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					Global System for Mobile
 Communication (GSM)
          Li-Hsing Yen
         Assistant Prof.
Dept. of CSIE, Chung Hua Univ.




GSM System Architecture
   Um
     MS        MSC
   (ME/SIM) E MSC  PSTN, ISDN, PSPD
                     CSPDN
    A-bis
        A      F
           C B
       A-bis
 BTS BSCHLRVLR EIR
    BSS      D
Um            G
  MS
 (ME/SIM) AuCVLRNSS



                                 1
             Nomenclature
• MS (Mobile Station) =
  MT (Mobile Terminal ) +
  TE (Terminal Equipment)
• BSS (Base Station Subsystem) =
  BTS (Base Transceiver Station) +
  BSC (Base Station Controller)
• NSS (Network Switching Subsystem)
• MSC (Mobile Switching Center): telephony
  switching function and authentication of user




             HLR and VLR
• HLR (Home Location Register)
  – a database to store and management
    permanent data of subscribers
• VLR (Visitor Location Register)
  – a database to store temporary information
    about subscribers
  – needed by MSC in order to service visiting
    subscribers




                                                  2
                AuC and EIR
• Authentication Center (AuC)
   – used in the security data management for
     the authentication of subscribers.
• Equipment Identity Register (EIR)
   – used to maintain a list of legitimate,
     fraudulent, or faulty MSs.
   – optional in GSM network, and is not used
     generally.




              GSM Interfaces
• Um
   – Radio interface between MS and BTS
   – each physical channel supports a number of
     logical channels
• Abis
   – between BTS and BSC (vender specific)
   – primary functions: traffic channel transmission,
     terrestrial channel management, and radio
     channel management




                                                        3
          Frequency Division Duplex
        n: Absolute Radio Frequency Channel Number (ARFCN). 1 ≤ n ≤ 124
     Uplink           890.0 MHz                           Guard band
                      890.2 MHz                                                        200kHz
F ul(n)=890+ 0.2*n    890.4 MHz
        MHz
                         ....


                                                              0




                                                              .
                                                              57
                                                              7
                      914.8 MHz




                                                              ms
                                                                       burst (contents of time slot)
    Downlink          935.0 MHz                           Guard band
                      935.2 MHz

F dl(n)=F ul(n)+45    935.4 MHz

        MHz
                         ....




                      959.8 MHz
                                  .   .   .   7   0   1   2    3   4   5   6   7   0   time slot




                     Time Division Duplex
       MS and BTS do not transmit simultaneously
       (MS transmits 3 time slots after the BTS)

          Downlink         5 6 7 0 1 2 3 4 5 6 7 0 1 2




           Uplink          2 3 4 5 6 7 0 1 2 3 4 5 6 7

     Timing advance: MS transmits its data a little earlier as
     demanded by the “three time slots delay rule”.




                                                                                                       4
                 Timing Advance
                 Propagation delay


Base station      send                   recv




Mobile station      recv                send
                                          send   Original timing


                             Timing advance
                             ~ Propagation delay * 2




         GSM Frame Structure
• 1 hyperframe = 2048 superframes (~3.5hr)
• For speech
    – 1 superframe = 51 multiframes = 6.12s
    – 1 multiframe = 26 frames = 120ms
• For Signaling
    – 1 superframe = 26 multiframes
    – 1 multiframe = 51 frames
• 1 frame = 8 time slots = 4.615 ms
• 1 time slot = 156.25 bit duration = 0.577ms




                                                                   5
                GSM Frame Hierarchy
                                                                              3.48hr
Hyper                                            …
          0 1                                                                      2047
frame

Super     0 1                         …                            48 49 50    6.12s
frame

Multi-                       …
          0 1                                    23 24 25 120ms
frame

Frame     0 1 2 3 4 5 6 7                 4.615ms

                                       28 bits
                                                                              0.57692ms
Time                                                                           8.25
                Encrypted bits                       Encrypted bits
Slot                                                                           guard bits
                   57 bits                               57 bits
    3 tail bits                                                      3 tail bits
                                 Training sequence        Stealing bit




                  Normal Burst Format
   • Trail bits
         – always (0,0,0); provide start and stop bit pattern
   • encrypted bits
         – data is encrypted
   • stealing bits
         – indicate whether the burst was stolen for urgent
           control signaling (FACCH signaling)
   • Guard bits
         – avoid overlapping with other bursts due to different
           path delay




