Overview of GSM Cellular Network and OperationsGanesh SrinivasanNTLGSPTN

Network and switching subsystemNSS is the main component of the public mobile network GSMswitching, mobility management, interconnection to other networks, system controlComponentsMobile Services Switching Center (MSC)controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSCDatabases (important: scalability, high capacity, low delay)Home Location Register (HLR)central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs)Visitor Location Register (VLR)local database for a subset of user data, including data about all user currently in the domain of the VLR

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Operation subsystemThe OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystemsComponentsAuthentication Center (AUC)generates user specific authentication parameters on request of a VLR authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system Equipment Identity Register (EIR)registers GSM mobile stations and user rightsstolen or malfunctioning mobile stations can be locked and sometimes even localizedOperation and Maintenance Center (OMC)different control capabilities for the radio subsystem and the network subsystem

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Mobile HandsetTEMPORARY DATA PERMANENT DATA

- Temporary Subscriber Identity Permanent Subscriber Identity

- Current Location Key/Algorithm for Authentication.

- Ciphering DataProvides access to the GSM n/wConsists ofMobile equipment (ME)Subscriber Identity Module (SIM)

The GSM Radio Interface

AIR INTERFACE

UPLINK 890 - 915 MHz

DOWNLINK 935 - 960 MHz

MOBILE

BASE TRANSCEIVER STATION

The GSM Network ArchitectureTime division multiple access-TDMA124 radio carriers, inter carrier spacing 200khz.890 to 915mhz mobile to base - UPLINK935 to 960mhz base to mobile - DOWNLINK8 channels/carrier

*270.833 kb/s per carrierGMSK with a time bandwidth product BT =0.3Slow frequency hoping 217/hops/second.Synchronization compensation for up to 233micro seconds absolute delayBlock and convolutional channel coding copuled with interleaving to combat channel perturbations- overall channel rate of 22.8 kb/sFull rate channel 13 kb/s voice coder rate using regular pulse excitation/linear predictive coding RPE/LPC, half rate channel 6.5 kb/s usingVector coder rate using vector sum excited linear predictivie coding VSELPOverall full rate channel bit rate of 22.8 kb/s.Each cell can have from 1 to 16 pairs of carriers.

GSM uses paired radio channels01240124890MHz915MHz935MHz960MHzUPLINKDOWNLINK

Access MechanismFDMA, TDMA, CDMA

Frequency multiplexSeparation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the

spectrum for the whole timeAdvantages:no dynamic coordination necessaryworks also for analog signalsDisadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces

k2k3k4k5k6k1ftc

k2k3k4k5k6k1Time multiplexA channel gets the whole spectrum for a certain amount of timeAdvantages:only one carrier in themedium at any timethroughput high even for many usersDisadvantages:precise synchronization necessary

fTime and Frequency MultiplexCombination of both methodsA channel gets a certain frequency band for a certain amount of time

tck2k3k4k5k6k1

fTime and Frequency MultiplexExample: GSM Advantages:Better protection against tappingProtection against frequency selective interferenceHigher data rates compared tocode multiplexBut: precise coordinationrequired

tck2k3k4k5k6k1

GSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand.

GSM uses paired radio channels01240124890MHz915MHz935MHz960MHzUPLINKDOWNLINK

Code MultiplexEach channel has a unique codeAll channels use the same spectrum at the same timeAdvantages:Bandwidth efficientNo coordination and synchronization necessaryGood protection against interference and tappingDisadvantages:Lower user data ratesMore complex signal regenerationImplemented using spread spectrum technology

k2k3k4k5k6k1ftc

Various Access Method

Cells

Capacity & Spectrum Utilization SolutionThe need:Optimum spectrum usageMore capacityHigh quality of serviceLow cost

Representation of CellsIdeal cellsFictitious cells

*The system capacity depends on :The total number of radio channelsThe size of the cellThe frequency re-use factor or distance

