Overview of GSM Cellular Network and Operations

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					    Overview of
GSM Cellular Network
       and
    Operations

          www.XpressKnowledge.com
              Network and switching subsystem
•   NSS is the main component of the public mobile network GSM
     – switching, mobility management, interconnection to other
       networks, system control
•   Components
     – Mobile 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 MSC
     – Databases (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
                       Operation subsystem
•   The OSS (Operation Subsystem) enables centralized operation,
    management, and maintenance of all GSM subsystems
•   Components
     – Authentication 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 rights
         • stolen or malfunctioning mobile stations can be locked and
           sometimes even localized
     – Operation and Maintenance Center (OMC)
         • different control capabilities for the radio subsystem and the
           network subsystem
 Mobile Handset
TEMPORARY DATA                    PERMANENT DATA

- Temporary Subscriber Identity   Permanent Subscriber Identity

- Current Location                Key/Algorithm for Authentication.

- Ciphering Data




        Provides access to the GSM n/w
        Consists of
            Mobile equipment (ME)
            Subscriber Identity Module (SIM)
The GSM Radio Interface

            AIR INTERFACE
                                                          BASE TRANSCEIVER STATION




                                               z
                                         MH
                                 - 960
                           935
                    NL INK
                 DOW
   MOBILE

                                                     Hz
                                                1  5M
                                         0   -9
                                     89
                                 K
                           L  IN
                         UP
 The GSM Network Architecture
• Time division multiple access-TDMA
• 124 radio carriers, inter carrier spacing
  200khz.
• 890 to 915mhz mobile to base - UPLINK
• 935 to 960mhz base to mobile -
  DOWNLINK
• 8 channels/carrier
         GSM uses paired radio channels




890MHz            915MHz    935MHz        960MHz




  0                   124    0             124
     Access Mechanism


– FDMA, TDMA, CDMA
                Frequency multiplex
• Separation of the whole spectrum into smaller frequency bands
• A channel gets a certain band of the
spectrum for the whole time
• Advantages:                               k1     k2   k3   k4   k5   k6

    – no dynamic coordination            c
       necessary                                                            f
    – works also for analog signals
• Disadvantages:
    – waste of bandwidth
       if the traffic is
       distributed unevenly
    – inflexible         t
    – guard spaces
                           Time multiplex
• A channel gets the whole spectrum for a certain amount of
  time
• Advantages:
   – only one carrier in the
     medium at any time
   – throughput high even
                                           k1   k2   k3   k4   k5   k6
     for many users
• Disadvantages:                       c
   – precise
                                                                         f
     synchronization
     necessary



                       t
  Time and Frequency Multiplex
• Combination of both methods
• A channel gets a certain frequency band for a certain
  amount of time                    k    k    k     k     k5   k6
                                      1   2    3    4


                                  c
                                                                    f




                  t
      Time and Frequency Multiplex
• Example: GSM
• Advantages:
   – Better protection against
     tapping
   – Protection against frequency
     selective interference              k1   k2   k3   k4   k5   k6

   – Higher data rates compared to   c
     code multiplex
                                                                       f
• But: precise coordination
  required

                     t
• 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 channels




890MHz            915MHz    935MHz        960MHz




  0                   124    0             124
                   Code Multiplex
                                           k1    k2   k3   k4       k5   k6


•   Each channel has a unique code
                                                                c
•   All channels use the same spectrum at the same
    time
•   Advantages:
      – Bandwidth efficient
      – No coordination and synchronization
         necessary
                                                                              f
      – Good protection against interference and
         tapping
•   Disadvantages:
      – Lower user data rates
      – More complex signal regeneration
•   Implemented using spread spectrum technology t
Various Access Method
Cells
 Capacity & Spectrum Utilization
            Solution
The need:
• Optimum spectrum                Network capacity at required QoS
  usage                            with conventional frequency plan
• More capacity                                       Out of
• High quality of                                     Capacity!!!
  service
• Low cost                                        Subscriber
                                                 growth
                                                      Time

          increase capacity
I wish I could
     without adding NEW BTS!
                 What can I do?
     Representation of Cells