                                                                                            6
             Training Sequence
• A known bit pattern that differs for different
  adjacent cells
• to adapt the parameters of the receiver to the
  current path propagation characteristics
• to select the strongest signal in case of
  multipath propagation
• for multipath equalization
   – extract the desired signal from unwanted
     reflections




            GSM Protocol Stack
   MS               Base                   Base                    MSC
                 Transceiver              Station
  CM               Station               Controller                CM
  MM               (BTS)                  (BSC)                    MM
                                                      DTAP
                                  RR                             BSSMAP/DTAP
   RR                                           BSSMAP
            RR            BTSM BTSM
                                                SCCP             SCCP
LAPDm       LAPDm      LAPD           LAPD
 Layer 1     Layer 1   Layer 1        Layer 1    MTP              MTP



          Um                   Abis                          A
    (air interface)




                                                                               7
        Layer 1 - Physical Layer
• Modulation
• Equalization
• Channel coding
    – block code
    – convolutional code
• Interleaving
    – to distribute burst error




 GSM Physical Layer (MS Side)
signaling                            voice     signaling
            voice
            speech                  speech
            coding                  decoding


 channel coding                     channel decoding


  interleaving                       de-interleaving

burst formatting                    burst de-formatting


   ciphering                          deciphering

   modulation        R/F      R/F    demodulation




                                                           8
              GSM Speech Transmission
                                                  20 ms

                                                                 speech encoding (RPE-LTP)
                                                  260 bits

               channel encoding
                                               456 bits

                                   0     57 114 171 228 285 342 399
                                   64   121 178 235 292 349 406 7

                interleaving        :    :
                                   392 449 50
                                              :    :   :     :   :
                                                   107 164 221 278 335
                                                                      :   57 rows




burst    57 57 57 57 57 57 57 57        57 57 57 57 57 57 57 57             57 57 57 57 57 57 57 57
formatting

                                                                                             frame
                                               burst




        GSM Speech Channel Coding
                                                                     260 bits
                                              Class 1a               Class 1b           Class 2
                                              50 bits                132 bits           78 bits
              Parity bits                              reordering                                     Tail
              protecting 1a                                                                           Bits
                                          91 bits            3       91 bits        4

                               Convolutional Coding
                                        378 bits                                        78 bits

                                             456 bits




                                                                                                             9
  Tailing Bits and Reordering
                     d(0)         u(0)
    d(0)             d(2)         u(1)
    d(1)             d(4)         u(2)
    d(2)               :     :     :
    d(3)    reorder d(178)       u(89)     Tailing Bits
                    d(180)       u(90)
     …




                     p(0)        u(91)    u(185)   0
                     p(1)        u(92)    u(186)   0
   d(179)            p(2)        u(93)    u(187)   0
   d(180)                        u(94)    u(188)   0
                    d(181)
   d(181)                        u(95)
                    d(179)
                                 u(96)
    p(0)            d(177)
                       :           :
    p(1)
                     d(3)        u(183)
    p(2)
                     d(1)        u(184)




                   Parity Bits
• The first 50 bits are protected by 3
  parity bits p(0), p(2), p(3)
• generator polynomial g(D)=D3+D+1
• the remainder of
  d(0)D52+d(1)D51+… +d(49)D3+p(0)D2+p(
  1)D+p(2) divided by g(D) should be
  1+D+D2




                                                          10
   Convolutional Encoder for
 GSM Speech (Rate=1/2, K=5)


U0 … U188
            ak    ak-1   ak-2   ak-3    ak-4




                  Interleaving
    0                                                455


      0      57   114    171    228    285     342     399
     64     121   178    235    292    349     406       7
    128     185   242    299    356    413      14      71
    192     249   306    363    420     21      78     135
    256     313   370    427     28     85     142     199
    320     377   434     35     92    149     206     263
    384     441    42     99    156    213     270     327
    448      49   106    163    220    277     334     391
     56     113   170    227    284    341     398     455
    120     177   234    291    348    405       6      63
    184     241   298    355    412     13      70     127
    248     305   362    419     20     77     134     191
    312     369   426     27     84    141     198     255
      :       :     :      :      :      :       :       :
      :       :     :      :      :      :       :       :
    392     449    50    107    164    221     278     335