The minimum distance which allows the same frequencies to be re-used will depend on many factors,The number of co-channel cells in the vicinity of the center cellThe geography of the terrain,The antenna heightThe transmitted power within each cell

Cell size and capacityCell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all usersCapacity within cell limited by available bandwidth and operational requirementsEach network operator has to size cells to handle expected traffic demand

Cell structureImplements space division multiplex: base station covers a certain transmission area (cell)Mobile stations communicate only via the base stationAdvantages of cell structures:higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc. locallyProblems:fixed network needed for the base stationshandover (changing from one cell to another) necessaryinterference with other cellsCell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

Capacity of a Cellular SystemFrequency Re-Use DistanceThe K factor or the cluster sizeCellular coverage or Signal to interference ratioSectoring

ij1234567Frequency re-use distance is based on the cluster size KThe cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent clusterK = i2 + ij + j2 K = 22 + 2*1 + 12K = 4 + 2 + 1 K = 7D = 3K * RD = 4.58R 123567DRThe K factor and Frequency Re-Use Distance

K = i2 + ij + j2 K = 22 + 2*0 + 02K = 4 + 0 + 0 K = 4D = 3K * RD = 3.46RThe Frequency Re-Use for K = 4

The Cell Structure for K = 7

Cell Structure for K = 4

Cell Structure for K = 12

Increasing cellular system capacityCell sectoringDirectional antennas subdivide cell into 3 or 6 sectorsMight also increase cell capacity by factor of 3 or 6

Increasing cellular system capacityCell splittingDecrease transmission power in base and mobileResults in more and smaller cellsReuse frequencies in non-contiguous cell groupsExample: cell radius leads 4 fold capacity increase

Tri-Sector antenna for a cell

HighwayTownSuburbRuralCell Distribution in a Network

Optimum use of frequency spectrumOperator bandwidth of 7.2MHz (36 freq of 200 kHz)TDMA 8 traffic channels per carrierK factor = 12What are the number of traffic channels available within its area for these three casesWithout cell splittingWith 72 cellsWith 246 cells

One Cell = 288 traffic channels72 Cell = 1728 traffic channels246 Cell = 5904 traffic channelsRe-use of the frequency8 X 36 = 2888 X (72/12 X 36) = 1728

*Due to assumptions 1MHz carrier 5 radio frequencies(radio channels) 5X200 kHz. Each radio frequency carries8 traffic channels = 40 traffic channels/MHzWithout cell splitting, traffic channels = 7.2MHzX40 = 288 traffic channelsWith 72 cells, 72/12(kfactor = 12) = 6 paterns(all spectrum may be used in a pattern), traffic channels = 6X288 = 1728 traffic channelsWith 246 cells, 246/12(K factor = 12) we will get 20 sectors + 6 cells, for 20 patterns and 6/12 we get 20X288 + 6/12*288 = 5904 traffic channels.

For the same channels spacing and re-use pattern, the number of re-used channels is increased when cell radius are reduced.

Concept of TDMA Frames and ChannelsGSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand.

GSM uses paired radio channels01240124890MHz915MHz935MHz960MHzUPLINKDOWNLINK

GSM delays uplink TDMA framesUplink TDMA Frame F1 + 45MHzDownlink TDMA F1MHzThe start of the uplink TDMA is delayed of three time slotsTDMA frame (4.615 ms)Fixed transmit Delay of three time-slots

*The start of the uplink TDMA frame is delayed with respect to downlink by a fixed period of three timeslots. Why ? Staggering TDMA frames allows the same timeslot number to be used in both the down and uplink while avoiding the requirement for mobile to transmit and receive simultaneously. Between T and R the MS is in the IDLE mode, makes measurement of signal strength of neighboring cells.