Ideal cells       Fictitious cells
        Cell size and capacity
• Cell size determines number of cells
  available to cover geographic area and (with
  frequency reuse) the total capacity available
  to all users
• Capacity within cell limited by available
  bandwidth and operational requirements
• Each network operator has to size cells to
  handle expected traffic demand
                       Cell structure
•   Implements space division multiplex: base station covers a certain
    transmission area (cell)
•   Mobile stations communicate only via the base station
•   Advantages of cell structures:
     – higher capacity, higher number of users
     – less transmission power needed
     – more robust, decentralized
     – base station deals with interference, transmission area etc. locally
•   Problems:
     – fixed network needed for the base stations
     – handover (changing from one cell to another) necessary
     – interference with other cells
•   Cell 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 System
• Frequency Re-Use Distance
• The K factor or the cluster size
• Cellular coverage or Signal to interference
  ratio
• Sectoring
        The K factor and Frequency Re-Use Distance
                                                          7
                                                 6                2
 K=   i2 +   ij +   j2                                    1
 K = 22 + 2*1 + 12                               5                3
                                                              j
 K=4+2+1                                 7                                   R
 K=7                                             2
                                  6
                                          1           i
                                                                      D
                                  5              3
                                         4
                                                                                 D = Ö`3K * R
Frequency re-use distance is based on the cluster size K                         D = 4.58R

The cluster size is specified in terms of the offset of the center of a cluster from the
center of the adjacent cluster
            The Frequency Re-Use for K = 4



                                      K = i2 + ij + j2
                                      K = 22 + 2*0 + 02
                                      K=4+0+0
                        D
                                      K=4
D = Ö`3K * R
                   R
D = 3.46R                   i
    The Cell Structure for K = 7

              7
          6       2
              1
          5       3
      7       4               1
6         2
                      2
      1                           7
5         3               6           2
      4                           1
          7               5           3
      6       2       7           4
          1       6       2
      5       3       1
          4       5       3
                      4
    Cell Structure for K = 4
                 1

                     2               1
             4
     1                       4           2
                 3
4            2       1               3

     3           4       2               1

             1       3           4           2

         4       2       1               3

             3       4           2

                         3
Cell Structure for K = 12


                9                   9
          8         10         8        10
                2        11         2
          7                                  11
                     3         7         3
                1        12         1
          6                                  12
                     4         6         4
      9         5         9
  8                                 5
          10         8        10
      2        11         2
 7                                 11
          3          7
      1                       3
               12         1        12
  6       4          6        4
      5                   5
     Increasing cellular system
              capacity
• Cell sectoring
  – Directional antennas subdivide cell into 3 or 6
    sectors
  – Might also increase cell capacity by factor of 3
    or 6
     Increasing cellular system
              capacity
• Cell splitting
  – Decrease transmission power in base and
    mobile
  – Results in more and smaller cells
  – Reuse frequencies in non-contiguous cell
    groups
  – Example: ½ cell radius leads 4 fold capacity
    increase
Tri-Sector antenna for a cell
Cell Distribution in a Network




  Rural
                  Highway


  Suburb         Town
     Optimum use of frequency
            spectrum
• Operator bandwidth of 7.2MHz (36 freq of 200
  kHz)
• TDMA 8 traffic channels per carrier
• K factor = 12
• What are the number of traffic channels available
  within its area for these three cases
   – Without cell splitting
   – With 72 cells
   – With 246 cells
          Re-use of the frequency



One Cell = 288 traffic channels
 8 X 36 = 288



                                                   72 Cell = 1728 traffic channels
                                                     8 X (72/12 X 36) = 1728


                                  246 Cell = 5904 traffic channels
  Concept of TDMA Frames and
            Channels
                          c
                                                      f




      t


• 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 channels




890MHz            915MHz    935MHz        960MHz




  0                   124    0             124
GSM delays uplink TDMA frames
The start of the uplink
 TDMA is delayed of                    TDMA frame (4.615 ms)
   three time slots


