Cellular Communications_3_ by hcj


Groupe Speciale Mobile/Global
System for Mobile
GSM Air Interface
   TDMA with FDD

   200Khz channels with 200KHz guard bands
   GSM 900 has 124 carriers
   GMSK modulation, 270kbps per carrier
   Up to 8 users, 24.8kbps per user
   FEC reduces to 13kbps per user for voice
Physical Channel
   RF carrier divided into 8 slots, numbered 0..7
   Timeslots carrying data
     At   most 8 traffic channels
   Control messages
     At   least 1 control channels
   More control (logical) channels
   Packed into RF carrier
Single Burst/Slot
Frame Structure
Logical Channel List
   Traffic          TCH/F: Full-rate Traffic Channel       Two-way
  channels          TCH/H: Half-rate Traffic Channel
                     FCCH: Frequency correction
             BCH     SCH: Synchronization
                     BCCH: Broadcast control
  Signaling          PCH: Paging                           mobile
  channel            AGCH: Access grant
                     RACH: Random access
                    SDCCH: Stand-alone dedicated control
                    SACCH: Slow associated control
                    FACCH: Fast associated control
Broadcast Control Channels
Common Control Channels
Dedicated Control Channels
Channel Coding
     What is a location area (LA)?

        A powered-on mobile is informed of an incoming call by a
         paging message sent over the PAGCH channel of a cell
        One extreme is to page every cell in the network for each call
         - a waste of radio bandwidth
        Other extreme is to have a mobile send location updates at
         the cell level. Paging cut to 1 cell, but large number of location
         updating messages.
        Hence, in GSM, cells are grouped into Location Areas –
         updates sent only when LA is changed; paging message sent to
         all cells in last known LA
International Mobile Station Equipment
Identity (IMEI)

   Type Approval Code (TAC): 6 decimal places,
    centrally assigned.
   Final Assembly Code (FAC): 6 decimal places,
    assigned by the manufacturer.
   Serial Number (SNR): 6 decimal places, assigned
    by the manufacturer.
   Spare (SP): 1 decimal place.
International Mobile Subscriber
Identity ( IMSI)
   Mobile Country Code (MCC): 3 decimal places,
    internationally standardized.
   Mobile Network Code (MNC): 2 decimal places,
    for unique identification of mobile network within
    the country.
   Mobile Subscriber Identification Number
    (MSIN): Maximum 10 decimal places, identification
    number of the subscriber in the home mobile
Mobile Subscriber ISDN Number (
   Country Code (CC) : Up to 3 decimal places.
   National Destination Code (NDC): Typically 2-3
    decimal places.
   Subscriber Number (SN): Maximum 10 decimal
   SMS allowed
       Two way communications of the text messages
       Maximum character length of 160 characters
           This can change though depending on the operator or the character set used
       Character sets supported are
           ASCII + additional European characters
           Unicode
   First Text
       Was sent in December 1992, to a Vodafone device
           Sent by Neil Papworth, saying “Merry Christmas”
   Standard
       Defined by ETSI and is known as “GSM 03.40”

   SMS Continued
     The success is SMS was never planned for!
     It was only ever intended as the Pager replacement, with limited
           This will explain some of the design decisions made
   GSM
       At a defined time interval in GSM all devices will listen to a
           This is when a Digital Control Channel (DCCH) packet of information is being
            sent across the network.
                 These DCCH packets are used to transfer essential information into the devices.
                 Information like a call is in coming
                 Paging signals from the Base stations, to work out if a handover is needed
                 One of these packet formats is called SMS point to point messaging, Paging,
                  access control channel (SPACH)
                      This message type can be used to carry a text message.

                 Advantage of this method is a text message can still be delivered during a phone

   SMS Packet format
       All data is transferred in a single DCCH SPACH packet

SCA         Service Centre       MR    Message Reference     PID   Protocol Identifier
PDU Type    Protocol Data Unit   DA    Destination Address   DCS   Data Coding Scheme
VP          Validity Period      UDL   User Data Length      UD    User Data
GPRS: General Packet Radio Service

   GSM data
     CSD:  circuit switched data
     Max 14kbps
     Similar to voice call
     Inefficient usage of spectrum

   GPRS packet-based service
     Upgrade  of infrastructure
     GGSN is a gateway to outside world
     SGSN is a gateway within the network
GPRS architecture
GPRS handset classes
   Class A

    Class A terminals have 2 transceivers which allow them to send / receive
    data and voice at the
    same time. This class of device takes full advantage of GPRS and GSM.
    You can be taking a call and receiving data all at the same time.

   Class B
    Class B devices can send / receive data or voice but not both at the same
    time. Generally if you are using GPRS and you receive a voice call you will
    get an option to answer the call or carry on.

   Class C
    This device only allows one means of connectivity. An example would be a
    GPRS data card in a laptop.
    Enhanced Data GSM Environment
   Packet switched
   Upgrades the modulation scheme
       From GMSK to 8-PSK
       Maximum speed ~59 Kb/sec per time slot, ~473.6 Kb/sec for all 8 time slots
       Variable data rate – depending on the channel conditions
   Defines several different classes of service and mobile terminals

                                                      EDGE enabled data mobile

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    Practically achievable data rates
   Theoretical rates are constrained by mobile
    power and processing capabilities
   Most mobiles support less than the maximum
    allowed by standard
                         Practically achievable data

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 GSM Migration Towards 3G
1.   High speed circuits      Data Rate                                                               2Mb/sec

     switched data
     (HSCSD)                                                                384 Kb/sec

2.   Packet switched data                                 GPRS
                                                        114 Kb/sec
3.   Integrated packet                     64 Kb/sec

     services – possibly
                                                       HSCSD   - High Speed Circuit Switched Data
                                                       GPRS    - General Packet Radio System
                               GSM 2+                  EDGE    - Enhanced Data GSM Environment
     under different access   9.6 Kb/sec               UMTS    - Universal Mobile Telephone Service

     scheme (UMTS)

                                             1Q            2Q                  3Q                         4Q
                                            2000          2000                2001                       2002

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