935-960 MHz124 channels (200 kHz)downlink890-915 MHz124 channels (200 kHz)uplinkfrequencytimeGSM TDMA frameGSM time-slot (normal burst)guardspaceguardspaceGSM - TDMA/FDMA

*Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding andblock interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality thanothers. The bits are thus divided into three classes:

Class Ia 50 bits - most sensitive to bit errors Class Ib 132 bits - moderately sensitive to bit errors Class II 78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightlyattenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder ofconstraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remainingClass II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocksare transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

Recall that each time-slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK).GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related topower consumption, which should be minimized for the mobile station.

LOGICAL CHANNELSTRAFFICSIGNALLINGFULL RATEBm 22.8 Kb/SHALF RATELm 11.4 Kb/SBROADCASTCOMMON CONTROL DEDICATED CONTROLFCCHSCHBCCHPCHRACHAGCHSDCCHSACCHFACCH

FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLYUPLINK ONLYBOTH UP & DOWNLINKS

Broadcast Channel - BCHBroadcast control channel (BCCH) is a base to mobile channel which provides general information about the network, the cell in which the mobile is currently located and the adjacent cellsFrequency correction channel (FCCH) is a base to mobile channel which provides information for carrier synchronizationSynchronization channel (SCH) is a base to mobile channel which carries information for frame synchronization and identification of the base station transceiver

Common Control Channel - CCHPaging channel (PCH) is a base to mobile channel used to alert a mobile to a call originating from the networkRandom access channel (RACH) is a mobile to base channel used to request for dedicated resourcesAccess grant channel (AGCH) is a base to mobile which is used to assign dedicated resources (SDCCH or TCH)

Dedicated Control Channel - DCCHStand-alone dedicated control channel (SDCCH) is a bi-directional channel allocated to a specific mobile for exchange of location update information and call set up information

Dedicated Control Channel - DCCHSlow associated control channel (SACCH) is a bi-directional channel used for exchanging control information between base and a mobile during the progress of a call set up procedure. The SACCH is associated with a particular traffic channel or stand alone dedicated control channelFast associated control channel (FACCH) is a bi-directional channel which is used for exchange of time critical information between mobile and base station during the progress of a call. The FACCH transmits control information by stealing capacity from the associated TCH

DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms = 0.577ms

*Normal burst 148 bits + 8.25 guard bitsFrequency correction burst 148 bits + 8.25 guard bitsSynchronizing burst 148 bits + 8.25 guard bitsAccess burst 88 bits +68.25 guard bits used to access a cell for the first time in case of a call set up or handoverThe data structure within a normal burst consists of 148 bits transmitted at a rate of 270.833 kb/s. Each burst in GSM system modulates one of the carriers assigned to a particular cell using GMSK.

0 1 2 3 4 5 6 2043 2044 2045 2046 2047

0 1 2 24 25

0 1 2 3 24 251 HYPER FRAME = 2048 SUPERFRAMES = 2 715 648 TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS )1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S ) LEFT (OR) RIGHT1 MULTI FRAME = 51 TDMA FRAMES (235 .4 ms )1 SUPER FRAME = 26 MULTI FRAMES1 SUPER FRAME = 51 MULTI FRAMES1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms )TDMA FRAME NO.0101HIERARCHY OF FRAMES

1 2 3 4 155 156 1 TIME SLOT = 156.25 BITS ( 0.577 ms)(4.615ms)(4.615 ms)1 bit =36.9 micro secTRAFFIC CHANNELSSIGNALLING CHANNELS

