01) gsm system survey.ppt

8/10/2019 01) GSM System Survey.ppt

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ZTE University www.univ.zte.com.cnThe information contained in the file is solely property of ZTE corporation. Any kind of disclosing without permission is prohibited.


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GSMSYSTEM OVERVIEW

Important Principles and

Technologies of GSM


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OBJECTIVES

To be aware of the developments of cellular mobiletechnology

To understand the entities that made up GSMMobile System

To know the principles and services that make upGSM


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EVOLUTION OF MOBILECOMMUNICATIONS

STAGES OF PUBLIC LAND MOBILENETWORK

GENERATION STAGE

First Analog Mobile Telephone SystemSecond Digital Mobile Communication System

Third International Mobile Telephony 2000


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FIRST GENERATION OF MOBILESYSTEM

ANALOG MOBILE TELEPHONESYSTEM

NMT 450 Nordic Mobile Telephony: used in 450 MHz, started in 1981

AMPS Advance Mobile Phone System: used in 800 MHz, started

in 1983 TACS

Total Access Communication System: used in 900 MHzstarted in 1985

NMT 900- A variant of NMT 450 at 900 MHz. Started in 1986


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FIRST GENERATION OF MOBILESYSTEM

CONS OF ANALOG MOBILETELEPHONE SYSTEM

No public air interface between various systems

Problem in inter-operability with digital networks

Low frequency availability thus reducing network capacity

Low degree of subscriber security and safety


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SECOND GENERATION OF MOBILESYSTEM (1)

DIGITAL MOBILE COMMUNICATIONSYSTEM

1. TIME DIVISION MULTIPLE ACCESS(TDMA)

2. NARROW BAND CODE DIVISIONMULTIPLE ACCESS (N-CDMA)


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DIGITAL MOBILE COMMUNICATIONSYSTEM (TDMA)

D-AMPS Digital AMPS: used in either 800 or 1900

MHz, started in 1991 PDC

- Personal Digital Cellular: used in 1900 MHz,started in 1994

GSM- Global Systems for Mobile

Communications: used in 900 MHz, started in1991


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DIGITAL MOBILE COMMUNICATIONSYSTEM (TDMA) GSM 1800

Digital Cellular System: used in 1800MHz, started in 1992 GSM 1900

Personal Communication System: usedin 1900 MHz, started in 1995


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INTRODUCTION TO GSM

GROUPE S PECIALE MOBILE

GLOBAL S YSTEM for MOBILECOMMUNICATIONS

WHAT IS GSM?


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INTRODUCTION TO GSM

Why was there a need to develop a cellularstandard?

Incompatibility among existing cellular systemswhich were mostly analog

Spectrum Efficiency International roaming capability Low mobile and Station Cost

Good subjective voice quality Ability to support new services


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MAIN FEATURES OF GSM (1)

1. Spectrum Efficiency

2. Capacity

3. Strong Anti-Interference Capability


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4. Open Interfaces

5. Security

6. Interconnection with other networks

7. Roaming

MAIN FEATURES OF GSM (2)


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THIRD GENERATION OF MOBILESYSTEM

INTERNATIONAL MOBILETELEPHONY 2000 (IMT 2000)

The future of mobile network that will allow themulti-media support of high flow of servicesand mobility, with performances higher thanthose currently offered by the GSM/ GPRS

Designation of frequency bands isharmonized universally in the vicinity of 2 GHz

Ideal transmission speed is 2 Mbps


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IMT 2000 MAIN FEATURES Multiple System Inter-operability With a high degree of consistency in

worldwide design Compatibility between IMT-2000 services

and fixed networks

High quality Small portable terminals used worldwide.


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IMT 2000

2 Mb/s

384 Kb/s

EDGEGPRS

160 Kb/s64 Kb/s

HSCSD

1997 1998 1999 2000 2001 2002 2003

Bit rate

CS data - SMS, 9.6Kbits/s

9.6 Kb/s

Technology


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GSMSYSTEM ARCHITECTURE


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GSM SYSTEM

NETWORK DIAGRAM

BTS

BSC

MSC

VLR HLR AuC EIR

PSTN, ISDN...