Downlink TDMA                     R1 R2 R3 R4 R5 R6 R7 R8
   F1MHz
                                                               Uplink TDMA
                    T1 T2 T3 T4 T5 T6 T7 T8                        Frame
                                                               F1 + 45MHz



                    R             T



                          R            T


                        Fixed transmit
                    Delay of three time-slots
                GSM - TDMA/FDMA
                                               935-960 MHz
                                               124 channels (200 kHz)
                                               downlink


                                               890-915 MHz
                                               124 channels (200 kHz)
                                               uplink
                                      higher GSM frame structures
                                                                          time

                     GSM TDMA frame

  1             2         3      4        5         6           7         8
                                                                                 4.615 ms

                     GSM time-slot (normal burst)
guard                                                                    guard
space    tail       user data   S Training S     user data          tail space
        3 bits       57 bits    1 26 bits 1        57 bits           3
                                                                         546.5 µs
                                                                               577 µs
                            LOGICAL CHANNELS



        TRAFFIC                                       SIGNALLING



 FULL RATE        HALF RATE
 Bm 22.8 Kb/S     Lm 11.4 Kb/S
                                  BROADCAST    COMMON CONTROL   DEDICATED CONTROL




                FCCH        SCH     BCCH
                                                    RACH
                                              PCH           AGCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNEL
BCCH -- BROADCAST CONTROL CHANNEL
PCH -- PAGING CHANNEL
RACH -- RANDOM ACCESS CHANNEL                              SDCCH     SACCH   FACCH
AGCH -- ACCESS GRANTED CHANNEL
SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL         DOWN LINK ONLY
SACCH -- SLOW ASSOCIATED CONTROL CHANNEL                                BOTH UP &
FACCH -- FAST ASSOCIATED CONTROL CHANNEL               UPLINK ONLY      DOWNLINKS
    Broadcast Channel - BCH
• Broadcast 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 cells
• Frequency correction channel (FCCH) is a base to
  mobile channel which provides information for
  carrier synchronization
• Synchronization channel (SCH) is a base to mobile
  channel which carries information for frame
  synchronization and identification of the base
  station transceiver
   Common Control Channel -
           CCH
• Paging channel (PCH) is a base to mobile channel
  used to alert a mobile to a call originating from the
  network
• Random access channel (RACH) is a mobile to base
  channel used to request for dedicated resources
• Access grant channel (AGCH) is a base to mobile
  which is used to assign dedicated resources
  (SDCCH or TCH)
  Dedicated Control Channel -
            DCCH
• Stand-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 -
             DCCH
• Slow 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 channel
• Fast 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        3              57         1      26      1             57      3      8.25
 - NB


  FREQUENCY
  CORRECTION         3                                       142                     3      8.25
  BURST - FB



SYNCHRONISATION     3            39                   64                    39      3     8.25
BURST - SB




 ACCESS
 BURST - AB              6               41                36         3          68.25

                                                      FIXED BITS            SYNCHRONISATION BITS
         TAIL BIT            GUARD PERIOD



        ENCRYPTION BIT                TRAINING BITS             FLAG BITS               MIXED BITS
                                                                 HIERARCHY OF FRAMES
        1 HYPER FRAME = 2048 SUPERFRAMES = 2 715 648 TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS )

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


        TRAFFIC CHANNELS   1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S )
                                        LEFT (OR) RIGHT
    1 SUPER FRAME = 51 MULTI FRAMES

0        1       2           3       4                                       48       49 50                        SIGNALLING CHANNELS

                                                                                                          1 SUPER FRAME = 26 MULTI FRAMES

                                                                                  0                   1            2                              24           25
    1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms )

0       1 2          3                                               24 25
                                                                                                          1 MULTI FRAME = 51 TDMA FRAMES (235 .4 ms )
                                                                                                      0    1 2         3   4                           48 49 50


