can reside in it

Can’t reside ,search neighbor cell

Come into idle mode

NO

YES

When MS is powered off, it will store some BCCH carrier information. When MS is powered on, it will first search the BCCHs which have been stored. If MS can decode the BCCH data of this cell but cannot reside, it will check the BA table of this cell and try these BCCHs. If it still cannot pass, MS will start the cell selection process without BCCH list.

72


Normal Cell SelectionSearch all frequency

Rank according to receiving level

Come into idle mode

Check whether this is a BCCH carrier, get SCH and decode system information

PLMN, C1>0, priority is normal

YES

Check next one

NO

NO

YES

Search all frequency

Judge whether it is BCCH

If MS can correctly read the system information and verify this cell belongs to the selected PLMN and its C1 is larger than 0, and also its cell selection priority is normal, then MS conducts location update. After passing, MS resides in that cell.

Otherwise, MS will tune to second strongest BCCH and go on with the same procedure. If after trying the strongest 30 (single-band) or 40 (dual-band) carriers it still can’t find a suitable cell to reside in, the MS will try to access the cells with low cell selection priority.

If still unsuccessful, the MS will try the cells of other PLMNs which are allowed by the SIM card.

If failed again, MS will stay at a cell (signal is the strongest, C1 is larger than 0, cell selection priority is not prohibited.) without considering the PLMN and enter the emergency call mode (service bar mode).

73


Normal Cell Selection

Check the next frequency by the same step

Check the cell which priority is low, C1>0,PLMN

Come into idle mode

Try the cell which PLMN is allowed in the SIM card

Stay at a cell which RX-LEV is strongest, C1>0 , cell priority is not prohibited Emergency

call mode

NO

NO

YES

YES

YES

NO

YES

Search all frequency

Judge whether it is BCCH

If MS can correctly read the system information and verify this cell belongs to the selected PLMN and its C1 is larger than 0, and also its cell selection priority is normal, then MS conducts location update. After passing, MS resides in that cell.

Otherwise, MS will tune to second strongest BCCH and go on with the same procedure. If after trying the strongest 30 (single-band) or 40 (dual-band) carriers it still can’t find a suitable cell to reside in, the MS will try to access the cells with low cell selection priority.

If still unsuccessful, the MS will try the cells of other PLMNs which are allowed by the SIM card.

If failed again, MS will stay at a cell (signal is the strongest, C1 is larger than 0, cell selection priority is not prohibited.) without considering the PLMN and enter the emergency call mode (service bar mode).

74



RLA_C: Receiving level of MS

RXLEV_ACCESS_MIN: Minimum receiving level of MS permitted to access

P: Maximum Physical supported transmitting power of MS.

C1=RLA_C-RXLEV_ACCESS_MIN-

MAX((MS_TXPWR_MAX_CCH - P), 0)

75


RXLEV_ACCESS_MIN

Value range: 0~63

Unit: Level

Content: It means the minimum receive signal level

required for MS to access a cell.

Recommendation: 8

To prevent MS from accessing the system when the receiving signal level is very low (this will make unsatisfactory communication quality and waste the radio resources of the network), GSM specifications require that the MS’s receiving level must be greater than a threshold when it needs to register in the network. The threshold is the RXLEV_ACCESS_MIN.

For some cells with high traffic, this parameter can be increased appropriately to lower the values of C1 and C2 of this cell. Accordingly the effective coverage range of the cell will be reduced. But the value of RXLEV_ACCESS_MIN should not be too big, otherwise coverage hole (with regard to idle mode MS) will be created at the edge of the cell. When this method is used to balance the traffic, the value of RXLEV_ACCESS_MIN is recommended to be no more than 20.

Except for the areas with densely distributed BTS and good coverage, generally it is not recommended to use RXLEV_ACCESS_MIN to adjust the traffic of the cell. For isolated BTS or BTS with poor coverage, this value should be set properly, otherwise the call drop rate may increase and QoS will be affected.

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MS TXPWR_MAX_CCH

Value range: 0~31

Unit: level

Content: This parameter determines the maximum

allowed output power of the MS when it begins to

access a cell and has not yet received power control

command.

Recommendation: 5(900MHz), 0(1800MHz)

During MS’s communication with BTS, its transmitting power is controlled by the network via power control command. This command is transmit in SACCH (There are two head bytes. One is power control byte, the other is time advance). MS must get the power control head from the downlink SACCH, and output the power as is indicated in the in the power control header. If the MS cannot support the power level in the power control header, it will use the nearest power level it supports.

