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  • 1. BTS functionModulation/De-modulationChannel Coding/DecodingInterleavingEncryption/CipheringFrequency HoppingTRAU Frame FormattingBCCH managementSignal Strength Measurement for active connectionsIdle Channel Measurements on free channelsTo identify a cell uniquely across PLMNs, an identity called the Cell GlobalIdentity (CGI) is defined. CGI is obtained by the concatenation of LAI and theCI.Frequency Correction Channel: FCCHOne way channel operating in forward direction and using frequency correctionburst formatBears information for Frequency Synchronization142 all 0 bits in this burst causes GMSK modulator to deliver an unmodulatedcarrier for the entire duration of the timeslotUpon detecting this sine wave the MS can adjust its frequency referenceappropriatelySynchronization Channel (SCH)Uses the synchronization burst formatRaw Data information for SCH is of 89 bits64 bits are same for each cell and helps them to achieve timing synchronization.6 bits are for the identification of BTS and mapped on Base Station IdentityCode (BSIC) = NCC (3 bit) + BCC (3 bit)BSIC avoids ambiguity or interference which can arise when a MS can receive SCHfrom two cells using the same BCCH frequency.Network Colour Code (NCC)Used to identity the BTS for which measurement is made.Base-Station Colour Code (BCC)Each 8 BCC value maps to a different Training Sequence.Different training sequences allow for a better transmission in case ofinterference19 bits represent the TDMA frame number (reduced frame number)Broadcast control channel (BCCH)One way channel operating in the forward direction and using the normal burstformatBCCH Occur in timeslot 0 of some specific carriers known as BCCH carriersAfter locking on to the frequency and frame structure in the cell, MS needs somemore general information broadcast on the BCCH for call setup purposesCell Identity (CI)Network Identity (LAI)Control Channel structureBCCH Frequencies of neighboring cellsGPRS Supported or not.Paging Channel: PCHOne way channel operating in the forward direction and using the normal burstformatMobile subscribers are paged this channel for incoming calls or short messagesEvery MS in a cell periodically listen to this channelUses same coding scheme as used for BCCHRandom Access Channel: RACHOne way channel operating in the reverse direction and using the access burstformatWhen MS wants to initiate dialogue with network, this channel is used to sendrequest to network for a dedicated resourceThe actual communication between the MS and the network will takes place lateron the dedicated channel.If the request is not granted within a specific time period, the MS repeats the
  • 2. request on RACHAccess Grant Channel: AGCHAGCH is a one way channel operating in forward direction and using the normalburst formatIn response to requests from different MS on RACH, the network allocates aspecific dedicated signaling channel (SDCCH) against each request for furthercommunication.The response to the request is sent on AGCH.Uses same coding scheme as used for BCCHStand-alone Dedicated Control Channel: SDCCHTwo way channel using normal burst formatAs per the allocation conveyed over the AGCH, both the MS & the BTS switch overto the assigned SDCCH for a further communicationThe Following tasks require the use of SDCCHLocation UpdatesCall SetupSMSUses the same coding scheme as used by BCCHSlow Associated Control Channel: SACCHTwo way channel using normal burst formatSACCH is always associated with TCH or SDCCHWhen associated with a TCH, the SACCH occurs in 12 or 25 frame of each 26-framemulti frameEach message comprises of 456 bits so 4 multi frames are required to transmit amessageSince a 26-frame multiframe requires 120ms, a SAACH message over 4 multi-framerequires 480ms.Thus, power control that is linked to SAACH exchanges is hindered by the lowrates of SAACH exchange. (But then, SAACH was meant to be slow!)Used to convey the periodic carrier-signal strength measurements to the networkWhile an MS is busy on a call over a traffic channel (TCH) or in communicationwith MSC on the SDCCH, MS takes periodic carrier-signal strength measurements onown base station & neighboring base stations.Based on the analysis of measurements taken by BTS & the MS, the BSC conveysinformation on timing advance & MS transmitter power controlUses the same coding scheme as used by BCCHFast Associated Control Channel: FACCHFACCH is a two way channel using normal burst formatFACCH can be associated with SDCCH or TCHFACCH works on the principle of stealingThe burst of speech