j'c'westlakestaffs'ac'ukk'k'Chaistaffs'ac'uk

1
GSM (Global Systems for Mobile Communications)

• You will need access to this online dictionary
  for the numerous acronyms used throughout the
  slides!
• http//www.mpirical.com/companion/mpirical_compani
  on.htmlhttp//www.mpirical.com/companion/GSM/BCCH
  .htm
• Provide ubiquitous network connection
• Anybody 500 million users
• Anywhere 168 countries
• Any media voice, messaging, data, multimedia
• The main services are
• Voice, 3.1 kHz
• Data, 9.6 kbps with and w/o error correction
• SMS-messages

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GSM (Global Systems for Mobile Communications)
Network Management Center
Home Location Register
AuC
BTS Base Transceiver Station AuC
Authentication Center OMC Operation and
Maintenance Center PSTN Public Switched
Telephone Network ME Mobile Equipment
Equipment ID
OMC
Visitor Location Register
Subscriber Identity Module
BTS
Mobile switching center
ME
Data communication network
Subscriber Identity Module
Base station controller
BTS
ME
Subscriber Identity Module
Source Stallings Source Mehrotra
BTS
ME
PSTN
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Base Station Subsystem

• Transcoding Rate and Adaptation Unit (TRAU)
• Performs coding between the 64 kpbs PCM coding
  used in the backbone network and the 13 kbps
  coding used for the Mobile Station
• Base Station Controller (BSC)
• Controls the channel (time slot) allocation
  implemented by the BTSes
• Manages the handoffs within the BSS area
• Knows which mobile stations are within the cell
  and informs the MSC/VLR
• Does now know the exact location of a MS before a
  call is made
• Base Transceiver System (BTS)
• Controls several transmitters
• Each transmitter has 8 time slots, some used for
  signaling, on a specific frequency
• Maximum amount of frequencies and transmitters in
  a cell is 6
• Maximum capacity of a cell is 45 calls ( 3 time
  slots for signaling)

4
Network Switching Subsystem

• The backbone of a GSM network is an ordinary
  telephone network with some added capabilities
• Mobile Switching Center (MSC)
• An ISDN exchange with additional capabilities to
  support mobile communications
• Visitor Location Register (VLR)
• A database, part of the MSC
• Contains the location of the active Mobile
  Stations
• Gateway Mobile Switching Center (GMSC)
• Links the system to PSTN and other operators
• Home Location Register (HLR)
• Contains subscriber information, including
  authentication information in Authentication
  Center (AuC)
• Equipment Identity Register (EIR)
• International Mobile Station Equipment Identity
  (IMEI) codes
• for e.g. blacklisting stolen phones

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Home Location Register

• One database per operator
• Contains all the permanent subscriber information
• MSISDN (Mobile Subscriber ISDN number) is the
  telephone number of the subscriber
• IMSI code is used to link the MSISDN number to
  the subscriber's SIM (Subscriber Identity Module)
• International Mobile Subscriber Identity (IMSI)
  is the 15 digit code used to identify the
  subscriber
• It incorporates a country and operator code
• Charging information
• Services available to the customer
• Also the subscriber's present Location Area Code,
  which refers to the MSC, which can connect to the
  MS

6
Mobile System

• MS is the user's handset and has two parts
• Mobile Equipment
• Radio equipment
• User interface
• Processing capability and memory required for
  various tasks
• Call signaling
• Encryption
• SMS messages
• Equipment IMEI number
• Subscriber Identity Module

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Subscriber Identity Module

• A small smart card
• Encryption codes needed to identify the
  subscriber
• Subscriber IMSI number
• Subscriber's own information (telephone
  directory)
• Third party applications (banking etc.)
• Can also be used in other systems besides GSM
• e.g. some WLAN access points accept SIM based
  user authentication

8
Mobile Registration

• Takes place whenever a mobile is turned ON
• After turning ON, it passively synchs with the
  nearest BS
• Frequency, frame and bit timings
• After this registration takes place
• Simple if MS is turned ON in the same area
• Complex if MS is turned ON in a new MSC area
  requiring changes to HLR and VLR

9
Call Establishment Mobile originated call
10
Call Establishment Mobile terminated call
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Handoff or handover

• Internal Handoff
• Handoff between two BTSs of the same BSS
• External Handoff
• Handoff between two BTSs of different BSS (base
  station system) but the same MSC (mobile
  switching centre)
• Handoff is also possible between two BSS under
  different MSCs
• Old MSC continues to handle the call
• Seldom handoff is possible between two MSCs in
  different networks (different providers)

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Handoff

• BTS (Base Transceiver Station) provides MS
  (mobile station) with list of available channels
  in neighboring cells via BCCH (Broadcast Control
  Channel)
• MS monitors RSS (received signal strength) from
  the BCCHs of the neighboring cells and reports it
  to MSC using SACCH
• Mobile assisted handoff is followed in GSM
• BTS also monitors RSS of MS
• Proprietary algorithms are used to decide when to
  handoff
• MSC negotiates new channel with the new BSS and
  indicates to the MS to initiate a handoff
• Upon completion of handoff MS informs the MSC
• Old BSS is informed to release the resources

13
GSM Protocol Architecture
14
GSM Physical packet bursts

• GSM uses 890-915 MHz for uplink and 935-960 MHz
  for downlink
• Each 245 MHz band is divided into 124 channel
• Each channel occupies 200 kHz with 100 kHz guard
  band at the edges
• Data rate of each carries is 270.833 kbps
• Packet burst duration is 577 usec or 156.25 bit
  times
• Four types of packet bursts
• Normal burst
• Frequency correction burst
• Synchronization burst
• Random access burst

15
GSM Physical packet bursts
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GSM Physical packet bursts

• Frequency correction burst
• Used for synchronizing the MS to the carrier
  frequency
• Synchronization burst
• Used for synchronizing the MS to the bit and
  frame timings
• MS also learn about the network identity
• Random Access burst
• Used during registration and for initiating a
  call
• Normal burst
• Used for sending traffic (voice and data)

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GSM Voice and Data traffic packets
18
Frame interleaving in GSM