Since SACCH is associated signaling channel, it must be combined with other channels, such as SDCCH or TCH. Therefore, the control of MS’s power begins after MS receives SACCH. While the power level used by MS before it receives SACCH (power used when RACH is sent) is determined by the “MS TXPWR_MAX_CCH“ (maximum power level of control channel).

This parameter will affect cell selection and cell reselection.

C1 = RLA_C - RXLEV_ACCESS_MIN - MAX((MS_TXPWR_MAX_CCH - P), 0)

RLA_C: mean receiving level of MS

RXLEV_ACCESS_MIN: minimum receiving level of MS permitted to access

P: maximum physical supported transmitting power of MS.

77


Power Class

Phase 2 MS output Power Class and the exact power relations.

power class

GSM900 Max peak power

DCS1800 Max peak power

Tolerance(dB)_normal

Tolerance(dB)_extreme

1 ------ 1W(30dBm) +/-2 +/-2.5

2 0.25W(24dBm) 2 2.5

3 5W(37dBm) 4W(36dBm) 2 2.5

4 2W(33dBm) 2 2.5

5 0.8W(29dBm) 2 2.5

In the above interface trace, we often see the MS classmark. In MS classmark, there is a “power class” parameter. MS power class is to stand for MS max transmitting power. Generally speaking, the new MS is phase 2 MS. Now let’s talk about Phase 2 MS output Power Class and the exact power relations.

Power class of 1800M MS is generally 1, and its transmitting power is 1W or 30dBm; power class of 900M MS is generally 4, and its transmitting power is 2W or 33dBm. In different power classes, transmitting power has a tolerance; the tolerance is generally 2dB and the extreme is 2.5dB.

78


Power Control Level

Phase 2 MS

Power control

level and the

exact power

relations.

power control level

output power (dBm)

Tolerance_normal

(dB)

Tolerance _extreme

(dB)

Power control level

output power (dBm)

Tolerance_normal

(dB)

Tolerance _extreme

(dB)0-2 39 2 2.5 29 36 2 2.53 37 3 4 30 34 3 44 35 3 4 31 32 3 45 33 3 4 0 30 3 46 31 3 4 1 28 3 47 29 3 4 2 26 3 48 27 3 4 3 24 3 49 25 3 4 4 22 3 410 23 3 4 5 20 3 411 21 3 4 6 18 3 412 19 3 4 7 16 3 413 17 3 4 8 14 3 414 15 3 4 9 12 4 515 13 3 4 10 10 4 516 11 5 6 11 8 4 517 9 5 6 12 6 4 518 7 5 6 13 4 4 5

19-31 5 5 6 14 2 5 615-28 0 5 6

In power control, we can often see another important parameter—power control level, or power level. This parameter is called MS power level.

In the front page, we have talked about power class. MS power class is a static parameter, and each MS has its own fixed power class.

Power level is adjustable; MS “power level” can be adjusted by power control algorithms, and this is subject to the distance between MS and base station, transmission condition, etc. Power control is to control power level of MS and base station.

Here we will focus on Phase 2 MS Power control level and the exact power relations.

Max transmitting power of 900M MS is generally 2W, this is to say, power class is 4; its power control level is generally 5, its transmitting power is 33dBm, so its max power control level value is 5. As performing power control, the value will be adjusted down from 5. For 1800M MS, its max transmitting power is 1W (30dBm).

79

Let’s have a look at the right part of the film. For 1800M MS, its power control level should be below 0, 1, 2, and 3; when power control level increase one level, MS transmitting power will lower down 2 dB. For 900M MS and 1800M MS, when power control level increase one level, MS transmitting power will lower down 2 dB. There is a tolerance problem, I.e., power control level error; the general tolerance is 3 dB, and max tolerance is 4dB or 5dB. For different power level, the tolerance are different.

80


CBQ

Value range: Yes , No

Unit: None

Content: Cell Bar Qualify. CBQ only affects the selection of

cells but is not related to cell reselection. It works with CBA

to define the access priorities of cells.

Recommendation: No.

For the area overlapped by cells, the operator often wants MS to preferably select certain cell during cell selection according to the cell capacity, traffic and cell functions, i.e. setting the cell priority. This function can be implemented by setting parameter “cell bar quality”. It works with parameters “cell bar access” together to determine the cell selection and cell reselection priority of the cell.