is replaced by FACCH signalingFACCH is used to conveyHandover informationUses the same coding scheme as used by BCCHFull Rate Traffic ChannelThis channel carries information at rate of 22.8 KbpsHalf Rate Traffic ChannelThis channel carries information at rate of 11.4 KbpsEnhanced Full Rate SpeechGMSK Adaptive Multi Rate (Half Rate and Full Rate)Circuit Switched Data (Transparent): 600/1200, 2400, 4800, 9600, 14400.Circuit Switched Data (Non-transparent): 9600, 14400.Group 3 Fax: 2400, 4800, 9600, 14400.CS 1 to 4MCS 1 to 9High-Speed Circuit Switched Data (HSCSD) for 9.6/14.4 kbpsEnhanced Circuit Switched Data 28.8/32.0/43.2 Kbps per TSGenerally two configurations are mainly usedSeparate SDCCH: FCCH + SCH + BCCH + CCCHAddresses a channel configuration in which no SDCCH are available on TS 0.In this case SDCCH sub channels are defined on TS 1Rest of the TS are used by Traffic channelsCombined SDCCH: FCCH + SCH + BCCH + CCCH + SDCCH/4
  • 3. Addresses a channel configuration in which all control channels are assigned toTS 0In this case TS1 is also available for Traffic channelsThe downlink direction of TS 0 of the BCCH-TRX is used by various channels.FCCHSCHBCCHFour SDCCH sub channels (optional);CCCHThis use is possible because the logical channels can time-share TS 0 indifferent TDMA frames of 51 frame Multi frameMultiplexing of FCCH + SCH + BCCH + CCCH on TS 0 of radio frequency C0 (51 FrameMultiframe)Cycle of 51 TDMA frame (0-50), The structure is repeated after IDLE frameIt contains one block of 4 frames for BCCH and 9 Blocks of 4 frames for CCCH(AGCH/PCH)Referred to as OMUSIG channelThere is one OMUSIG channel per BTS (typically 16 to 64Kbps).BSC controls operations of the whole BTS through this channel.There is O&M SW running on BTS that handles all command on O&M channel.OMUSIG uses LAPD as link layer protocol.Typical operations include:InitializationConfigurationSW DownloadAlarm handling and Fault reportingBlock and ResetLoop Test and other test operationsReferred to as TRXSIG channelThere is one TRXSIG channel per TRX (typically 16 Kbps).All mobile signalling (including RR signalling) is carried over TRXSIG channel.There is RR SW running on each TRX that handles all command on TRX signalingchannel.TRXSIG uses LAPD as link layer protocol.Typical operations is explained in subsequent slides.Apart from the OMUSIG and TRXSIG channels, there are traffic channels for eachTimeslot of a TRX.Thus, if there are N TRX, then there are 8N traffic channels, each of 16Kbpslink speed.A host of traffic types can be carried, each with different channel coding. Thefollowing are some of the basic traffic channel types:Full RateHalf Rate SpeechThe traffic channels are carried over either TRAU frame format or GPRS Layer 1frame format.These frame formats define how BTS and BSC exchange stream of information atlayer 1.For example, while 13Kbps is required for carrying user traffic on Abis, theTRAU frame is allocated 3Kbps. Thus, there is bandwidth for signaling traffic.Note: There is no LAPD for traffic channels. LAPD provides reliable delivery andis not feasible for user traffic.PAGE119At L3, Abis Interface defines a set of sub-procedures (48.058)At L2, the LAPD protocol is followed based on Q.931(48.056)At L1, the E1 PCM structure is followedThe TRXSIG Layer 3 Signaling has four basic set of procedures:Common Channel ManagementDedicated Channel Management
  • 4. Radio Link ManagementTRX ManagementRefer 3GPP 48.058 for detailsCommon Channel Management includes procedures for enabling mobile to obtaindedicated channels (I.e. SDCCH and TCH) for communication. Till such state isreached, the communication takes place through RACH and AGCH.The procedures belonging to this category include:System Information Broadcast (over BCCH)Paging (over PCH)Initial Channel Request (via RACH)Immediate Assignment (over AGCH) Note: The logical channels shown in brackets do not apply for Abis. Itonly indicates for example that Paging message received over Abis is sent overPCH channel or random access received over RACH is relayed over Abis.Dedicated Channel Management includes procedures for managing the state ofdedicated channels (I.e. SDCCH and TCH).The procedures belonging to this category include:Channel Activation/releaseHandover supportMode modifyPower ControlMeasurement ReportingCiphering ControlRadio Link Management procedures relay the actions on air interface to BSC overAbis or receive instructions from BSC to trigger actions over air interface.The procedures belonging to this category include:Request/Indication for Link EstablishmentRequest/Indication for