OMCOMC

NMCX.25 links

GSM interfaces

Voicemail Serv er

SM-SC

MS (Mobile Station)

BSS (Base Station System)

NSS (Network SwitchingSubsystem)

OMM (Operations & MaintenanceManagement)

MS
http://www.nokia-asia.com/nokia/0,,48192,00.html


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GSM SYSTEM

LOGICAL NETWORK CONFIGURATION

BSC A

Abis Um

BTS

BSC

TRAU

MSC/VLR/GMSC

MSC/VLR/GMSC

NSMU FSMU

TRAU

Ater A

OMC

Qx

HLR/AUC

EIR SMC

PSTN / PLMN /

PSPDN / ISDN

BTS

BTS

BTS


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GSM NETWORK ENTITIES HLR:Home Location Register VLR:Visitor Location Register

MS:Mobile Station ISDN:Integrated Services Digital Network PSTN:Public Switched Telephone Network PSPDN:Packet Switched Public DataNetwork PLMN Public Land Mobile Network

GSM SYSTEM


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MOBILE STATION (MS) Mobile stations are not fixed to one subscriber MS comprises of the following:

1. Mobile Equipment2. Subscriber Identity Module (SIM)

Personal Identification Number (PIN) is used to

prevent unauthorized use of SIM Card

GSM SYSTEM


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MOBILE STATION (MS)

GSM SYSTEM

Mobi le Equipm ent

SIM


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MOBILE STATION IDENTIFICATIONNUMBERS

International Mobile Equipment Identifier (IMEI) Consists of GSM Type permission code and related

manufacturer product number International Mobile Subscriber Identifier (IMSI)

This number is also stored in Home Location Register(HLR)

GSM SYSTEM


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NETWORK SWITCHING SUBSYSTEM(NSS)

Performs the following main functions:

Switching functions Database functions Mobility Management Safety Management

Manages communication among GSM Mobilesubscribers and other communication networksubscribers

GSM SYSTEM


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GSM SYSTEM


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NSS ARCHITECTURE

MOBILE SERVICES SWITCHING CENTER (MSC)

GSM SYSTEM

The core of GSM Network which provides

switching functions Connects mobile subscribers with fixed network

subscribers or with other mobile subscribers

Provides interfaces to other communicationnetworks and interconnection with other MSCs


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GSM SYSTEM


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NSS ARCHITECTURE

VISITOR LOCATION REGISTER (VLR)

GSM SYSTEM

Dynamic subscriber database

Stores all related information of mobilesubscribers entering the MSC Service Area

Obtains and stores necessary data from the HLRof a mobile subscriber

GSM SYSTEM


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NSS ARCHITECTURE

HOME LOCATION REGISTER (HLR)

GSM SYSTEM

Central Database of GSM System Stores all related data of a mobile subscribers controlled

by the same HLR Important Static Data:

IMSI, Access capability, subscriber type, and Supplementaryservices

HLR also stores and provides MSC(A) with (dynamic)information of the MSC(B) area into which a mobile stationhas roamed, so that any incoming call is immediately sentto the called subscriber on a selected path.


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GSM SYSTEM


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NSS ARCHITECTURE

EQUIPMENT IDENTIFCATION REGISTER (EIR)

GSM SYSTEM

Stores International Mobile Equipment Identifier(IMEI) of mobile equipment

3 Types of List:1. WHITE Authorized Mobile Equipment2. GRAY MEs that should be monitored in case of

faults3. BLACK Unauthorized Mobile equipment

Mobile operators used EIR information thelocation of reported stolen MS and block it

GSM SYSTEM


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WIRELESS COVERAGE STRUCTURE

GSM SYSTEM

GSM Service Area

PLMN Service Area

MSC Service Area

Location Area

Cell

CELL

LOCATION AREA

MSC SERVICE AREA

PLMN SERVICE AREA

GSM SERVICE AREA


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BSSSYSTEM ARCHITECTURE


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ZXG-10 BSS ARCHITECTURE

GSM SYSTEM

BTSBIE

BIE

BTS SM SM TC

BSC

MSC

OMC-RMS

UmInterface

Ab is Interface Q3 Inter face

BS Interface Ater Interface A Interface

TC: TransCoderSM: SubMultiplexing

BIE: Base station InterfaceEquipment


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FUNCTIONAL LAYERS OF GSM

OAM

RR

MM

CM

TRANSMISSION

Service Carrier

Subscriber


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Transmission: data transmission function ,providing methods of carrying subscriber data andtransmitting signalings between different entities invarious segments along the communication path.