CBQ CBA Cell selection priority Cell reselection priority

No Yes Normal Normal

No No Prohibited Prohibited

Yes Yes Low Normal

Yes No Low Normal

Usually the priorities of all cells should be set as “Normal”. But in some special cases such as micro-cell and dual-band network, the operator may expect MS to preferably enter the cells of a certain type. In this case, the network operator can set the priority of this type of cell as “Normal” while setting the other cells as “Low”. MS will select the cell with lower priority only when there is no appropriate cell with the priority as “Normal”. During the network optimization by means of cell priority, it is necessary to note that CBQ only influences the cell selection. Therefore, in order to achieve the target, C2 (cell reselection parameter) must be taken into consideration.

81


CBA


Content: Cell Bar Access, worked together with CBQ to set the

priority status of the cell in idle mode for cell selection and

reselection.

Recommendation: Yes

The network operator can set the cell access is permitted or not by the parameter. Usually all cells allow MS to access, thus it is set as “Yes”. But in the special cases, the operator may want a cell to be used for handover service only, which can be realized by setting the parameter as “No” (CBQ should be “No” in this case).

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Application of CBQ

The traffic of cell A and B is heavy. Set these two cells with CBA=“No”, CBQ=“No”.

A B

Each circle in the diagram indicates a cell. For some causes, the traffic in cell A and that of cell B are obviously higher than those of the adjacent cells. To make the traffic of the entire area distributed on average, set the priorities of cell A and cell B as “Prohibited” and those of other cells as “Normal”. In this way, the services in the shadow areas in the diagram will be shared by the adjacent cells. It must be pointed out that this setting will reduce the actual coverage areas of cell A and cell B, which is different from decreasing the transmitting powers of cell A and cell B.

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Application of CBA

B is a micro cell. Set B to “Normal” and A to “Low”.

BA

Assume that micro-cell B and macro-cell A together cover an area. In order to make micro-cell B share more traffic of macro-cell A, the priority of cell B can be set as “Normal” and that of cell A as “Low”. Thus in the coverage area of cell B, MS will select cell B as long as the level of cell B reaches the RXLEV_ACCESS_MIN, no matter cell B has a lower signal level than cell A’s or not. And then reselection parameters can be set appropriately to make MS not to reselect cell A.

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Application of CBA

CBQ(A) = “NO” CBA(A) = “NO”

A

C

BD

Assume that area D in the diagram is a hot spot. Usually micro cell is adopted in order to have a better capacity with the limited frequency resources. And dual-layer network is usually adopted in order to reduce the handover times. When MS moves at a high speed, it can use site A to avoid too many handovers between the micro-cells. But in general case, MS should work in the micro cell (this can be realized by setting the priority and proper reselection parameters of the cell). When MS moves at a high speed during the conversation, the network will force MS to hand over to cell A. If MS stays close to cell A and at the edge of micro cell when the conversation ends, as the signal quality of cell A is much better than that of micro cell, MS will not initiate the cell reselection process according to GSM specification. As cell A usually have a small capacity, it will be congested in the above case. The solution is to set the cell bar access of cell A as “No”, certainly CBQ should also be set as “No” to prohibit MS from accessing cell A directly and only permit MS to enter the coverage area of cell A by HO. In this way, MS will reselect the micro cell when the conversation ends.

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Cell Reselection ProcessCell reselection for cells in same location area

If the C2 value of the target cell is higher than that of the serving cell for longer

than 5 seconds, a cell reselection process will be performed and the MS tunes to

the new cell.

Cell reselection for cells in different location areas

If the C2 value of the target cell is higher than that of the serving cell by at least

the value of CRH for longer than 5 seconds, a location update process and the cell

reselection process will be performed.

Two consecutive cell reselections caused by C2 have a time interval of 15 seconds.

In other words to say, if because of C2 a MS reselected to a cell, then the MS

cannot reselect to another cell by the cause of C2 within 15 seconds.

If current serving cell is prohibited, or down link fails, or C1 is less than 0

continuously for 5s, cell reselection will also be triggered.

MS starts a cell reselection if the access times exceed the MAX retrans.

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Cell Reselection Parameter

C2=C1+ CRO -TO*H(PT-T) When PT is not equal to 31

C2=C1- CRO When PT is equal to 31

For neighbor cell

H(x)=0, when x<0;

H(x)=1, when x>=0

For serving cell

H(x)=0

C2=C1+CRO-TO*H(PT-T) When PT is not equal to 31:

C2=C1-CRO When PT is equal to 31.

