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INTRODUCTION TO GSM

ContentsGSM IntroductionTechnical SpecificationGSM ArchitectureChannelsFrame StructureAdvantagesDisadvantagesIntroductionGlobal System for Mobile Communication (GSM) is a second generation (2G) cellular standard.Developed by European Telecommunication Standards Institute(ETSI) in 1982.Supports eight time slotted users for each 200 KHz radio channel Deployed in Europe, Asia, Australia, South America and some parts of US.Technical SpecificationSpectrumGSM 900 Mobile to BTS (uplink): 890-915 MHz BTS to Mobile(downlink):935-960 MHz Bandwidth : 2* 25 MHz

GSM 1800 Mobile to BTS (uplink): 1710-1785 MHz BTS to Mobile(downlink) 1805-1880 MHz Bandwidth : 2* 75 MHz

Technical SpecificationCarrier Separation : 200 KHzDuplex Distance : 45 MHzNo. of RF carriers : 124Access Method : TDMA/FDDModulation Method : GMSKModulation data rate : 270.833 Kbps

Type of Communication

SimplexDuplexOne way onlyTwo wayGSM Architecture

7Mobile Station(MS)Mobile Equipment

Portable, vehicle mounted, hand held device Uniquely identified by an IMEI (International Mobile Equipment Identity)Voice and data transmission Power level : 0.8W 20 W160 character long SMS

Mobile Station (MS) contd.Subscriber Identity Module (SIM)

Smart card contains the International Mobile Subscriber Identity (IMSI), Personal Identity Number(PIN), PIN Unblocking Number(PUK). Allows user to send and receive calls and receive other subscribed servicesEncoded network identification details - Key Ki,Kc and A3,A5 and A8 algorithmsProtected by a password or PIN

Base Station Subsystem(BSS) The BSS performs functions liketo maintain radio connections to an MS, coding/decoding of voice, and rate adaptation to/from the wireless network

Base Station Subsystem is composed of two parts:-Base Transceiver Station (BTS)Base Station Controller (BSC)Base Station Subsystem(BSS) contd.Base Transceiver Station (BTS):It consist of Transreceivers, Directional Coupler, Combiner etc.

The main functions are :-Air Interface SignalingBaseband/Radio Frequency transformationModulation of voice/dataCipheringSpeech ProcessingGeneration of Alarm Report

Base Station Subsystem(BSS) contd.Base Station Controller (BSC)Manages BTS and radio resources for BTSAssigns Frequency and time slots for all MSs in its areaHandles call set up and handover for each MSRadio Power control It communicates with MSC and BTSReallocation of frequenciesIntercell handover

Network Switching Subsystem(NSS) It is the heart of GSM Networkconnects the wireless network with standard public networks, performs handovers between different BSSs andsupports charging, accounting, and roaming of users between different providers in different countriesconsist of three major parts:-Mobile Switching Center(MSC)Home Location Register(HLR)Visitor Location Register(VLR) Network Switching Subsystem(NSS) contd. Mobile Switching Center (MSC)

Main functions arecall handlingLocation updatingHandover managementBilling for all subscribers based in its areaReallocation of frequencies to BTSs in its area to meet heavy demandsSignaling interface to databases like HLR, VLR.Gateway to SMS between SMS centers and subscribersHandle interworking function while working as GMSC

Network Switching Subsystem(NSS) contd. Home Location RegisterStores user relevant informationStatic information includes- Mobile Subscriber ISDN(MSISDN)- Subscriber information(such as call forwarding, roaming restrictions, GPRS)- International Mobile Subscriber Identity(IMSI)Dynamic information includes :-- current location of MS- Mobile Subscriber Roaming Number(MSRN)Also supports charging and accounting.Network Switching Subsystem(NSS) contd. Visitor Location Register(VLR)It is a dynamic database which stores all important information needed for the MS users currently in the Local AreaIf a new MS comes into an Local Area, the VLR is responsible for it and copies all data for this user from HLROperation Support Subsystem(OSS) It contains all necessary functions for network operation and maintenance