                                                             11
   GSM Normal Burst Formatting
              A                               B                             C
  57 57 57 57 57 57 57 57        57 57 57 57 57 57 57 57        57 57 57 57 57 57 57 57




                                      burst                                           frame

                                       28 bits
                                                                                          8.25
        BABA…               …   BAB               ABAB…              …   ABA              guard
                                                                                          bits
                  57 bits                                  57 bits
3 tail bits                                                                 3 tail bits
                                Training sequence           Stealing flag




     Physical Vs. Logical Channels
  • Physical channels are all the available time
    slots of a BTS
        – a BTS with 6 carriers has 48 physical channels
  • Logical channels are piggybacked on the
    physical channels
        – logical channels are laid over the grid of physical
          channels
        – each logical channel performs a specific task




                                                                                                  12
    GSM Logical Channels (I)
• Speech traffic channels (TCH)
  – Full-rate TCH (TCH/F)
  – Half-rate TCH (TCH/H)
• Broadcast channels (BCH)
  – Frequency correction channel (FCCH)
  – Synchronization channel (SCH)
  – Broadcast control channel (BCCH)
• Cell broadcast channel (CBCH)




   GSM Logical Channels (II)
• Common control channels (CCCH)
  – Paging channel (PCH)
  – Access grant channel (AGCH)
  – Random access channel (RACH)
• Dedicated control channel (DCCH)
  – Slow associated control channel (SACCH)
  – Stand-alone dedicated control channel (SDCCH)
  – Fast associated control channel (FACCH)




                                                    13
   Broadcast Channels (BCH)
• Frequency correction channel (FCCH)
  – the “lighthouse” of a BTS
• Synchronization channel (SCH)
  – PLMN/base identifier of a BTS plus
    synchronization information (frame number)
• Broadcast control channel (BCCH)
  – to transmit system information 1-4, 7-8 (differs in
    GSM 900, GSM 1800, and PCS 1900)




            CBCH and CCCH
• CBCH (Cell Broadcast Channel)
  – transmits cell broadcast messages
• PCH (Paging Channel)
  – carries PAG_REQ message
• AGCH (Access Grant Channel)
  – SDCCH channel assignment
• RACH (Random Access Channel)
  – communication request from MS to BTS




                                                          14
      Mapping of Logical Channels
    • Each BTS has a particular frequency carrier
      called BCCH-TRX to transmit BCCH info
    • The following channel structure can be found
      on time slot 0 of carrier BCCH-TRX
        –   FCCH
        –   SCH
        –   BCCH information 1-4
        –   Four SDCCH subchannels (optional)
        –   CBCH (optional)




        Example Mapping of Logical
     Channels on Time Slot 0 (Downlink)
              FCCH + SCH             Block 4
FN= 0 - 5           +                         FN= 26 - 29
                                   CCCH/SDCCH
               BCCH 1 - 4
                 Block 0            FCCH/SCH FN= 30 - 31
FN= 6 - 9      reserved for
                  CCCH               Block 5
               FCCH/SCH            CCCH/SDCCH FN= 32 - 35
FN= 10 - 11
                 Block 1             Block 6
               reserved for                   FN= 36 - 39
FN= 12 - 15                        CCCH/SDCCH
                  CCCH
                 Block 2            FCCH/SCH FN= 40 - 41
FN= 16 - 19    reserved for
                  CCCH               Block 7  FN= 42 - 45
FN= 20 - 21    FCCH/SCH            CCCH/SACCH
              Block 3                 Block 7 FN= 46 - 49
FN= 22 - 25 CCCH/SDCCH             CCCH/SACCH
                                     not used FN= 50




                                                            15
        Example Mapping of Logical
     Channels on Time Slot 2 (Downlink)


 FN= 0 - 11    TCH



    FN= 12    SACCH



FN= 13 - 24    TCH



    FN= 25    not used




              GSM Layer 2: LAPDm
    • Functions
       – organization of Layer 3 information into
         frames
       – peer-to-peer transmission of signaling data
         in defined frame formats
       – recognition of frame formats
       – establishment, maintenance, and
         termination of one or more (parallel) data
         links on signaling channels