Wherein:

1. Function H(x)=0, when x<0; H(x)=1, when x>0.

2. T is a timer with initial value 0. When a certain cell becomes one of the six strongest neighbor cells, T corresponding to thiscell begins to count. When the cell is out of the six strongest neighbor cells, the corresponding timer is reset.

3. CRO is used to revise the C2 intentionally.

4. The function of TO is to reduce the value of C2 from T beginsto T reaches the stipulated PENALTY_TIME.

5. PT is the time that TO functions on C2. But if PT=31, the C2 formula is changed as C2=C1-CRO.

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PI

Value range: Yes (1), No (0).

Unit: None

Content: Cell Reselect Parameters Indication, sent on

the broadcast channel of the cell. It is an indication to

the existence of “Cell Reselect Offset(CRO)”,

“Temporary Offset(TO)” and “Penalty Time(PT)”.

Recommendation: Yes

This parameter is to inform MS whether C2 is used as the standard for cell reselection. The minimum interval between cell reselections caused by C2 parameter is 15s to avoid too frequent cell reselection.

If PI=Yes, C2 is used for cell reselection standard; if PI=No, C1 is used for cell reselection.

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CRO

Value range: 0~63, the corresponding level value:

0~126dB, stepped every 2dB.

Unit: None

Content: Cell Reselect Offset. It is a parameter in C2

calculation to give an intentional modification in MS

cell reselection.

Recommendation: 0

After cell selection, MS will reselect another better cell in idle mode. It is C2 parameter that determines cell reselection. The principle for MS reselection is: select the cell with the maximum C2 as the serving cell. C2 is determined by the following factors:

C2=C1+CRO-TO*H(PT-T) (PT <31)

C2=C1-CRO (PT = 31)

H(x)=0 if x<0；

H(x)=1 if x≥0

As shown above, C1 indicates the quality of radio channel. The larger C1 is, the better the channel is. C2 value is based on C1, through CRO, C2 of various cells can be adjusted. Thus C2 value can be calculated according to CRO, TO and PT in order to prefer selecting the cell in reselection process. That is, in dual-band network,several parameters that influence C2 value can be set to make C2 value of GSM1800 larger than that of GSM900. Therefore, even though the signal strength of GSM1800 cell is weaker than that of GSM900 cell, MS can still reside in GSM1800 by the aid of these parameters. Besides CRO, there are another two parameters influencing C2:TEMPORARY_OFFSET(TO) and PENALTY_TIME(PT).

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CRO is a artificial modification on C2. Reasonable setting of this parameter can reduce handover times and realize assignment to a better cell. Usually it is not set as larger than 25dB. Generally the cells with the same priority in the network have basically the same CRO. Setting of this parameter only affects MS of GSM Phase II and above.

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TO

Value range: 0~7 , the corresponding value is 0~60dB,

7 corresponds to “Infinite”.

Unit: None

Content: Cell Reselect Temporary Offset. It is a

parameter in C2 criterion to give a temporary

modification within PT time.

Recommendation: 0

TO indicates the temporary modification on C2. Temporary means that it functions for C2 only within a duration which is determined by PT parameter. Setting of this parameter only affects MS of GSM Phase II and above.

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PT

Value range: 0~31, the corresponding time is 20~620s.

31 is to change the formula of C2.

Unit: None

Content: Cell Reselect Penalty Time.

Recommendation: 0

If the communication in a cell is affected due to very heavy traffic or some other reasons, this cell should not be the preferable cell that MS works in (a repulse should be made for this cell). In this case, PT can be set as 31, which causes TO invalid and C2=C1 –CRO. Therefore, C2 value of this cell is decreased. MS will reselect this cell with little possibility. Besides, the network operator can set CRO according to the repulse degree to this cell. The higher the repulse degree, the larger CRO.

For the cell with very low traffic, MS should prefer to work in this cell. In this case, CRO is recommended to be between 0-20dB. It can be set according to the preference degree to this cell. The higher the preference degree and the larger CRO. Generally TO is recommended to be the same as or a little more than CRO. PT is mainly used to prevent MS’s too frequent cell reselections. Generally it is recommended to be 0 (20s) or 1 (40s).

For the cell with medium traffic, generally CRO is recommended to be 0 and PT be 31 as a result of C2=C1.

