gsm overview

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Global System for Mobile

Communication Overview


Overview

• Introduction

• Evolution of Mobile communication

• Multiple Access Technologies

• Features of GSM

• GSM System Architecture

• Wireless coverage area structure

• Functional layers of GSM


• Mobile communication existed half a century ago, but it was in the 1980’s that it was really developed.

• The main goal of mobile communication is to realize communication among any objects at any time, and in any place.

Introduction


Evolution of Mobile Communication

Land cellular mobile communication system

(PLMN) has gone through 3 stages:

1. First Generation ---Analog Mobile Telephone System

2. Second Generation---Digital Mobile Communication System

3. Third Generation---IMT-2000


Analog Mobile Telephone System

• Adopts cellular networking technology.• Currently practical, high capacity systems worldwide i

ncludes:

（ 1） North-American’s AMPS；（ 2） North European’s NMT-450/900；（ 3） Britain’s TACS；

• Operating frequency bands locate around 450MHz and 900MHz with carrier spacing less than 30 kHz.


• Analog cellular mobile communication system has following fatal weaknesses:

A) There is no public air interface between various systems;

B) It can not adapt itself to the digitization of fixed networks, and digital bearer services are hard to develop;

Analog Mobile Telephone System


C) It has a low frequency availability, thus unable to meet large capacity requirements;

D) It has a low degree of safety, thus easy to be eavesdropped, and easy to be “copied” with “false” handsets.

• Analog cellular mobile communication system has been replaced by digital cellular mobile communication in China.


Digital Mobile Comm. System

• Due to the various defects of analog systems such as TACS, mobile telephone systems were developed in the 1990’s embodied by digital transmission, TDMA and narrow-band CDMA, which are called the Second generation mobile telephone system.


Types of TDMA Systems

• The comparatively mature and most typical system in the TDMA series are: the pan-European GSM, American D-AMPS and the Japanese PDC.

（ 1 ） D–AMPS was formally launched into commercial application in 1993 after the American Electronic Industry Association (EIA) completed its technical standardization in 1989).


（ 2 ） Japanese JDC (now renamed as PDC) had its technical standards completed in 1990. It was put into service in 1993, but is restricted to Japan only.

（ 3 ） The special mobile communication group (SMG) of CEPT laid down the phase 1 standard of GSM in 1988, with the working frequency band around 900MHz. It was launched into commercial use in 1990.


N-CDMA System

• The N–CDMA (Narrow band Code Division Multiple Access) system is a mobile communication system developed mainly by companies leaded by the Quacomm Co. on basis of IS–95. The specifications of North American digital cellular systems are drawn up by the American Telecom Industry Association (TIA).

This system has been adopted by China Unicom in 2001.


Third Generation - IMT-2000

• With the ever-lasting increase in subscribers and the development of digital communications, the second generation mobile telephone systems has gradually exposed its shortcomings:

Narrow bandwidth: unable to provide various broadband information services such as high-speed data and television pictures.


• With the development of sciences and technologies as well as communication services, what is needed is an integrated service system that can integrate all current mobile telephone system functions and provide multiple services.


Features of IMT-2000

• Including multiple systems;• With a high degree of consistency in worldwide

design;• Compatibility between IMT-2000 services and fixed

networks;• High quality;• Small portable terminals used worldwide.


MULTIPLE ACCESS

Transmission

Medium

Multiplexing （ Multiplex Access Technology ）： Transmit two or more voice signals through one pair of cables without mutual interference at the same time.


• Each carrier frequency equals one traffic channel

• Voice is transmitted continuously in time domain

FDMA

US

ER

1 US

ER

2 US

ER

3

Time

FrequencyFDMA


• Increase frequency spectrum efficiency• Voice is transmitted uncontinuously in time domain• System synchronous is very import

USER1

USER2

USER3

Frequency

Time

TDMA


• Large capacity• High voice quality• Soft hand-over• Low sensitive to radio fading

USER2

USER3

USER1

CDMA Frequency

TimeCode

CDMA


FrequencyTime

Power

FrequencyTime

Power

FrequencyTime

Power

FDMA

TDMA

CDMA

FDMA---Different user use different frequency

TACS、 AMPS

TDMA---Different user use different time slot of one frequency

GSM、 DAMPS

CDMA---Different user use same frequency at the same time,but with different spreading code

Large capacity: 8 -10 times than AMPS and 4-6 times than GSM


Main Features of GSM (1)

(1) Spectrum efficiency: due to the adoption of high-efficiency modulator, channel coding, interleaving, equalization and voice coding technologies, the system has a high spectrum efficiency.

