GSM Basics Final

7/28/2019 GSM Basics Final

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GSM Basics and KPIs




GSM NETWORK ELEMENTS

Page 1

Um Interface is the central interface of every mobile network and typically

the only one to which the customer is exposed




Air interface characteristics

Page 2

•ETSI(European Telecommunications Standards Institute) defined all the

characteristics of the air interface.

Examples of the air interface characteristics defined

by ETSI are:

• The output power of by the MS and the BTS • The transmit and receive frequencies of the MS and

the BTS

• The structure of the burst in which the data is sent

• When which type of messages may be sent.




Mobile Equipment (ME or MS)

Page 3

•The ME is the only part of the GSM network which the subscriber will really

see.

• Mobile types have distinct features which must be known by the network, for

example their maximum transmission power and the services they support.

•The ME is therefore identified by means of a class mark.

•The class mark is sent by the ME in its initial message.



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The following pieces of information are held in the class mark:

Revision Level –

Identifies the phase of the GSM specifications that the mobile complies with.

RF Power Capability –

The maximum power the MS is able to transmit, used for power control and

handover preparation. This information is held in the mobile power class number.

Ciphering Algorithm –

Indicates which ciphering algorithm is implemented in the MS.

Frequency Capability –

Indicates the frequency bands the MS can receive and transmit on. Currently all

GSM MSs use one frequency band, in the future this band will be extended but not

all MSs will be capable of using it.

Short Message Capability –

Indicates whether the MS is able to receive short messages.




Subscriber Identity Module (SIM)

Page 5

The SIM contains several pieces of information:

International Mobile Subscriber Identity (IMSI)

This number identifies the MS subscriber. It is only transmitted over the air during

initialization.

Temporary Mobile Subscriber Identity (TMSI)

This number identifies the subscriber, it is periodically changed by the system

management to protect the subscriber from being identified by someoneattempting to monitor the radio interface.

Location Area Identity (LAI)

Identifies the current location of the subscriber.

Subscriber Authentication Key (Ki)

This is used to authenticate the SIM card.

Mobile Station International Services Digital Network (MSISDN)

This is the telephone number of the mobile subscriber. It is comprised of a country

code, a network code and a subscriber number.


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Frequency Spectrum




Absolute Radio Frequency Channel Number

Page 9

•The Absolute Radio Frequency Channel Number

(ARFCN) is a unique number given to each radio channel

in GSM. The ARFCN can be used to calculate the exact

frequency of the radio channel

Band Name ARFCN Uplink Downlink

(MHz) (MHz)

GSM900

Primary GSM 1 ≤ n ≤ 124 890 + 0.2×n f up(n) + 45

Extended GSM 0 ≤ n ≤ 124 890 + 0.2×n f up(n) + 45

975 ≤ n ≤ 1023 890 + 0.2×(n-1024)

GSM Rail 0 ≤ n ≤ 124 890 + 0.2×n f up(n) + 45

955 ≤ n ≤ 1023 890 + 0.2×(n-1024)

GSM1800 GSM1800 512 ≤ n ≤ 885 1710.2 + 0.2×(n-512) f up(n) + 95

(DCS1800)

GSM1900 GSM1900 512 ≤ n ≤ 810 1850.2 + 0.2×(n-512) f up(n) + 80

(PCS1900)

http://www.telecomabc.com/g/gsm.html

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Physical channels

Page 10

•FDMA and TDMA

To achieve a high spectral efficiency in the cellular network a

combination of FDMA (Frequency Division Multiple Access) and

TDMA (Time Division Multiple Access) is used.

Each of the carrier frequencies is divided in time, using a TDMA

scheme to increase the number of channels per carrier

frequency. This results that each carrier frequency channel carries

eight time-division multiplexed physical channels (eight timeslots)

A physical channel is determined by the carrier frequency and

the timeslot number.




Logical Channels

Page 12




Broadcast Channel – BCH(Down Link)

Broadcast control channel (BCCH) is a base tomobile channel which provides general informationabout the network, the cell in which the mobile is

currently located and the adjacent cells Frequency correction channel (FCCH) is a base to

mobile channel which provides information forcarrier synchronization

Synchronization channel (SCH) is a base to mobilechannel which carries information for framesynchronization and identification of the base stationtransceiver




Common Control Channel - CCH

Paging channel (PCH) is a base to mobilechannel used to alert a mobile to a call

originating from the network

Random access channel (RACH) is a mobileto base channel used to request for dedicated

resources

Access grant channel (AGCH) is a base tomobile which is used to assign dedicated

resources (SDCCH or TCH)




Dedicated Control Channel - DCCH

Stand-alone dedicated control channel (SDCCH) is a bi-directionalchannel allocated to a specific mobile for exchange of location update

information and call set up information




Dedicated Control Channel - DCCH

Slow associated control channel (SACCH) is a bi-

directional channel used for exchanging control

information between base and a mobile during the

progress of a call set up procedure. The SACCH is

associated with a particular traffic channel or stand

alone dedicated control channel

Fast associated control channel (FACCH) is a bi-

directional channel which is used for exchange of

time critical information between mobile and base

station during the progress of a call. The FACCH

transmits control information by stealing capacity

from the associated TCH




Logical channel combinations

Page 17




Frame Structure

Page 18

Graphical representation of the logical




Graphical representation of the logical

channel combinations 4 and 5.