Setting of PT can effectively prevent the fast moving MS from accessing the micro-cell. This parameter can be set according to the size of micro-cell. And it is recommended to be 20s for the ordinary micro-cells. When PT is set as 31, it is used to change the direction of CRO.

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CRH

Value range: 0~14 , the step size is 2

Unit: dB

Content: Cell Reselection Hysteresis. It is the parameter used

when cell reselection happens between two location areas.

Recommendation: 4

This parameter aims to prevent the frequent location update that may increase the network signaling flow and to reduce the possibility of paging message loss. If the value of this parameter is too small, the location update will have “ping-pong” effect and the signaling load on SDCCH will increase. Moreover, the call setup successful rate of the system will become lower because MS will not respond paging during location update. When it is too large, the cell where MS resides for a long time may not be the best when MS enters a new location area. When MS reselects a cell from a different location, MS will start a location update. Due to the fading of radio channel, C2 values of two cells at the their edges will fluctuate, which causes MS to reselect frequently. To reduce the influence, GSM specifications define a parameter called cell reselection hysteresis. It is required that MS start cell reselection only when C2 value of the adjacent cell (in a different location area) is greater than that of serving cell and their difference is greater than the value of reselection hysteresis.

Appropriate cell reselection hysteresis is important for network optimization. Usually it is recommended to be 8~10dB and can be adjusted in the following cases:

When the traffic of an area is very heavy and the signaling overload often occurs, it is recommended to increase the value of cell reselection hysteresis of the adjacent cells belonging to different LACs.

When the overlapping coverage of the adjacent cells belonging to different LAC are wide, it is recommended to increase the value of cell reselection hysteresis.

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Network Function Parameter

PWRC (Power Control Permitted)

UL DTX (Uplink Discontinuous Transmission)

Call Re-establishment Allowed (RE)

EC Allowed (Emergency Call Allowed)

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PWRC

Value range: Yes, No.

Unit: None

Content: It is used to indicate MS weather to reduce received level

value obtained from the timeslots of BCCH when MS measures

receiving level during base band hopping mode. When it is set to

“No”, it means the influence of receiving level on BCCH is not in

consideration. If it is set to “Yes”, process it in the way mentioned

above.

Recommendation: Yes

In order to monitor the communication quality of radio link and conduct power control, both MS and BTS must have measuring function. Butthe measurement will encounter some problems when several independent functions of GSM specification are combined for use. First, GSM specification allows BCCH frequency to attend frequency hopping (excluding the BCCH timeslot); secondly, GSM specification allows to conduct downlink power control for the frequency hopping channel; lastly, the power of BCCH frequency is not allow to be changed because MS needs to measure the signal level of the adjacent cells. In the above conditions, the downlink power control of a channel can only be applicable to a subassembly of the frequency assembly that the channel is using, that is, the BCCH frequency during the frequency hopping process cannot be power-controlled. If MS measures the downlink channel level in ordinary mode, the measurement result involving the BCCH frequency will be inaccurate for the power control. To reduce the influence of this problem upon power control, MS is required to deduct the receiving level value obtained from the timeslot of BCCH carrier when calculating the average value of receiving level during frequency hopping. To make MS execute the above operation, the parameter “PWRC” should be set as “yes” in the system.

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UL DTX

Value range: May use, Shall use, Shall not use.

Unit: None

Content: It is used to indicate whether MS uses DTX function.

In System Information 3, it is 2-bit coding. In System

Information 6, it is 3-bit coding. It shows the DTX state of the

MS in the previous measurement period.

Recommendation: Shall use

DTX has very limited influence on the conversation quality. But its application has two advantages: one is that the interference on radio channel is effectively reduced and a better average conversation quality can be achieved; the other is that DTX can considerably reduce the power consumption of MS. Therefore DTX is recommended on the network.

According to the protocol, MS reports BTS with two kinds of measurement report. One is called full measurement report. It will average the levels and qualities of 100 timeslots in the whole measurement cycle (one measurement cycle involves 4 TCH-multi-frames except idle frames). The other is called sub measurement. It averages the levels and qualities of 12 timeslots, including eight “consecutive” TCH bursts and four SACCH bursts.

According to GSM specification, both BTS and MS should conduct this two kind of measurement (FULL and SUB) no matter whether the uplink/downlink DTX of the system is activated or not. Each measurement report of SACCH indicates whether DTX is active or not. According to this indication, BTS can make the right selection, either FULL or SUB.