*Speech in GSM is digitally coded at a rate of 13 kbps, so-called full-rate speech coding. This is quite efficient compared with the standard ISDN rate of 64 kbps. One of the most important Phase 2 additions will be the introduction of a half-rate speech codec operating at around 7 kbps, effectively doubling the capacity of a network. This 13 kbps digital stream (260 bits every 20 ms) has forward error correction added by a convolutional encoder. The gross bit rate after channel coding is 22.8 kbps (or 456 bits every 20 ms). These 456 bits are divided into 8 57-bit blocks, and the result is interleaved amongst eight successive time slot bursts for protection against bursty transmission errors. Each time slot burst is 156.25 bits and contains two 57-bit blocks, and a 26-bit training sequence used for equalization. A burst is transmitted in 0.577 ms for a total bit rate of 270.8 kbps, and is modulated using Gaussian Minimum Shift Keying (GMSK) onto the 200 kHz carrier frequency. The 26-bit training sequence is of a known pattern that is compared with the received pattern in the hope of being able to reconstruct the rest of the original signal. Forward error control and equalization contribute to the robustness of GSM radio signals against interference and multipath fading. The digital TDMA nature of the signal allows several processes intended to improve transmission quality, increase the mobile's battery life, and improve spectrum efficiency. These include discontinuous transmission, frequency hopping and discontinuous reception when monitoring the paging channel. Another feature used by GSM is power control, which attempts to minimize the radio transmission power of the mobiles and the BTS, and thus minimize the amount of co-channel interference generated.

GSM FrameFull rate channel is idle in 25SACCH is transmitted in frame 120 to 11 and 13 to 24Are used for traffic dataFrame duration = 120msFrame duration = 60/13msFrame duration = 15/26ms

*The full rate TCH uses 24 out of the 26 available in the multiframe The duration of the multiframe is therefore 26X60/13ms = 120ms

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extractthe desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal.This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.

114 bits are available for data transmission.The training sequence of 26 bits in the middle of the burst is used by the receiver to synchronize and compensate for time dispersion produced by multipath propagation.1 stealing bit for each information block (used for FACCH)

*Distinct training sequences will therefore be allocated to channels using the same frequencies in cells which are close enough to interfere with one another.

LOGICAL CHANNELSTRAFFICSIGNALLINGFULL RATEBm 22.8 Kb/SHALF RATELm 11.4 Kb/SBROADCASTCOMMON CONTROL DEDICATED CONTROLFCCHSCHBCCHPCHRACHAGCHSDCCHSACCHFACCH

FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLYUPLINK ONLYBOTH UP & DOWNLINKS

Mobile looks for BCCH after switching onRACH send channel request AGCH receive SDCCHSDCCH authenticateSDCCH switch to cipher modeSDCCH request for location updatingSDCCH authenticate responseSDCCH cipher mode acknowledgeSDCCH allocate TMSISDCCH acknowledge new TMSISDCCH switch idle update modeLocation update from the mobile

*When a mobile station is first switched on it is necessary to read the BCCH in order to determine its orientation within the network.The mobile must first synchronize in frequency and then in time. The FCCH, SCH and BCCH are all transmitted on the same carrier frequency which has a higher power density than any of the other channels in a cell because steps are taken to ensure that it is transmitted information at all times. The mobile scans around the available frequencies, picks the strongest and then selects the FCCH. Fc+67.7kHz

Mobile looks for BCCH after switching onRACH send channel request AGCH receive SDCCHSDCCH do the authentication and TMSI allocationSDCCH require traffic channel assignmentSDCCH send call establishment requestSDCCH send the setup message and desired numberFACCH switch to traffic channel and send ack (steal bits)FACCH receive alert signal ringing soundFACCH acknowledge connect message and use TCHTCH conversation continuesFACCH receive connect messageCall establishment from a mobile

Mobile looks for BCCH after switching onReceive signaling channel SDCCH on AGCHReceive alert signal and generate ringing on FACCHReceive authentication request on SDCCHGenerate Channel Request on RACHAnswer paging message on SDCCHAuthenticate on SDCCHReceive setup message on SDCCHFACCH acknowledge connect message and switch to TCHReceive connect message on FACCHReceive traffic channel assignment on SDCCHMobile receives paging message on PCHFACCH switch to traffic channel and send ack (steal bits)Call establishment to a mobile