(2) Capacity: due to the increase of the transmission bandwidth of each channel, the requirement of co-channel reuse carrier-to-interference ratio is lowered to 9dB,


So,the cofrequency reuse mode of the GSM system

can be narrowed down to 4/12 or 3/9 or even less

(for a analog system, it is 7/21).

GSM system capacity is 3~5 times higher than that

of a TACS system.


(3) Voice quality: due to digital transmission technology, GSM system has a strong anti-interference capability compared with analog system, thus the voice quality is guaranteed.

(4) Interfaces openness: the open interfaces provided by GSM standards refer not only to air interface (Um), but also A interface.


(5) Security: security is guaranteed with authentification, encryption and TMSI.

(6) Interconnection with ISDN, PSTN, etc.: interconnections with other networks normally use the existing standard interfaces, such as ISUP or TUP.

(7) Roaming: roaming is realized on basis of SIM cards.


GSM System Architecture


Block Diagram of GSM System

BSC

BTS

BTS

HLR/AUC EIRSC

ISDN

PLMN

PSTN

PSPDN

OMC

BSS(1)

……

BSS（n）

MS

A interface

Abis interfaceUm interface

Fig.1-1 GSM Mobile Communication Network Structure

MSC/VLR


• BSS: Base Station Subsystem• BSC: Base Station Controller• BTS: Base Transceiver Station• MSC: Mobile Services Switching Center• OMC: Operation and Maintenance Center• AUC: Authentication Center• EIR: Equipment Identification Register


• 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


MSC

VLR

B

MSC

VLR

B

G E

EIR

HLR/AUC

BSCBTS

NSS

A

BSS

F

C

D

Network and Switching Subsystem (NSS)


Network and Switching Subsystem (NSS)

• The Network and Switching Sub-system mainly includes switching functions of the GSM system, and database functions used for subscriber data and mobility management as well as safety management. It manages the communications among GSM mobile subscribers and those between GSM mobile subscribers

and other communication network subscribers.


The network sub-system is divided into six function

units:

• Mobile Service Switching Center (MSC)• Home Location Register (HLR)• Visitor Location Register (VLR)• Authentification Center (AUC)• Equipment Identification Register (EIR)• Operation and Maintenance Center (OMC)

NSS Architecture


Mobile Services Switching Center (MSC)

• As the core of a network, MSC provides switching functions, and connects mobile subscribers with fixed network subscribers, or with mobile subscribers.

Thus, it provides interfaces to fixed networks (such as PSTN, ISDN, etc.) and interfaces for interconnection with other MSCs.


Functions of MSC

• MSC obtains all the data for processing subscriber call requests from 3 types of databases: HLR, VLR and AUC.

• MSC can provides a series of services for subscribers: - Telecom services, such as phone, fax, and emergency calls

- Bearer services

- Supplementary services, such as call transfer, call restriction.


Visitor Location Register（ VLR）

• VLR stores all related information of mobile subscribers having entering into its coverage area, which enables MSC to set up incoming/outgoing calls. It can be taken as a dynamic subscriber database.

VLR obtains and stores necessary data from the HLR of a mobile subscriber. Once a mobile subscriber leaves the coverage area of this VLR, it will be re-registered in another VLR, the temporarily recorded data of this mobile subscriber stored in the original VLR will be deleted.


Home Location Register（ HLR）

• As the central database of the GSM system, HLR stores the related data of all existing mobile subscribers controlled by the same HLR. One HLR can control several mobile switching areas or the whole mobile communication network and the important static data of all subscribers are stored in the HLR, including MSISDN,IMSI, access capability, subscriber type and supplementary services. Furthermore, HLR also stores the location information of all subscribers, so that any incoming call is immediately sent to the called subscriber on a selected path.


Authentication Center（ AUC）

• As a function unit of HLR, AUC is specially used for security management of the GSM system.

AUC stores authentication information and encryption keys for subscriber authentication, encryption of voice, data, signaling messages on radio interfaces, preventing unauthorized subscribers access and guaranteeing the safety of mobile subscriber communication.


Equipment Identification Register（ EIR）

• EIR stores the international mobile equipment identifier (IMEI) of mobile equipment.

• By checking 3 types of lists, i.e., white lists, black lists, and gray lists, it respectively lists the mobile equipment identifiers that are authorized, that should be monitored in case of faults, and that are unauthorized in case of theft.

• Service operators can use such information to locate the location of a stolen mobile station and block it.