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Call Re-establishment Allowed


Unit: None

Content: Informing MS whether call reestablishment is allowed

or not.

Recommendation: No

In case of call drop, MS can start call re-establish process to resume the conversation. The network has the right to decide whether to re-establish or not. This function is achieved by setting “Call Re-establishment Allowed”.

Call Re-establishment Allowed” is contained in “RACH control parameters”, and sent on system information of each cell.

In some special cases (e.g. the cell has coverage hole at a fixed position), call drop will occur if MS passes by coverage hole during the conversation. If call re-establishment is permitted, the dropped call be resumed. However, call re-establishment takes relatively long time. Most of the subscribers have already released manually before the re-establishment is completed. Therefore, the re-establish of call not only fails to achieve the target, but also waste a lot of radio resources. So it is recommended that except for some special cells, it is not permitted to re-establish the call on the network.

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EC Allowed


Unit: None

Content: Emergence Call Allowed . When EC Allowed is set to

“Yes”, it means emergency call is permitted. Otherwise it is

prohibited.

Recommendation: Yes

For MS with common access control class 0~9, when “EC Allowed” is “Yes”, it indicates to allow emergency call. For MS with access levels 11~15, the emergency call will not be allowed only when the corresponding access control class bit and “EC Allowed” are all set as “not allow” at the same time.

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Others in Data Configuration System

System Information Usage

Regular Transmission

Regular Transmission Interval

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System Information Usage

Value Range: It is a check box, including system information

1~12, 2bis, 2ter, 5bis, 5ter, 10bis.

Unit: None

Content: It is used to determine whether a type of system

information should be sent. A selected box means to send.

Recommendation: Select system information 1~6, 2bis, 2ter,

5bis, 5ter

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Regular Transmission


Unit: None

Content: It shows whether BSC chooses to update system information

contents transmitted by BTS. If it is “Yes”, BSC will send system

information to BTS at a certain interval (it is decided by “Regular

Transmission Interval”). Otherwise, system information can only be

updated manually by using BSC maintenance system.

Recommendation: Yes

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Regular Transmission Interval

Value range: 0~255

Unit: Minute

Content: It stipulates the time interval that BSC updates system

information to BTS.

Recommendation: 10 minutes

BSC needs to periodically refresh the system information in BTS to avoid the mis-change or dead-lock of the system information.

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Power on

BA2 RLTNCC AllowedMBREC AllowedCall Re-establishment AllowedPWRC UL DTX

MS MAX Retrans / ECSCTX-IntegerCommon/Special Access Control Class

CGI, BSIC

ATTT3212CCCH_CONFBS_AG_BLKS_RESBS_PA_MFRAMES

C2 (CRO,TO,PT,PI)CRHDSC

C1(RXLEV_ACCESS_MIN/MS TXPWR_MAX_CCH)CBA, CBQ

BA1

SI Mapping in Different Statuses of MSSelect Network

(Network Identity Para. )

Cell Selection(Cell Selection

Para.)

Access Network(Call Control Para.)

Idle Mode(Idle Mode Para.)

Dedicated Mode(Call Control Para.)

Paging Mode(Idle Mode Para.)

Cell Reselection Mode(Cell Reselection Para.)

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Contents



3. Case Study

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Case StudyCase 1: Island effect

Case Description

During the process of optimization in a certain area, it is found out that

the signal is poor in a certain section of suburban highway. The signal

strength measured is less than -95dBm.

Handling Procedure

Modify the data in BSC. Add neighbor relationship to Cell A and B for

each other. The coverage problem is thus solved.

C

BA-92

-80-95

Procedure analysis:

The place is located in a highway section in suburb and is about three kilometers away from downtown. The terrain structure does not fluctuate seriously. Theoretically, the signal strength should be about -80dBm, which is very different from the actual value measured. After conducting frequency scanning here, we find that cell A, B and C are covering here. Their frequencies are Fa, Fb, Fc and signal strength is -95, -80dBm, -92dBm respectively. After checking the data, it is found that cell B is not configured as cell A’s neighbor, thus an island is formed between cell A and B. When MS comes from cell A goes towards B, it cannot have cell reselection or handover to B and this leads to as if the coverage near cell B is not good.

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Case StudyCase 2: Incorrect CGI causes MS unable to register on the network.