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GSM speech coding

AIR INTERFACE

UPLINK 890 - 915 MHz

DOWNLINK 935 - 960 MHz

MOBILE

BASE TRANSCEIVER STATION

Transmit Path BS Side8 bit A-Law to 13 bit UniformRPE/LTP speech Encoder

To Channel Coder 13Kbps8 K spsMS Side LPFA/DRPE/LTP speech Encoder

To Channel Coder 13Kbps8 K sps,Sampling Rate - 8KEncoding - 13 bit Encoding (104 Kbps)RPE/LTP - Regular Pulse Excitation/Long Term PredictionRPE/LTP converts the 104 Kbps stream to 13 Kbps

GSM Speech CodingGSM is a digital system, so speech which is inherently analog, has to be digitized.The method employed by current telephone systems for multiplexing voice lines over high speed trunks and is pulse coded modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link.

GSM FrameFull rate channel is idle in 25SACCH is transmitted in frame 120 to 11 and 13 to 24Are used for traffic dataFrame duration = 120msFrame duration = 60/13msFrame duration = 15/26ms

*The full rate TCH uses 24 out of the 26 available in the multiframe The duration of the multiframe is therefore 26X60/13ms = 120ms

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extractthe desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal.This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.

GSM Speech CodingSpeech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. Regular pulse excited -- linear predictive coder (RPE--LPC) with a long term predictor loop is the speech coding algorithm.

*The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption onceimplemented) before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder (RPE--LPC) with a Long Term Predictor loop. Basically, information from previous samples, which does notchange very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted andactual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. This is the so-called Full-Rate speech coding.Recently, an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13kbps bit rate.

The 260 bits are divided into three classes:Class Ia 50 bits - most sensitive to bit errors.Class Ib 132 bits - moderately sensitive to bit errors.Class II 78 bits - least sensitive to bit errors.Class Ia bits have a 3 bit cyclic redundancy code added for error detection = 50+3 bits.132 class Ib bits with 4 bit tail sequence = 132 + 4 = 136.Class Ia + class Ib = 53+136=189, input into a 1/2 rate convolution encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolution encoder thus outputs 378 bits, to which are added the 78 remaining class II bits.Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolution encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

GSM Protocol Suite

BTSRadio interfaceHLRMSCVLRBSCRRMM + CMSS

Link LayerLAPDm is used between MS and BTSLAPD is used between BTS-BSCMTP2 is used between BSC-MSC/VLR/HLR

Network LayerTo distinguish between CC, SS, MM and RR protocol discriminator (PD) is used as network address.CC call control management MS-MSC.SS supplementary services management MS-MSC/HLR.MM mobility management(location management, security management) MS-MSC/VLR.RR radio resource management MS-BSC.Messages pertaining to different transaction are distinguished by a transaction identifier (TI).

Application Layer protocolsBSSMAP between BSC and MSCDTAP messages between MS and MSC.All messages on the A interface bear a discrimination flag, indicating whether the message is a BSSMAP or a DTAP.DTAP messages carry DLCI(information on type of link on the radio interface) to distinguish what is related to CC or SMS.MAP protocol is the one between neighbor MSCs. MAP is also used between MSC and HLR.

A-Bis InterfaceUmBase Station System

GSM Functional Architecture and Principal Interfaces

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GSM protocol layers for signalingCMMMRRMMLAPDmradioLAPDmradioLAPDPCMRR BTSM

CMLAPDPCMRRBTSM 16/64 kbit/sUmAbisASS7PCMSS7PCM64 kbit/s /2.048 Mbit/sMSBTSBSCMSCBSSAPBSSAP

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Protocols involved in the radio interfaceLevel 1-PhysicalTDMA frame Logical channels multiplexingLevel 2-LAPDm(modified from LAPD)No flagNo error retransmission mechanism due to real time constraintsLevel 3-Radio Interface Layer (RIL3) involves three sub layersRR: paging, power control, ciphering execution, handoverMM: security, location IMSI attach/detachCM: Call Control(CC), Supplementary Services(SS), Short Message Services(SMS),