Page 19

rap ca represen a on o e og ca




rap ca represen a on o e og cachannel combinations 1,2,and 7

Page 20




Mobile looks for BCCH after switching on

RACH send channel request

AGCH receive SDCCH

SDCCH do the authentication and TMSI allocation

SDCCH require traffic channel assignment

SDCCH send call establishment request

SDCCH send the setup message and desired number

FACCH switch to traffic channel and send ack (steal bits)

FACCH receive alert signal ringing sound

FACCH acknowledge connect message and use TCH

TCH conversation continues

FACCH receive connect message

Call establishment from a mobile




GetServi

ce

GetSDC

CH

Establish SD

Channe

l

Est. TCHConn. Call PhoneRef. Phon

SD

Blocking

SDCCH Seizer

TCH

Blocking

CSSR or TCHAssignment

Call Establishment

Call

completion

Rate (TCH

Drop call)




Main KPIs

KPIs Can be classified in to two based on

Accessibility KPIs

▪ SD Blocking

▪ SD Drop

▪ Immediate Assignment Success Rate

▪ TCH Assignment Success Rate

▪ TCH Blocking

▪ Call Setup Success Rate

Retainability KPIs

▪ TCH Drop

Handover Success Rate

Page 23




SD Blocking

Main Reasons•Location Area not optimized

•SMS usage

•SDCCH availability




Main reasons

Page 26

following issues that could be possible reasons for SDCCH Drops:

•Low signal strength

•Interference

•Path loss Imbalance between UL/DL

•High Timing Advance

•MS error or Subscriber behavior •TCH Congestion

•Transmission Congestion (LAPD Concentration)

•Hardware or Transmission failures




Call Setup Success Rate (%)

KPI Formula

Call Setup Success Rate (%) = ((Immediate Assignment Success Rate x TCH

Assignment Success Rate) x (1 - SDCCH Drop Rate)) x 100%

Counters

CA303J: CELL_IMM_ASS_SUCC,

CA300J: CELL_IMM_ASS_REQ

CA313: CELL_ASS_SUCC

CA310: CELL_ASS_REQ

CM30:CELL_SD_CALL_DROPS

K3003:CELL_KPI_SD_SUCC




Immediate Assignment Success Rate

KPI Formula

Immediate Assignment Success Rate = Call Setup Indications (Circuit Service)

/ Channel Requests (Circuit Service) x 100%

Immediate assignment procedure

CA300J

CA303J




Immediate Assignment Success Rate

Related Factor

Coverage/Interference/Transmission/Device/System Traffic Volume

Statistics

TRX power 、TA、 Antenna system、Rxlevel&Power level

Interference Band、RxQuality、Rxlevel、RACH Overload

Alarm

Cell/TRX/SDCCH/TCH Availability、TCH Assignment Successful Rate

、TCH channel active NACK/Timeout

SDCCH/TCH Traffic Volume、SDCCH/TCH Congestion、Immediate

Assignment Measurement、Location Update




TCH Assignment Success Rate

Formula

TCH Assignment Success Rate = Successful Assignments / Assignment

Requests x 100%

Assignment procedure on the radio interface

CA310

CA313

• Description

•

The number of successfulassignments is equal to the number

of ASS REQ messages received by

the BSC minus the number of ASS

FAIL messages reported by the BSC

to the MSC in various scenarios,

such as air interface assignment,

Mode Modify, and directed retry.




TCH Assignment Success Rate

Related Factor

Coverage/Interference/Transmission/Device/System TrafficVolume

Statistics

TRX power 、TA、 Antenna system、Rxlevel&Power Controllevel、UL&DL Balance

Interference Band、RxQuality、Rxlevel

Alarm

Cell/TRX/TCH Availability、TCH channel activeNACK/Timeout

TCH Traffic Volume、TCH Congestion、Handover (ping-pong)




TCH Blocking

Page 32




TCH Drop Rate

Page 33




Radio Handover Success Rate (%)

Formula Handover Success Rate (%) = (Successful Outgoing Internal Inter-Cell Handovers + Successful

External Outgoing inter-Cell Handovers)/ (Outgoing Internal Inter-Cell Handover Commands +

Outgoing External Inter-Cell Handover Commands) x 100%

Description TCH handover (Intra BSC/Inter BSC)

CH311 :Outgoing Internal Inter-Cell Handover Commands

CH331 :Outgoing External Inter-Cell Handover Commands

CH313 :Successful Outgoing Internal Inter-Cell Handovers

CH333 :Successful Outgoing External Inter-Cell Handovers



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Radio Handover Success Rate (%)

Related Factor

Coverage/Interference/Transmission/Device/System Traffic Volume

r Boost before handover 、Direct Retry

Statistics

TRX power 、TA、 Antenna system、Rxlevel、UL&DL Balance

Interference Band、RxQuality、Rxlevel、Rxquality Handover 、Power Control

Alarm、BSC clock、 A interface circuit

Cell/TRX/TCH Availability、TCH channel active NACK/Timeout、Handover measurement

TCH Traffic Volume、TCH Congestion、Neighbor Cell Traffic

GSM Basics Final

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