RR : radio resources management, setting up andreleasing stable connections between mobile

stations and MSC at the call setup stage, which ismainly performed by MS and BSC;


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MM: refers to mobility and safety management ,mobile station processing environment changing,making choices of cells possibly belonging todifferent networks, so that the calling subscriber isable to set up a valid process; infrastructures arerequired to manage subscriber location data(location updating)


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CM : refers to communication management , i.e.,under subscriber requests, setting up connectionsbetween subscribers, maintaining and releasingcalls (which can be divided into CC call control,SSM supplementary service management, andSMS short messages service)

OAM: Operation, administration and maintenanceplatform , providing operation methods for operators . The service is supplied by the transmission layerdirectly.


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PROTOCOL STACK STRUCTURE

Um

MTP2

MTP3

SCCP

BSSMAP

MM

CM

LAPDMTP2

MTP3

SCCP

BSSAP

BTSM

RR

LAPDm LAPD

BTSMRR

LAPDm

RR

MM

CM

MS BTS BSC MSC

Radio Carrier E1/ T1E1/ T1

Abis A


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2 KINDS OF BSSAP SIGNALING

MS BTS/ BSC MSC

DTAP

BSSMAP

DTAP:transfer transparently through BSS (MM +CM)

BSSMAP:BSS management message


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A interface SIGNALING PROTOCOL MODEL (1)

The A interface is used in the message betweenBSC and MSC as well as the message cominginto/out of MS

LAYER 1 Physical and electrical parameterand channel architecture

Integration of MTP1 in Common ChannelSignaling No.7 (CSS7), adopting 2 Mbps

PCM digital link as the transmission


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A interface SIGNALING PROTOCOL MODEL (3)LAYER 3 Application layer which mainlyincludes BSS application part (BSSAP)

This layer maintains and manages the resourcesand the connections in BSS as well as controlsboth the connection and the disconnection ofservice.


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Abis interface SIGNALING PROTOCOL MODEL (2)

LAYER 2 - The data link layer employs theLAPD protocol , which is a point to multi-pointcommunication protocol. LAPD utilizes the frame structure including theflag field, the control field, the information field,the check field and the flag sequence. Theservice access point identification (SAPI) and theterminal equipment identification (TEI) are bothfound in the flag field, used to discriminaterespectively the service and the entity to access.


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Abis interface SIGNALING PROTOCOL MODEL (3)FRAME STRUCTURE OF LAPD

FLAG ADDR FCS FLAGCONTL MESSAGE

SAPI TEI N(S) N(R)


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Abis interface SIGNALING PROTOCOL MODEL (4)FRAME STRUCTURE OF LAPD

FLAG: Flag

ADDR: Address FCS: Frame check sequence

SAPI: Service access point identifier

TEI: Terminal equipment identifier N(S): Sending frame No. N(R): Receiving frame No.


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Abis interface SIGNALING PROTOCOL MODEL (4)

LAYER 3 - The upper layer part, transmits theapplication part of BTS, including the radio link

management (RLM) function and the operationand maintenance function.

Through the Abis interface, BSC provides such

signaling control information as BTSconfiguration, BTS monitoring, BTS testing, andservice control.



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Um interface SIGNALING PROTOCOL MODEL (1)

LAYER 1 Transmission layer (physical layer),the lowest layer of Um interface Provides transmission channel for radio linkand provides differently functional logic channels(control channel and traffic channel ) for higherlayer.


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LAYER 2 Data link layer, provides reliablededicated data link for and between MS and BTS.

Its based on link access protocol of D channel(LAPD), but add some protocols of mobileapplications that are unique to GSM ( LAPDm,the difference is that no FCS and sync flag inLAPDm )


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FRAME STRUCTURE OF LAPDm

ADDR CONTL MESSAGE

SAPI N(S) N(R)

1 1 21

SAPI 0 : Signaling Application

SAPI 3: Short Message Application


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LAYER 3 Application layer, performscontrolling and management functions. Itincludes three sub-layers - CM, MM and RR ,each realizes call control, supplementary servicemanagement and short message managementrespectively.