Case description

The subscriber complained that MS could not register to the network, or it was

difficult to register to the network between BTS A and B, or no network mark was

displayed on the MS, but MS indicated that the signal was very good. Some

subscribers also complained that MS could make MTC could not make MOC.

Cause analysis

CGI of the cell was modified. So the network register flow was not normal. Although

the signal was very good, MS could not get registered in the network.

Recommendation and summary

CGI is a very important parameter and should be configured correctly, thus to

ensured that the CGI data in BSC and MSC are the same.

Procedure analysis:

This BTS site originally worked normally. The problem arose recently. Since the subscriber complained that the problem were most serious near BTS A and B, we decided to check these two BTS first. First we checked the alarm on BTS A and B, it was normal. Then we checked the traffic statistics data and found out that the SDCCH requests in cell 2 of BTS A rose from about 3000 in 17th May to about 6000 in 18th May. The obvious change began from 7:00 AM on 18th May. At the same time, SDCCH attempt failures rose suddenly to more than 3000. It was obviously abnormal. And SDCCH congestion rate in cell 3 of BTS A was about 30%. But the traffic volume, congestion rate, and call drop rate of BTS A is normal. No abnormal situation is found in BTS B’s traffic measurement. After checking the SDCCH traffic measurement of all the adjacent cell of BTS A, we found a cell in which times of SDCCH attempt decreased dramatically. The SDCCH statistics result showed the problem should exist in cell2 and 3 of BTS A.

Where did the 3000 times of SDCCH requests come from? Analyzed from the failure, we suspected that the data was modified, so we checked the BSC data operation log. Using MA10 to trace the network access flow of MS, and we found a lot of LOCATION UPDATE REJECTED messages. Since there was no record of modification of BSC data, we checked MSC’s related cell parameters, and found that CGI of cell 2 and 3 of BTS A were modified. All the three cells in BTS A were using exactly the same CGI. After correcting the CGI of cell 2 and 3, everything was back to normal.

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Case Study

Case 3: RXLEV_ACCESS_MIN causes signal fluctuation.

Case description

Subscribers complain that the signal fluctuates in a certain area. On-site test shows that this

phenomenon does exist. From the testing mobile, we can see that there are a lot of cells’

signals in this area. Some frequencies are the suburb BTS signals far away from this area. The

cell reselection of MS is rather frequent.

Handling procedure

Modify parameter “RXLEV_ACCESS_MIN” of all cells in urban area and make them consistent

with each other. The problem of signal fluctuation is solved.

Recommendation and summary

In area with complicated coverage, the “RXLEV_ACCESS_MIN” of each cell should be kept

same.

Cause analysis

We check the parameter “RXLEV_ACCESS_MIN” in data management system, and find out that “RXLEV_ACCESS_MIN” of correlative cells are not consistent, and some even have great differences. Then in this area, MS is quite likely to reselect the cell with poor signals. And subscribers will feel the obvious fluctuation of signals.

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Case Study

Case 4：Too small T3212 causes MS lose connection with network.

Case description

A local GSM network has more than 6000 subscribers. There are four BTSs. The

network has been working properly. But from sometime on, some MS often lost

connection with network suddenly. The complaint from subscribers in urban area

increased.

Handling procedure

Modify T3212 to 10, i.e. 60 minutes. Modify the corresponding time of MSC to

180 minutes. Two days’ observation shows no subscriber complaint and the

problem is solved.

Cause analysis:

Recently, no modification is conducted on the BSC. The current network only differs from original network in the number of subscribers. In VLR, there are more than 4000 local subscribers and 5000 roaming subscribers. Therefore, the problem might be caused by the sudden increase of subscribers.

The pressure brought by subscriber increase on network is mainly shown in two aspects: (1) TCH (traffic channel) congestion rate increases; (2) SDCCH (signaling channel) congestion rate increases. MS maybe cannot make MOC or MTC, and at the same time cannot conduct location updating successfully. The direct result of location update failure is MS loses connection with the network. Check BSC system information data table. It is found that T3212 is set to 2 (unit: 6 minutes), i.e. 12 minutes, and MSC corresponding time is set to 30 minuets. This kind of configuration will cause the result that all activated MS will originate a periodic location update every 12 minutes. When the number of subscriber reaches a certain limit, SDCCH will be fully seized. If at this time some MS originates periodic location update, it will fail because there is no spare SDCCH available. Thus MS loses the connection with the network.

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OMF010001 GSM System Information ISSUE2.0

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