It is consist of following components:-1. Operation and Maintenance Center(OMC)2. Authentication Center(AUC)3. Equipment Identity Register(EIR)Operation Support Subsystem(OSS) contd.Operation and Maintenance Center(OMC)- It monitors traffic - Contains status report of network entities- Subscriber and security management- Accounting and billingAuthentication Center (AUC) Protects user identity and data transmissionAlgorithms for authentication, keys for encryption and values for authorizationEquipment Identity Register (EIR)- It is a database of all IMEIs of MSMade up of three sub-classes: The White List- contains list of valid IMEIsThe Black List- contains list of stolen or locked devicesThe Gray List- list of malfunctioning devicesHandover/HandoffThere are three types of handovers that can occur 1. between calls by the same BSC2. between cells by different BSCs, but same MSC/VLR3. between cells by different MSC/VLRs

The process of changing cells during a call is called handover/ handoff.Types of GSM ChannelsThere two types of GSM channels :-Traffic Channels(TCHs)- Half Rate TCH- Full Rate TCHControl Channel(CCHs)- Broadcast channel(BCH)- Common Control Channel(CCCH)- Dedicated Control Channel(DCCH)

Traffic Channel(TCH)It carries digitally encoded user speech data.

Two types of TCH are:-1. Half Rate TCH(TCH/h) carries user speech at data rate of 11.4 Kbps.2. Full Rate TCH(TCH/f)carries user speech at data rate of 22.8 Kbps.Control Channels(CCH)It controls medium access, allocation of traffic channels and mobility management. It is of three types:-

Broadcast Channel(BCH)BTS uses this channel to signal information to all the MSs within the cell. There are three types :-Broadcast Control Channel(BCCH)- It is used to broadcast information on forward channel such as cell and network identity etc.Frequency Correction Channel(FCCH)-The BTS sends information for frequency correction via this channelSynchronization Channel(SCH)-The BTS sends information about time synchronization via this channelControl Channels(CCH) contd.2. Common Control Channel(CCCH)All information regarding connection setup between MS and BS is exchanged via the CCCH. These are of three types :-Paging Access Channel(PCH)-the BTS uses this channel for paging the appropriate MS.Random Access Channel(RACH)- If an MS wants to set up a call, it uses this channel to send data to the BTS.Access Grant Channel(AGCH)-The BTS uses this channel to signal an MS that it can use a TCH or SDCCH for further connection setup.

Control Channels(CCH) contd.3. Dedicated Control Channel(DCC) These Channels are bidirectional in nature. These are of three types:-Stand Alone Dedicated Control Channels (SDCCH)-, MS uses this channel with a low data rate for signaling comprising of authentication, registration or other data needed for setting up a TCH.Slow Associated Control Channels(SACCH)-carries general information between the MS and BTS at slow rate.Fast Associated Control Channels(FACCH)-carries urgent messages like handover between MS and BTS where large amount of data have to be sent in less time.Frequency Division Duplexing(FDD)

Frequency Division Duplexing(FDD)It is a technique by which simultaneous access to the medium by base station and mobile station in cellular networks can be made.It forms a duplex channel that allows simultaneous transmission in both directions.The two directions, mobile station to base station and vice versa are now separated using different frequencies. This scheme is hence called frequency division duplex (FDD)FDD VS TDD

GSM TDMA Frame Structure

GSM TDMA Frame StructureGSM 800 has 124 channels each of 200 KHz.Channel 1 and 124 are not used for transmission.Another 32 channels are reserved for organizational data and remaining 90 channels are used for customers.Each 248 channels are divided into 200 KHz frames having a duration of 4.615 ms. Each frame is again divided into 8 GSM times slots where each slot represents a physical TDM channel and lasts for 577 s. GSM Time-Slot(Normal Burst)The burst is 546.5 s long and contains 148 bits.The remaining 30.5 s are used by guard space to avoid overlapping.The first and last three bits of a normal burst (tail) are all set to 0 The training sequence in the middle of a slot is used to adapt the parameters of the receiverA flag S indicates whether the data field contains user or network control data.GSM Advantages Over 1GBetter QualityHigher Speed and BandwidthsMore SecuredLess Power ConsumptionMiniaturizationLess prone to interferenceIntroduction of digital data services, such as SMS and email.

Disadvantages of GSM Calls made through GSM mobiles can be tampered. So there are many concerns regarding security. Due to pulse nature of TDMA it causes electronic interference with some devices like hearing aid, pacemaker etc. As multiple users share same bandwidth, it causes interference with the transmission.GSM has a fixed maximum cell site range of 35 km, which is imposed by technical limitations

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