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f


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Air Interface CHANNELS

Logical

Channels

Control

Channels

Common ControlChannels

Traffic

Channels

BroadcastChannels

Dedicated ControlChannels

FCCH

SCH

BCCH

RACH

AGCH

PCH

SDCCH

SACCH

FACCH

Ai f C A S


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Air Interface CHANNELSTRAFFIC CHANNELS (TCH)

Traffic channel carries encoded speech or subscriber data,including full-rate traffic channel and half-rate traffic channel:

Full-rate traffic channel (TCH/F): total rate is 22.8

kbit/s

Half-rate traffic channel (TCH/H): total rate is 11.4kbit/s

1) Speech channelTCH/FS: full-rate speech traffic channelTCH/HS: half-rate speech traffic channel

2) Data channelTCH/F9.6: 9.6kbit/s full-rate data traffic channel

Ai I f CHANNELS


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CONTROL CHANNELS

Control channels carry signaling informationused by the MS to locate a BTS, synchronizeitself with the BTS, and receive information

required to perform call setup.

There are three categories of control channels:

1. Broadcast Channel (BCH)2. Dedicated Control Channel (DCCH)3. Common Control Channel (CCCH)


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CONTROL CHANNEL COMMON CONTROL CHANNEL(CCCH)

CCCH are shared by all mobile stations in the network. Thereare 3 types of such channels:

a. PCH: paging channels, used by a base station to pagemobile stations (downward).

b. RACH: random access channel, used by mobile stationsfor random access network application, i.e., requesting theallocation of SDCCH channels (upward).

c. AGCH: access granted channel, used by a base stationto respond to random access requests of mobile stations,i.e., to assign one SDCCH or directly assign one TCH(downward).


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Ai I t f CHANNELS


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CONTROL CHANNEL DEDICATED CONTROLCHANNEL (DCCH)

c. FACCH : fast associated control channel, combining withone traffic channel to carry the same signals as SDCCH,but it is assigned only when SDCCH has not beenassigned. Call connection is realized via frames borrowedfrom traffic channels to transmit such commands ashand -over

Ai I t f CHANNELS


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TDMA FRAME

f i

1 TDMA frame

1 2 3 4 5 760

TDMA1 TDMA25 or 50

1 Multiframe

Channel 0

Channel 1

Channel 7

Ai I t f CHANNELS


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CHANNEL COMBINATION

Item Configuration Combination1 TCHFull TCH/F + FACCH/F + SACCH/TF2 TCHHalf TCH/H)+FACCH/H+ SACCH/TH

3 TCHHalf2 TCH/H + FACCH/H+ SACCH/TH+ TCH4 MainBCCH FCCH + SCH + BCCH + CCCH

5 BCCHCombined FCCH+SCH+BCCH+CCCH+SDCCH/4+

SACCH/C46 BCH BCCH + CCCH7 SDCCH SDCCH/8+ SACCH/C8

8 BCCHwithCBCH FCCH+SCH+BCCH+CCCH+ SDCCH/4+

SACCH/C4 + CBCH9 SDCCHwithCBCH SDCCH/8+SACCH/C8+CBCH

Ai I t f CHANNELS


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Structure of 26-Frame Traffic Channel

Ai I t f CHANNELS


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Air Interface CHANNELSStructure of 51-Frame Control Channel

SF B C

R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R

51

SF C C SF C C SF C C I

R R R R R R R R R R

D0 D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A3

SF C C

R R R R R R R R R R

III

D0 D1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A7 III

A1 A2 A3 III

A5 A6 A7 III

D0 D1 D2 D3 D4 D5 D6 D7 A0

D0 D1 D2 D3 D4 D5 D6 D7 A4

SF B C SF C C SF D0 D1 SF D2 D3 ISF A0 A1

SF B C SF C C SF D0 D1 SF D2 D3 ISF A2 A3

D3

D3

R R

R R

A2 A3

A0 A1

D2

D2

SF

SF

D0 D1

D0 D1

R R R R R R R R R R R R R R R R R R R R R R R

R R R R R R R R R R R R R R R R R R R R R R R

FFCCH SSCHBBCCH CCCCHCCCH=PCH+AGCH+RACHRRACH DSDCCH

ASACCH/C Iidle

BCCH+CCCH

BCCH+CCCH

8 SDCCH/8

8 SDCCH/8

BCCH+CCCH+4SDCCH/4

BCCH+CCCH+4SDCCH/4

(a) FCCH+SCH+BCCH+CCCH

(b) SDCCH/8(0,...,7)+SACCH/C8(0,...,7)