LAPDm on radio interfaceIn LAPDm the use of flags is avoided.LAPDm maximum length is 21 octets of information. It makes use of more bit to distinguish last frame of a message.No frame check sequence for LAPDm, it uses the error detecting performance of the transmission coding scheme offered by the physical layer

LAPDm Message structure

LAPDm on radio interfaceThe acknowledgement for the next expected frame in the indicator N(R ).On radio interface two independent flows(one for signaling, and one for SMS) can exist simultaneously.These two flows are distinguished by a link identifier called the SAPI(service access point identifier).LAPDm SAPI=0 for signaling and SAPI=3 for SMS.SAP1=0 for radio signaling, SAPI=62 for OAM and SAPI=63 for layer 2 management on the Abis interface.There is no need of a TEI, because there is no need to distinguish the different mobile stations, which is done by distinguishing the different radio channels.

Protocols involved in the A-bis interfaceLevel 1-PCM transmission (E1 or T1)Speech encoded at 16kbit/s and sub multiplexed in 64kbit/s time slots.Data which rate is adapted and synchronized.Level 2-LAPD protocol, standard HDLCRadio Signaling Link (RSL)Operation and Maintenance Link (OML).Level 3-Application ProtocolRadio Subsystem Management (RSM)Operation and Maintenance procedure (OAM)

Presentation of A-bis InterfaceMessages exchanges between the BTS and BSC.Traffic exchangesSignaling exchangesPhysical access between BTS and BSC is PCM digital links of E1(32) or T1(24) TS at 64kbit/s.Speech:Conveyed in timeslots at 4X16 kbit/sData:Conveyed in timeslots of 4X16 kbit/s. The initial user rate, which may be 300, 1200, is adjusted to 16 kbit/s

LAPD message structure

LAPDThe length is limited to 260 octets of information.LAPD has the address of the destination terminal, to identify the TRX, since this is a point to multipoint interface.Each TRX in a BTS corresponds to one or several signaling links. These links are distinguished by TEI (Terminal Equipment Identities).SAPI=0, SAPI=3, SAPI=62 for OAM.

Presentation of the A-ter interface

BSCTRAUMSCOMCOAMTranscodingLAPD TS1Speech TSCCS7 TSX.25 TS2Speech TSCCS7 TSX.25 TS2

PCMLINKPCMLINK

Presentation on the A-ter interfaceSignaling messages are carried on specific timeslots (TS)LAPD signaling TS between the BSC and the TCUSS7 TS between the BSC and the MSC, dedicated for BSSAP messages transportation.X25 TS2 is reserved for OAM.Speech and data channels (16kbit/s)Ater interface links carry up to:120 communications(E1), 4*3092 communications(T1).The 64 kbit/s speech rate adjustment and the 64 kbit/s data rate adaptation are performed at the TCU.

Presentation of the A interface

Signaling Protocol Model

Presentation on the A-InterfaceBSSMAP - deals with procedures that take place logically between the BSS and MSC, examples:Trunk Maintenance, Ciphering, Handover, Voice/Data Trunk AssignmentDTAP - deals with procedures that take place logically between the MS and MSC. The BSS does not interpret the DTAP information, it simply repackages it and sends it to the MS over the Um Interface. examples:Location Update, MS originated and terminated Calls, Short Message Service, User Supplementary Service registration, activation, deactivation and erasure