    0    1       2       3       4       5   6   7       0   1           2   3        4       5   6        7   0
                             (4.615ms)           TDMA FRAME NO.
                              0                            1
1 TIME SLOT = 156.25 BITS
      ( 0.577 ms)                                            0       1       2    3           4   5        6   7       0   1     2    3   4   5        6   7   0
                                                                                                                                (4.615 ms)
1 2      3 4               155 156                                                        0                                           1
          1 bit =36.9 micro sec
                                     GSM Frame                                                  Full rate
                                                                                               channel is
                                                                    SACCH is                   idle in 25
                                                                   transmitted
0 to 11 and 13 to 24                                               in frame 12
Are used for traffic data                                                                                       Frame
                                                                                                              duration =
                            0        1           2                12             24       25                    120ms




                                                                                                              Frame
                                                                                                            duration =
                                0        1           2        3       4   5           6    7                 60/13ms




                                                                                                                      Frame
                                                                                                                    duration =
                                                                                                                     15/26ms
             3                  57           1           26       1               57            3      8.25
• 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)
                            LOGICAL CHANNELS



        TRAFFIC                                       SIGNALLING



 FULL RATE        HALF RATE
 Bm 22.8 Kb/S     Lm 11.4 Kb/S
                                  BROADCAST    COMMON CONTROL   DEDICATED CONTROL




                FCCH        SCH     BCCH
                                                    RACH
                                              PCH           AGCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH -- SYNCHRONISATION CHANNEL
BCCH -- BROADCAST CONTROL CHANNEL
PCH -- PAGING CHANNEL
RACH -- RANDOM ACCESS CHANNEL                              SDCCH     SACCH   FACCH
AGCH -- ACCESS GRANTED CHANNEL
SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL         DOWN LINK ONLY
SACCH -- SLOW ASSOCIATED CONTROL CHANNEL                                BOTH UP &
FACCH -- FAST ASSOCIATED CONTROL CHANNEL               UPLINK ONLY      DOWNLINKS
Location update from the mobile
     Mobile looks for BCCH after switching on

    RACH send channel request

              AGCH receive SDCCH


SDCCH request for location updating

               SDCCH authenticate

   SDCCH authenticate response

          SDCCH switch to cipher mode


 SDCCH cipher mode acknowledge

              SDCCH allocate TMSI


  SDCCH acknowledge new TMSI

         SDCCH switch idle update mode
         Call establishment from a mobile
               Mobile looks for BCCH after switching on

              RACH send channel request

                        AGCH receive SDCCH

        SDCCH send call establishment request

           SDCCH do the authentication and TMSI allocation

  SDCCH send the setup message and desired number

               SDCCH require traffic channel assignment

FACCH switch to traffic channel and send ack (steal bits)

               FACCH receive alert signal ringing sound

                    FACCH receive connect message

  FACCH acknowledge connect message and use TCH

                      TCH conversation continues
           Call establishment to a mobile
               Mobile looks for BCCH after switching on

                Mobile receives paging message on PCH

          Generate Channel Request on RACH

             Receive signaling channel SDCCH on AGCH

          Answer paging message on SDCCH

               Receive authentication request on SDCCH

                Authenticate on SDCCH

                   Receive setup message on SDCCH

             Receive traffic channel assignment on SDCCH

FACCH switch to traffic channel and send ack (steal bits)

          Receive alert signal and generate ringing on FACCH

                  Receive connect message on FACCH
FACCH acknowledge connect message and switch to TCH
GSM speech coding

          AIR INTERFACE
                                                        BASE TRANSCEIVER STATION




                                             z
                                       MH
                               - 960
                         935
                  NL INK
               DOW
 MOBILE

                                                   Hz
                                              1  5M
                                       0   -9
                                   89
                               K
                         L  IN
                       UP
                                        Transmit Path