(c) FCCH+SCH+CCCH+SDCCH/4(0,...,3)+SACCH/C4(0,...,3)


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BURST FORMATTING


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BURST FORMATTING

TYPES OF BURST (1)

BURST the information contained in one TS of TDMAframe over the air interface

5 DIFFERENT TYPES OF BURST

1. Normal Burst2. Frequency Correction Burst3. Synchronization Burst4. Access Burst5. Dummy Burst


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TrainingSequence

64

TrainingSequence 26

BURST FORMATTING


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BURST FORMATTING

TYPES OF BURST (2)

1. NB (Normal Burst): used for traffic channeland control channels except for RACH, SCH,FCCH.

2. AB (Access Burst): Transmitted on RACHchannel and used as access request made by

MS to BTS. AB is the sole short BP sequencedefined by GSM protocol.

BURST FORMATTING


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BURST FORMATTING

TYPES OF BURST (3)

3. SB (Synchronization Burst): Transmitted onSCH channel and used for initialsynchronization seizing by MS.

4. FB (Frequency Correction Burst) : Used forcarrier frequency correction of MS.

5. DB (Dummy Burst): Has the same format withNB, mainly used for bit filling

TRAINING SEQUENCE OF GSM


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TRAINING SEQUENCE OF GSM

BinaryBN61 - BN86

0 970897 00100101110000100010010111

1 B778B7 00101101110111100010110111

2 10EE90E 01000011101110100100001110

3 11ED11E 01000111101101000100011110

4 6B906B 00011010111001000001101011

5 13AC13A 01001110101100000100111010

6 29F629F 1010011110110001010011111

7 3BC4BBC 11101111000100101110111100

TSC Hexadecimal

TRAFFIC


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TRAFFICTraffic refers to the numbers of subscribers the network can

support and is usually described as follows:

A= n T/3600where,n- calls made by a subscriber within an hour

T- average duration of each call(in seconds) A - Traffic, in Erlang

Problem: If one call is made by a subscriber within anhour and last 120 seconds, the traffic is calculated as:

=1 120/3600=33mErl

For convenience of engineering calculation, the traffic isusually defined as 25mErl per subscriber.

GRADE OF SERVICE (1)


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Grade of Service GOS GOS refers to thedegree of network congestion or call loss rate.GOS=2% means that 98% of subscribers canmake calls successfully and 2% ofsubscribers will end up with unsuccessfulness.

For network operators, 2-5% of GoS is adopted.



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0.020 0.050 0.001 0.0022 0.22 0.38 0.05 0.074 1.09 1.52 0.44 0.545 1.66 2.22 0.76 0.9010 5.08 6.22 3.09 3.4320 13.18 15.25 9.41 10.0724 16.63 19.03 12.24 13.0140 31.00 34.60 24.44 25.6070 59.13 64.67 49.24 50.98100 87.97 95.24 75.24 77.47

Erlang B GoS CapacityNumber of Channels



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#Trunks Erlangs #Trunks Erlangs #Trunks #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs #Trunks Erlangs

1 0.0204 26 18.4 51 41.2 76 64.9 100 88 150 136.8 200 186.2 250 235.82 0.223 27 19.3 52 42.1 77 65.8 102 89.9 152 138.8 202 188.1 300 285.73 0.602 28 20.2 53 43.1 78 66.8 104 91.9 154 140.7 204 190.1 350 335.74 1.09 29 21 54 44 79 67.7 106 93.8 156 142.7 206 192.1 400 385.95 1.66 30 21.9 55 44.9 80 68.7 108 95.7 158 144.7 208 194.1 45