Inter MSC presentation

OAM

LAPD

BTS

MTP2SCCPMTP3LAPDOAMRRDTAPBSSMAPBSSAPBSC

MTP1MTP3MTP2SCCPMTP2MTP3SCCP

BSSAPDTAP/BSSMAPTCAPMMCMMAPNSS

RRMMCMMS

UmInterfaceA bisInterfaceAInterface

SCCP Ref=R2

TRX:TEI=T1

Channel ID = N1

SCCP Ref=R1

DTAP

DLCI: SAPI=3

DLCI: SAPI=0

Channel=C1

Link: SAPI=3

Link: SAPI=0

PD=CCTI=aTI=bPD=MMPD=RRTI=AMSBSCMSC

Channel=C2

Channel ID = N1

Radio InterfaceAbis InterfaceA Interface

PD: protocol discriminatorTI: Transaction Identifier for RIL3-CC protocolDLCI: Data Link connection IdentifierSAPI: Service Access Point Identifier on the radio InterfaceTEI: Terminal Equipment Identifier on the Abis I/F

*The first one of them Msa is the only one where all the levels of detail are given:The mobile station has two calls in progress (TI=a and b on PD=CC, on SAPI=0)And one SMS transaction (TI=A on SAPI=3).

Bearer ServicesTelecommunication services to transfer data between access pointsSpecification of services up to the terminal interface (OSI layers 1-3) Different data rates for voice and data (original standard)Data service Synchronous: 2.4, 4.8 or 9.6 kbit/sAsynchronous: 300 - 1200 bit/s

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Tele ServicesTelecommunication services that enable voice communication via mobile phones.All these basic services have to obey cellular functions, security measurements etc.Offered services.Mobile telephonyprimary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz.Emergency numbercommon number throughout Europe (112); Mandatory for all service providers; Free of charge; Connection with the highest priority (preemption of other connections possible).Multinumberingseveral ISDN phone numbers per user possible.

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Performance characteristics of GSMCommunication mobile, wireless communication; support for voice and data servicesTotal mobility international access, chip-card enables use of access points of different providersWorldwide connectivityone number, the network handles localizationHigh capacity better frequency efficiency, smaller cells, more customers per cellHigh transmission qualityhigh audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains)Security functions access control, authentication via chip-card and PIN

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Disadvantages of GSMNo full ISDN bandwidth of 64 kbit/s to the userReduced concentration while drivingElectromagnetic radiationAbuse of private data possibleHigh complexity of the systemSeveral incompatibilities within the GSM standards

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Thank You

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*270.833 kb/s per carrierGMSK with a time bandwidth product BT =0.3Slow frequency hoping 217/hops/second.Synchronization compensation for up to 233micro seconds absolute delayBlock and convolutional channel coding copuled with interleaving to combat channel perturbations- overall channel rate of 22.8 kb/sFull rate channel 13 kb/s voice coder rate using regular pulse excitation/linear predictive coding RPE/LPC, half rate channel 6.5 kb/s usingVector coder rate using vector sum excited linear predictivie coding VSELPOverall full rate channel bit rate of 22.8 kb/s.Each cell can have from 1 to 16 pairs of carriers.*The system capacity depends on :The total number of radio channelsThe size of the cellThe frequency re-use factor or distance

The minimum distance which allows the same frequencies to be re-used will depend on many factors,The number of co-channel cells in the vicinity of the center cellThe geography of the terrain,The antenna heightThe transmitted power within each cell

*Due to assumptions 1MHz carrier 5 radio frequencies(radio channels) 5X200 kHz. Each radio frequency carries8 traffic channels = 40 traffic channels/MHzWithout cell splitting, traffic channels = 7.2MHzX40 = 288 traffic channelsWith 72 cells, 72/12(kfactor = 12) = 6 paterns(all spectrum may be used in a pattern), traffic channels = 6X288 = 1728 traffic channelsWith 246 cells, 246/12(K factor = 12) we will get 20 sectors + 6 cells, for 20 patterns and 6/12 we get 20X288 + 6/12*288 = 5904 traffic channels.