BS Side
            8 bit A-Law      8 K sps
                  to
                                            RPE/LTP speech Encoder
           13 bit Uniform                                            To Channel Coder 13Kbps




MS Side


                             8 K sps,
          LPF          A/D                  RPE/LTP speech Encoder
                                                                     To Channel Coder 13Kbps




 Sampling Rate - 8K
 Encoding - 13 bit Encoding (104 Kbps)
 RPE/LTP - Regular Pulse Excitation/Long Term Prediction
 RPE/LTP converts the 104 Kbps stream to 13 Kbps
           GSM Speech Coding


• GSM 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 Frame                                                  Full rate
                                                                                               channel is
                                                                    SACCH is                   idle in 25
                                                                   transmitted
0 to 11 and 13 to 24                                               in frame 12
Are used for traffic data                                                                                       Frame
                                                                                                              duration =
                            0        1           2                12             24       25                    120ms




                                                                                                              Frame
                                                                                                            duration =
                                0        1           2        3       4   5           6    7                 60/13ms




                                                                                                                      Frame
                                                                                                                    duration =
                                                                                                                     15/26ms
             3                  57           1           26       1               57            3      8.25
        GSM Speech Coding
• Speech 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 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.

           3   57 bits   1 26   1   57 bits   3

           3   57 bits   1 26   1   57 bits   3

           3   57 bits   1 26   1   57 bits   3

           3   57 bits   1 26   1   57 bits   3

           3   57 bits   1 26   1   57 bits   3

           3   57 bits   1 26   1   57 bits   3

           3   57 bits   1 26   1   57 bits   3

           3   57 bits   1 26   1   57 bits   3
GSM Protocol Suite
                        SS
                                         HLR



             MM + CM
                                   MSC
                                   VLR

             RR
                             BSC




                  BTS

Radio interface
              Link Layer
• LAPDm is used between MS and BTS
• LAPD is used between BTS-BSC
• MTP2 is used between BSC-
  MSC/VLR/HLR
              Network Layer
• To 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 protocols
• BSSMAP between BSC and MSC
• DTAP 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.
GSM Functional Architecture and Principal Interfaces

 Mobile Application Part   A Interface


          MAP              Q931 BSSAP

          TCAP                SCCP

                           CCS7 MTP                                Um
      CCS7 SCCP


      CCS7 MTP                                                     Q.921
                                         Base Station System

                                                               Radio Interface




                                                 Q.931

                                                 Q.921



                                            A-Bis Interface
        GSM protocol layers for
        Um
             signaling             Abis                         A
 MS                   BTS                        BSC                 MSC

 CM                                                                   CM

 MM                                                                   MM

                                                       BSSAP
                                                                    BSSAP
 RR                                       RR’
             RR’            BTSM          BTSM         SS7           SS7
LAPDm        LAPDm      LAPD              LAPD

radio         radio     PCM                PCM         PCM           PCM



                              16/64 kbit/s                   64 kbit/s /
                                                             2.048 Mbit/s
    Protocols involved in the radio
               interface
•   Level 1-Physical
     – TDMA frame
     – Logical channels multiplexing
•   Level 2-LAPDm(modified from LAPD)
     – No flag
     – No error retransmission mechanism due to real time constraints
•   Level 3-Radio Interface Layer (RIL3) involves three sub layers
     – RR: paging, power control, ciphering execution, handover
     – MM: security, location IMSI attach/detach
     – CM: Call Control(CC), Supplementary Services(SS), Short
       Message Services(SMS),
    LAPDm on radio interface
• In 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
  ADDRESS CONTROL   INFORMATION 0-21 OCTETS




 SAPI

                    N(S)       N(R)
      LAPDm on radio interface
• The 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
             interface
• Level 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 HDLC
   – Radio Signaling Link (RSL)
   – Operation and Maintenance Link (OML).
• Level 3-Application Protocol
   – Radio Subsystem Management (RSM)
   – Operation and Maintenance procedure (OAM)
  Presentation of A-bis Interface
• Messages exchanges between the BTS and BSC.
   – Traffic exchanges
   – Signaling exchanges
• Physical 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/s
• Data:
   – 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
FLAG   ADRESS   CONTROL   INFORMATION 0 – 260 OCT   FCS   FLAG