0436.1

6 2.28 31 22.8 56 45.9 81 69.6 110 97.7 160 146.6 210 196.1 500 486.4

7 2.94 32 23.7 57 46.8 82 70.6 112 99.6 162 148.6 212 198.1 600 587.28 3.63 33 24.6 58 47.8 83 71.6 114 101.6 164 150.6 214 200 700 688.29 4.34 34 25.5 59 48.7 84 72.5 116 103.5 166 152.6 216 202 800 789.310 5.08 35 26.4 60 49.6 85 73.5 118 105.5 168 154.5 218 204 900 890.611 5.84 36 27.3 61 50.6 86 74.5 120 107.4 170 156.5 220 206 1000 999.112 6.61 37 28.3 62 51.5 87 75.4 122 109.4 172 158.5 222 208 1100 109313 7.4 38 29.2 63 52.5 88 76.4 124 111.3 174 160.4 224 21014 8.2 39 30.1 64 53.4 89 77.3 126 113.3 176 162.4 226 21215 9.01 40 31 65 54.4 90 78.3 128 115.2 178 164.4 228 213.916 9.83 41 31.9 66 55.3 91 79.3 130 117.2 180 166.4 230 215.917 10.7 42 32.8 67 56.3 92 80.2 132 119.1 182 168.3 232 217.918 11.5 43 33.8 68 57.2 93 81.2 134 121.1 184 170.3 234 219.919 12.3 44 34.7 69 58.2 94 82.2 136 123.1 186 172.4 236 221.920 13.2 45 35.6 70 59.1 95 83.1 138 125 188 174.3 238 223.921 14 46 36.5 71 60.1 96 84.1 140 127 190 176.3 240 225.922 14.9 47 37.5 72 61 97 85.1 142 128.9 192 178.2 242 227.923 15.8 48 38.4 73 62 98 86 144 130.9 194 180.2 244 229.924 16.6 49 39.3 74 62.9 99 87 146 132.9 196 182.2 246 231.825 17.5 50 40.3 75 63.9 100 88 148 134.8 198 184.2 248 233.8

Erlan gs

Erlang-B Traffic Tables

Abbreviated For 2% Grade of Service Only


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GSM CALL TRANSMISSION PROCESS


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1. Analog to Digital Conversion2. Segmentation

3. Speech Encoding4. Channel Encoding5. Channel Interleaving

6. Ciphering7. Burst formatting8. Modulation and Transmission

8 STAGES OF CALL TRANSMISSION

CHANNEL ENCODING


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CHANNEL ENCODING

Chanel encoding in GSM uses the 260 bits fromspeech coding as an input and outputs 456encoded bits.- Special redundancy technologies adopted to

increase the bulk of transmitted information whichcan be inserted at a certain pattern (encoding) atthe sending end and extracted at an agreedpattern (decoding) at the receiving end in order toenhance the anti-interference capacity andtransmission

- Commonly used channel coding methods are: 1)convolutional coding; 2) Fire coding; 3) paritycheck coding.

CHANNEL ENCODING


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CONVOLUTIONL CODING

50 bit (Ia) 132 bit(Ib)

78 bit (Ic)

260 bits /20ms

50 132 783 4

FireCode

Protectionbits

136 bit53 bit

189bit

CONVOLUTIONALCODER (1:2)

378bit

78 bit

456bit

CHANNEL INTERLEAVING


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Interleaving technique is adopted, by which thecontinuous bits in an information block aresegmented and transmitted individually according tocertain rules.

The original continuous block in the transmissionprocess becomes discontinuous, forming a group ofinterwoven message transmitting blocks, which are to

be recovered (de-interleaving) into the originalinformation blocks at the receiving end.



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456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

456 bit

0 1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

1 2 3 4 5 6 7 8

57 1 57 1 57 1 57 1 57 1 57 1 57 1 57 1

Block A Block B Block A+1 Block B+1

116-bit block 116-bit block 116-bit block 116-bit block

Odd N+4 Odd N+5Odd N+6

Odd N+7Even N Even N+1 Even N+2 Even N+3


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DISCONTINUOUS TRANSMISSION


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Two aims can be achieved by adopting DTX mode.1) Lower the total interference level in the air2) Save transmitter power.

The DTX mode and the normal mode are optional, sincethe former will slightly lower the transmission quality.

DTX


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01) gsm system survey.ppt

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