For the same channels spacing and re-use pattern, the number of re-used channels is increased when cell radius are reduced.*The start of the uplink TDMA frame is delayed with respect to downlink by a fixed period of three timeslots. Why ? Staggering TDMA frames allows the same timeslot number to be used in both the down and uplink while avoiding the requirement for mobile to transmit and receive simultaneously. Between T and R the MS is in the IDLE mode, makes measurement of signal strength of neighboring cells.*Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding andblock interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality thanothers. The bits are thus divided into three classes:

Class Ia 50 bits - most sensitive to bit errors Class Ib 132 bits - moderately sensitive to bit errors Class II 78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightlyattenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder ofconstraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remainingClass II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocksare transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.

Recall that each time-slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK).GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related topower consumption, which should be minimized for the mobile station.*Normal burst 148 bits + 8.25 guard bitsFrequency correction burst 148 bits + 8.25 guard bitsSynchronizing burst 148 bits + 8.25 guard bitsAccess burst 88 bits +68.25 guard bits used to access a cell for the first time in case of a call set up or handoverThe data structure within a normal burst consists of 148 bits transmitted at a rate of 270.833 kb/s. Each burst in GSM system modulates one of the carriers assigned to a particular cell using GMSK.*Speech in GSM is digitally coded at a rate of 13 kbps, so-called full-rate speech coding. This is quite efficient compared with the standard ISDN rate of 64 kbps. One of the most important Phase 2 additions will be the introduction of a half-rate speech codec operating at around 7 kbps, effectively doubling the capacity of a network. This 13 kbps digital stream (260 bits every 20 ms) has forward error correction added by a convolutional encoder. The gross bit rate after channel coding is 22.8 kbps (or 456 bits every 20 ms). These 456 bits are divided into 8 57-bit blocks, and the result is interleaved amongst eight successive time slot bursts for protection against bursty transmission errors. Each time slot burst is 156.25 bits and contains two 57-bit blocks, and a 26-bit training sequence used for equalization. A burst is transmitted in 0.577 ms for a total bit rate of 270.8 kbps, and is modulated using Gaussian Minimum Shift Keying (GMSK) onto the 200 kHz carrier frequency. The 26-bit training sequence is of a known pattern that is compared with the received pattern in the hope of being able to reconstruct the rest of the original signal. Forward error control and equalization contribute to the robustness of GSM radio signals against interference and multipath fading. The digital TDMA nature of the signal allows several processes intended to improve transmission quality, increase the mobile's battery life, and improve spectrum efficiency. These include discontinuous transmission, frequency hopping and discontinuous reception when monitoring the paging channel. Another feature used by GSM is power control, which attempts to minimize the radio transmission power of the mobiles and the BTS, and thus minimize the amount of co-channel interference generated. *The full rate TCH uses 24 out of the 26 available in the multiframe The duration of the multiframe is therefore 26X60/13ms = 120ms

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extractthe desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal.This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications. *Distinct training sequences will therefore be allocated to channels using the same frequencies in cells which are close enough to interfere with one another.*When a mobile station is first switched on it is necessary to read the BCCH in order to determine its orientation within the network.The mobile must first synchronize in frequency and then in time. The FCCH, SCH and BCCH are all transmitted on the same carrier frequency which has a higher power density than any of the other channels in a cell because steps are taken to ensure that it is transmitted information at all times. The mobile scans around the available frequencies, picks the strongest and then selects the FCCH. Fc+67.7kHz*

*The full rate TCH uses 24 out of the 26 available in the multiframe The duration of the multiframe is therefore 26X60/13ms = 120ms

At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extractthe desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal.This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications. *The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption onceimplemented) before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder (RPE--LPC) with a Long Term Predictor loop. Basically, information from previous samples, which does notchange very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted andactual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. This is the so-called Full-Rate speech coding.Recently, an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13kbps bit rate. *

*Mike*The first one of them Msa is the only one where all the levels of detail are given:The mobile station has two calls in progress (TI=a and b on PD=CC, on SAPI=0)And one SMS transaction (TI=A on SAPI=3).*

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