Link ReleaseTransparent data transfer from MS to BSCTransparent data transfer from BSC to MSTRX Management procedures include few other procedures related to measurementhandling.The procedures belonging to this category include:Interference on idle channels Note: Measurement of interference on idle channels is different frommeasurements for dedicated channels. The latter form part of dedicated channelhandling procedures.Some form of flow controlError controlLAPD frame format is specified by ITU Q.920 and Q.921.3GPP TS 48.056 adapts this to provide signaling on Abis interface.LAPD provides reliable data link layer for information transfer.Option is available to send relatively less important messages in un-acknowledged mode.The mobile sees various protocol layers:Layer 1: GSM Physical layer with BTS (already covered)Layer 2: LAPDm layer with BTSLayer 3a: RR layer with BSCLayer 3b: MM/CC layer with MSCBased on LAPD protocol, but adapted for mobile environment.That is why it is referred to as modified LAPD or simply LAPDm.The modifications from LAPD are as follows:There is no Frame CheckThis functionality is provided by channel coding/interleavingThere is no start and end flagFrame delineation is done by use of burstThe first set of messages (I.e. SABME) can carry layer 3 data to save airinterface resources.LAPDm carries ‘maximum‘ message size of 18 to 23bytes.LAPDm is defined in 3GPP TS 04.05 and 04.06There are different types of LAPDm format:A-format: A frame in the A-format generally can be sent on any DCCH in both
  • 5. directions, uplink and downlink. The A-format frame is sent as a fill frame whenno payload is available on an active connection.B-format: The B-format is used on the Air-interface to transport the actualsignaling data; hence, every DCCH and every ACCH use this format. If theinformation to be transmitted requires less space, this space has to be filledwith fill-in octets.Bbis format: Most simple one is the Bbis in which there is no header/trailer(just the information field). This is used for BCCH, PCH, and AGCH. For this,addressing is not necessary, since these are CCCHs, in which addressing is notrequired. In contrast to the DCCH, the CCCH transports only point-to-multipointmessages. Note: For CCCH, the LAPDm is effectively short-circuited (I.e. no LAPDm). Note: For traffic channels, there is no LAPDm.Important LAPDm parameters:SAPI:0 for RR, MM and CC3 for SMS and SSUm Protocols: L3 protocolAt L3, the protocols residing on Um interface are:Radio Resource ProtocolMobility Management ProtocolCall ControlSupplementary ServiceShort Message ServiceThe message type is identified by the Protocol Discriminator.L3 Um Protocols: Radio Resource ManagementThe RR protocol is specified in 04.18The messages are processed in the BSS or even in the MSC.The RR protocol is required for managing physical and logical channels of GSMAir interfaceImportant procedures are:Channel Request ProceduresSystem Information Broadcast ProceduresPaging ProceduresCipher mode HandlingHandover Related ProceduresMeasurement ReportsSome of these procedures are explained in Procedure section.L3 Um Protocols: Mobility ManagementThe MM protocol is specified in 24.008The MM messages are processed between MS and MSC/VLR.The MM protocol is required for managing terminal mobility, temporary identitymanagement, authentication, etc.Important procedures are:IMSI Attach/Location UpdatedIMSI DetachAuthentication Request/ResponseIdentity RequestTMSI Re-allocationService RequestL3 Um Protocols: Call ControlThe CC protocol is specified in 24.008The CC messages are processed between MS and MSC/VLR.The CC protocol is required for managing calls.Important procedures are:SetupCall ProceedingAlertingConnectRelease
  • 6. Release CompleteL3 Um Protocols: SS and SMSThe SS and SMS related L3 protocol is specified in 24.010/011These messages are processed between MS and MSC/VLR.Important procedures are:Register/Facility for SS procedureSimilarly, there are procedures for sending/receiving SMSA Interface Protocol: BSSAPUsed between MSC and BSSThe BSS Application Part (BSSAP) is split into two sub application parts, theseare:BSS Management Application Part (BSSMAP)Supports the procedures between MSC and BSS for call handling and resourcemanagement.For e.g. Paging, Reset, etc.Direct Transfer Application Part (DTAP).This includes the MM and CC messages as discussed earlier.These messages are transparent to the BSSDefined in 3GPP TS 08.08A Interface Protocol: BSSmAPUsed between MSC and BSSDefined in 3GPP TS 08.08Important procedures includeAssignmentHandoverResetCipher ModeOthers