Existing Wireless Systems: 2G, GSM System and PCS

Yschen, CSIE, CCU 1

Chapter 9:Existing Wireless Systems:

2G, GSM System

Associate Prof. Yuh-Shyan Chen

Dept. of Computer Science and Information Engineering

National Chung-Cheng University

Yschen, CSIE, CCU 2

Introduction

Global System for Mobile communication or Groupe Speciale Mobile (GSM) communication

• Initiated by European Commission• Second-generation mobile cellular system

Aimes at developing a Europe-wide digital cellular system

• Created in 1982 to have a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz

• The main objective of GSM is to remove any incompatibility among the systems by allowing the roaming phenomenon for any cell phone

Yschen, CSIE, CCU 3

Introduction

• It also supports speed transmissions between MSs, emergency calls, and digital data transmission

Specific functions of different constituents are as follows

• Base Station Controller (BSC):• Mobile Switching Center (MSC):• Authentication Center (AC):• Equipment Identity Register (EIR):

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GSM infrastructure

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Frequency Bands and Channels

GSM has been allocated an operational frequency from 890 MHz and 960 MHzMSs employ 890 MHz to 915 MHzBS operates in 935 MHz and 960 MHz

GSM follows FDMA and allows up to 124 MSs to be serviced at the same timeThe frequency band of 25 MHz is divided into 124

frequency division multiplexing (FDM) channels, each of 200 kHz (Fig. 10.9)

A guard frame of 8.25 bits is used between any two frames transmitted either by the BS or the MS

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Frequency band used by GSM

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GSM uses a variety of multiplexing techniques

GSM uses a variety of multiplexing techniques to create a collection of logical channels

Three control channels are used for broadcasting some information to all MSsBroadcast control channel (BCCH)Frequency correction channel (FCCH)Synchronization channel (SCH)

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Channel in GSM

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Cont.

Three common control channels are used for establishing links between the MS and the BSRandom access channel (RACH):

• Used by the MS to transmit information regarding the requested dedicated channel from GSM

Paging channel:• Used by the BS to communicate with individual MSs in

the cell

Access grant channel:• Used by the BS to send information about timing and

synchronization

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Cont.

Three dedicated control channels are usedSlow associated control channel (SACCH):Stand-alone dedicated control channel (SDCCH):Fast associated control channel (FACCH):


Frames in GSM

GSM system uses the TDMA scheme with a 4.615 – ms long frameDividing into eight time slots each of 0.557 msEach frame measured in terms of time is 156.25 bits

long, of which 8.25 period bits are guard bits for protection

The 148 bits are used to transmit the informationThe frame contains 26 training bits allow the

receiver to synchronize itselfMany such frames are combined to constitute

multiframes, superframes, and hyperframes


Frame structure of TDMA


Identify Numbers used by a GSM System

International Mobile Subscriber Identity (IMSI)


Format of IMSI


Cont.

Subscriber Identify Module (SIM)Every time the MS has to communicate with a BS, it

must correctly identify itself. A MS does this by storing the phone number,

personal identification number for the station, authentication parameters, and so on in the SIM card

Smart SIM cards have a flash memory The main advantage of SIM is that is supports

roaming with or without a cell phone, also called SIM roaming

Mobile System ISDN (MSISDN)


Format of MSISDN


Cont.

Location Area Identify (LAI)The GSM service area is usually divided into a

hierarchical structure that facilitates the system to access any MS quickly

Each PLMN is divided into many MSCsEach MSC typically contains a VLR to tell the system

if a particular cell phone is roamingIf it is roaming, the VLR of the MSC in which the cell

phone is reflects the factEach MSC is divided into many location areas (LAs)A location area is a cell or a group of cells and is

useful when the MS is roaming in a different cell but the same LA


Cont.

Since any LA has to be identified as the part of the hierarchical structure

The identifier should contain the country code, mobile network code, and LA code


GSM layout


Cont.

International MS Equipment Identity (IMSEI)


Format of IMSEI


Cont.

MS Roaming Number (MSRN)


Format of MSRN


Layout, Planes, and Interfaces of GSM


Interface of GSM


Cont.

The GSM system can be divided into five planesOAMCMMMRRPhysical


Functional planes in GSM


Authentication

Authentication is done with the help of a fixed network that is used to compare the IMSI (International Mobile Subscriber Identity) of the MS reliably


Authentication process in GSM


Handoff in GSM

Intracell/intra-BTS handoffIntercell/intra-BSC handoffInter-BSC/intra-MSC handoffInter-MSC handoff


Inter-MSC handoff


Personal Communication Service (PCS)

PCS employs an inexpensive, lightweight, and portable handset to communicate with a PCS BS

The PCS is classified into high-tier and lower-tier standards

High-tier system includes high-mobility units with large batteries

• An MS in a carLow-tier system includes system with low mobility,

capable of providing high-quality portable communication service over a wide area

The PCS lower-tier standards based on PACS (Personal Access Communication Systems) and DECT (Digital European Cordless Telecommunication) are given in Table 10.6.


FCC view of PCS


PCS High-Tier Standards


PCS Low-Tier Standards


Chronology of PCS Development

CT2 (Cordless Telephone)Using FDMA with a speed rate of 32 kbps using

Adaptive Differential Pulse Code Mudulation (ADPCM)

The transmitter data rate is 72 kbpsUses TDD, which allows BS and MS share one

channelD is called D-channel which includes 4 bits of control

information

DECT (Digital European Cordless Telecommunication)


CT2 TDD Slot (First Generation)


DECT (Digital European Cordless Telecommunication)

The second-generation cordless telephone system

DECT operates on frequencies ranging from 1880 MHz to 1900 MHz

Uses ADPCM with 32 kbps speed rateUses TDD with two frames with 10-ms periodsSupports both voice and data transmission


DECT TDD Slot (Second Generation)


Bellcore View of PCS

The Bellcore view of PCS is based on five different service provided between the Bellcore client company (BCC), BCC network, and the PCS wireless provider networkPCS access service for networks (PASN)

• A connection service to and from the PCS service provider (PSP)

PCS access service for controllers (PASC)• A service for use with PCS wireless provider (PWP)

across radio channels and some type of automatic link transfer capability


Bellcore View of PCS

PCS access service for ports (PASP)• An interface into PWP

PCS access service for data (PASD)• A database information transport service

PCS access service for external service providers (PASE)

• Is used to support specialized PCS service like voice mail and paging


Bellcore PCS Architecture

Radio Port Radio PortControl Unit

SignalingSystem 7

Operation, Administration,& Maintenance


Description of the PCS Air Interface:Forward TDMA Frame for PCS

Business

Existing Wireless Systems: 2G, GSM System and PCS