BSS System Architecture


ZXG-10 BSS Structure

TC: TransCoderSM: SubMultiplexingBIE: Base station Interface Equipment

MS

Ater interface

Um interface

BSC

BIE

BTS

BTS

BIE

Abis interface

SM SM TC

A interface

MSC

OMC

Q3 interface

Fig.1-3 BSS Structure


Base Station Controller (BSC)

• As the control part of BSS, BSC performs the switching function in BSS. BSC may be connected with multiple BTSs at one end, and MSC and OMC at the other end.

• BSC mainly manages radio network and radio resources, supervises and manages radio base station, controls the establishment, connection and disconnection of radio links in MS and BTS and the location updating, hand-over and paging of mobile station, provides functions such as voice encoding, transcoding, rate adaptation, as well as the operation and maintenance functions of BSS.


Base Transceiver Station (BTS)

• It is the radio tranmission part of the base station system.

• Controlled by BSC, it serves cell radio transceiver equipment, handles conversion between BSC and radio channels, and performs radio transmission between BTS and MS via air interfaces as well as related control functions.


Mobile Station

• Mobile stations are not fixed to one subscriber. On any mobile station in the system, we can identify the subscriber with the SIM card (Subscriber Identity Module). The personal identification number (PIN) can be used to prevent unauthorized use of the SIM card.


• Each mobile station has its own identification number, i.e., the International Mobile Equipment Identifier (IMEI).

IMEI mainly consists of the type permission code and the related manufacturer product number.

• Each mobile subscriber has its own international mobile subscriber identifier (IMSI), which is stored in the SIM card and in HLR.


TransCoder (TC)

• TC mainly completes voice conversion between the 16kbit/s RPE-LTP (Regular Pulse Excited Long-Term Prediction) codes and 64kbit/s A-law PCM codes.

In a typical application pattern, ZXG10-TC is located between MSC and BSC.


GSM service area

PLMNservice area

MSCService area

cell

Location area

Wireless Coverage Area Structure


Functional Layers of GSM

OAM

Service carrier

CM

MM

RR

Subscriber

Transmission


• Transmission: data transmission function, providing methods of carrying subscriber data and transmitting signalings between different entities in various segments along the communication path.

• RR: radio resources management. setting up and releasing stable connections between mobile stations and MSC at the call setup stage, which is mainly performed by MS and BSC.Such as: assignment a channel ,handover,system informations ,measurment report.


• MM: refers to mobility and safety management, mobile station processing -- environment changing, making choices of cells possibly belonging to different networks, so that the calling subscriber is able to set up a valid process; infrastructures are required to manage subscriber location data (location updating), authentification.


• CM: refers to communication management, i.e., under subscriber requests, setting up connections between subscribers, maintaining and releasing calls (which can be divided into CC - call control, SSM - supplementary service management, and SMS - short messages service);

• OAM: Operation, Administration and Maintenance platform, providing operation methods for operators. The service is supplied by the transmission layer directly.


• Mobility：– flexible and convenient ， global personal

communication• Poor environment and conditions ：

– Co-channel interference, multi-path(space and time), power change and other noise

• Multiple MS and channels：– Interference 、 near and far effect

• Limit of frequency resources• Reliability is important

– registration, handoff, switching

Characteristics of Mobile Communication


If Frequency reuse factor is 1;network design and expanding become much easierEx: CDMA

If Frequency reuse factor is 1;network design and expanding become much easierEx: CDMA

Frequency Reuse Factor

AMPS, D-AMPS, N-AMPS

CDMA

30 30 10 kHz

200 kHz

1250 kHz

1 3 1 Users

8 Users

20 Users1

1

11

1

11

11

1

11

1

1

12

34

43

2

56

17

Typical Frequency Reuse N=7



Vulnerability:C/I 17 dB

Vulnerability:C/I 12-14 dB

Vulnerability:Eb/No 6--7 dB

GSM


11GG 22GG 33GG

Analog cellular

DDiiggiittaall cceelllluullaarr DDiiggiittaall cceelllluullaarr

VVooiiccee VVooiiccee //ddaattaa VViiooccee // hhiigghh ssppeeeedd ddaattaa

AAMMPPSS CCDDMMAA 11XXRRtttt CCDDMMAA22000000

TTAACCSS GGSSMM GGPPRRSS WW__CCDDMMAA

8800’’ 11999922 11999999 22000011 22000033

Evolution of Mobile Communication System

AMPS: Advanced Mobile Phone SystemTACS: Total Access Communication SystemGPRS: General Packet Radio Services

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