                                                       16
    Layer 3 Protocol Architecture:
         Mobile Station Side
       MNREG-SAP             MNCC-SAP                     MNSS-SAP MNSMS-SAP

      CM
                                     CC                          SS                 SMS



                                                                  TI          TI                TI
                                                 MM CC                     SS           SMS
           MM                                                             PD



                                                                         PD

           RR


             SAPI=0                                                                                  SAPI=3
                                      AGCH+PCH

                                                 SDCCH

                                                         SACCH

                                                                 FACCH



                                                                                SDCCH

                                                                                        SACCH
                      RACH

                              BCCH




      Layer 3 - RR Sublayer
• The RR sublayer handles all the procedures
  necessary to establish, maintain, and release
  dedicated radio connections
   – channel allocation
                                                                                                              B
   – handover
                              A
   – timing advance
   – power control           power
                             level
   – frequency hopping
                                                                                                                  time
                                                                                                 A        B




                                                                                                                         17
       Three Cases of Hand-over
            MSC                          MSC


      BSC         BSC              BSC


 BTS        BTS         BTS   BTS

 MS     MS                    1. different BTS, same BSC


        MS          MS             2. different BSC, same MSC

                                       3. different MSC, same PLMN
                    MS        MS
                                               (old MSC=anchor MSC
                                                new MSC=relay MSC)




        Layer 3 - MM Sublayer
• The MM sublayer copes with all the
  effects of handling a mobile user that
  are not directly related to radio functions
  – location area
  – location registration & call delivery
  – location update & paging




                                                                     18
   Authentication & Encryption/Decryption in GSM
                        Mobile Station                                Home System
                                                                       RAND
                        SIM           Ki                                        Ki


                               A8           A3                             A3         A8

                                                           accept        SRES          Kc
                                                                  Y

                                           SRES              =?          SRES
                                                                  N
                                                           reject                                authentication

                                                                        frame number        Kc    encryption
                               Kc
                                                         Visited
                                  A5                     System                 A5
                          S1               S2                             S1           S2

           plain text                             ciphered data                                  plain text




                                                          MS           BTS BSC MSC VLR HLR
                                                              channel request
                   HLR                                       channel activation command
                                                             channel activation acknowledge
cancellation 5 ack            3       location update         channel assignment
                 subscriber                                   location update request
                 information                                      authentication request
       old         2                  new                         authentication response
       VLR       IMSI,                VLR                         comparison of authentication parameters
               auth. para.
                                                                  assignment of TMSI
                                  1        4 ack
                 old TMSI,                                   acknowledgement of TMSI
                old VLR ID            new TMSI
                                                                                     entry of the new area and
                                      MS                                             identity into the VLR & HLR

                                                              channel release




                                                                                                                   19
      Layer 3 - CM Sublayer
• The CM sublayer manages all the functions
  necessary for circuit-switched call control
   – call establishment procedures for mobile-
     originated calls and mobile-terminated calls
   – in-call modification
   – call reestablishment
   – Dual Tone Multi Frequency (DTMF) control
     procedure for DTMF transmission




            Contents of CM
• Call Control (CC)
• Short Message Service (SMS)
• Supplementary Service (SS)




                                                    20
                 Paging Procedure

             MS                                                  BSS
                       Paging Request Message on PCH

                         Channel Request on RACH

                           Assign SDCCH on AGCH

                          SABM (Paging Response)




      Call Setup Procedure: Mobile
             Terminated Call
            +886935...                request roaming number
                          GMSC        1                          1
                                                     HLR             VLR
          dial MSISDN     (INTX)      2                          2
             1      1                        allocate MSRN
                               3                                     MSC
             other          routing        other
                                                             3
            switches                      switches                        3

                                                                     MS
INTerrogating eXchange (INTX)
Mobile Station ISDN Number (MSISDN) (Country Code, see E.164)
Mobile Station Roaming Number (MSRN) (Mobile Country Code, see E.212)




                                                                              21
  Dual Tone Multiple Frequency
        (DTMF) in PSTN
                                      Switch
                     DTMF




Dialing

                             Switch   PBX




Connected




                    DTMF in GSM
                                         MSC

                             SETUP



   Dialing

                               MSC       PBX


                START_DTMF

                 STOP_DTMF

    Connected




                                               22

				
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posted:12/16/2011
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