  SAPI          TEI


                                     N(S)       N(R)
                     LAPD
• The 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
                               TRAU

BSC


      LAPD TS1
                         OAM


      Speech TS
                    Transcoding       Speech TS
                                                  MSC
       CCS7 TS                        CCS7 TS



      X.25 TS2                         X.25 TS2   OMC

             PCM
             LINK                     PCM
                                      LINK
          Presentation on the A-ter
                  interface
•   Signaling messages are carried on specific timeslots (TS)
     – LAPD signaling TS between the BSC and the TCU
     – SS7 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*30
     – 92 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-Interface
  BSSMAP - deals with procedures that take place logically between the BSS and
  MSC, examples:

           Trunk Maintenance, Ciphering, Handover, Voice/Data Trunk
  Assignment

  DTAP - 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
MS                                                                          NSS


CM                                                                     CM      M
                                                                               A
MM                                                                     MM      P
                 BTS                           BSC

                       O                           BSSAP                          T
R                                      O                            BSSAP
                       A                       R   D   B                          C
R                                      A               S            DTAP/
                       M                       R   T   S                          A
                                       M           A   M           BSSMAP
                                                       A                          P
                                                   P   P
                       L                   L       SCCP                SCCP   SCCP
                       A                   A
                       P                           MTP3                MTP3   MTP3
                                           P
                       D                   D       MTP2                MTP2   MTP2
                                                                              MTP1


        Um                   A bis                             A
     Interface             Interface                       Interface
      MS            BSC               MSC


                          PD=RR

                                         PD=MM

                                          TI=a
                                          TI=b
                                         PD=CC
Link: SAPI=0                        DLCI: SAPI=0

                                        TI=A
 Link: SAPI=3                       DLCI: SAPI=3
     Channel=C1   Channel ID = N1            DTAP
                                                    PD: protocol discriminator
                                    SCCP Ref=R1     TI: Transaction Identifier for
                                                        RIL3-CC protocol
                                                    DLCI: Data Link connection
                                                           Identifier
                                                    SAPI: Service Access Point
                                                          Identifier on the radio
                                                          Interface
     Channel=C2   Channel ID = N1    SCCP Ref=R2    TEI: Terminal Equipment
                                                         Identifier on the Abis I/F

                      TRX:TEI=T1

Radio Interface   Abis Interface     A Interface
               Bearer Services
• Telecommunication services to transfer data
  between access points
• Specification 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/s
      • Asynchronous: 300 - 1200 bit/s
                      Tele Services
•   Telecommunication services that enable voice communication via
    mobile phones.
•   All these basic services have to obey cellular functions, security
    measurements etc.
•   Offered services.
     – Mobile telephony
         primary goal of GSM was to enable mobile telephony offering the
         traditional bandwidth of 3.1 kHz.
     – Emergency number
         common number throughout Europe (112); Mandatory for all
         service providers; Free of charge; Connection with the highest
         priority (preemption of other connections possible).
     – Multinumbering
         several ISDN phone numbers per user possible.
          Performance characteristics of GSM
•   Communication
     – mobile, wireless communication; support for voice and data
       services
•   Total mobility
     – international access, chip-card enables use of access points of
       different providers
•   Worldwide connectivity
     – one number, the network handles localization
•   High capacity
     – better frequency efficiency, smaller cells, more customers per cell
•   High transmission quality
     – high 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
         Disadvantages of GSM
•   No full ISDN bandwidth of 64 kbit/s to the user
•   Reduced concentration while driving
•   Electromagnetic radiation
•   Abuse of private data possible
•   High complexity of the system
•   Several incompatibilities within the GSM
    standards
Thank You

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DOCUMENT INFO
Description: Overview of GSM Cellular Network and Operations