Wireless Communications Engineering

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Wireless Communications Engineering

• Lecture 10 GSM
• Prof. Mingbo Xiao
• Dec. 2, 2004

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Basic Components of GSM
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1. Mobile Station

• MS is the physical equipment used by a GSM
  subscriber (their mobile handset).
• It comprises two parts
• a Subscriber Identity Module (SIM) and
• the Mobile Equipment (ME).

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2. Subscriber Identity Module

• (SIM) is a smart card which carries all the
  subscriber specific information used by an MS.
  Major functions are to identify the current user
  of an MS and to take part in security and
  confidentiality procedures. It also stores recent
  location data and may also store personal
  information for the user such as abbreviated
  dialling codes (telephone directory). Specific
  functions include
• Permanent storage of a subscribers International
  Mobile Subscriber Identity (IMSI) and
  Authentication key (Ki)
• Semi permanent storage of system information e.g.
  current Location Area Identity (LAI), encryption
  key Kc and lists of preferred / forbidden GSM
  networks
• Semi permanent storage of user data, telephone
  directory, short messages
• Participation in mobility procedures e.g. user
  authentication, generation of ciphering key,
  instigation of location updates.
• Protected by PIN

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3. Mobile Equipment

• (ME) provides the radio and processing needed to
  access the GSM network, plus a man machine
  interface MMI to enable the user to access
  services. Specific functions
• radio transceiving and signal processing
• radio related operations power control timing
  advance discontinuous transmission (DTX) slow
  frequency hopping (SFH).
• Call handling
• man-machine interface, display, keypad, speech
  transducers.
• interfaces to external equipment e.g. laptops /
  palmtops

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4. Base Transceiver Station

• (BTS) provides GSM radio coverage within a cell.
  It comprises radio transmitting and receiving
  equipment (including antennas) and associated
  signal processing. A sophisticated radio modem.
  Specific functions include
• antennas (with diversity)
• radio transceiving and signal processing
• sending TDMA timing advance instructions to MS
• implementation of radio related operations slow
  frequency hopping (SFH) discontinuous
  transmission (DTX) ciphering and power control.

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5. Base Station Controller

• (BSC) is a small switch with enhanced processing
  capability. It acts as a local concentrator of
  traffic and provides local switching to effect
  handoff between a number of BTSs. It manages the
  radio channel and undertakes control of a variety
  of radio related procedures ensuring that
  reliable radio links are maintained. Its
  responsibilities include
• co-ordination and control of a number of BTSs
• traffic concentration
• low level switching operations
• network management interface to all radio elements

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5. Base Station Controller 2

• radio channel management
• handoff between BTSs within its domain
• scheduling of paging request
• passing system information e.g. LAI (Location
  Area Identity) to BTSs for onward transmission
• queuing
• control of radio related operations slow
  frequency hopping (SFH) discontinuous
  transmission (DTX) ciphering and power control
• service transcoding, changing air interface rates
  to network rates e.g. converting speech from 13
  kbit/s to 64 kbit/s

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6. Mobile Switching Centre

• (MSC) is an ISDN switch with (significantly)
  enhanced processing capability. An MSC will
  parent a number of BSCs. Responsible for call
  handling of the mobile subscribers within its
  domain this includes generating call charging
  records (for billing). Specific responsibilities
  of the MSC include
• call control
• generation of call records
• supplementary service execution
• inter BSC and inter MSC handoff
• Whilst they are identified as distinct entities
  within the specifications, the Mobile-services
  Switching Centre and Visitor Location Register
  (VLR) are invariably implemented as a single
  MSC/VLR unit.

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7. Visitor Location Register

• (VLR) is an intelligent database and service
  control function. It stores (on a temporary
  basis) the information needed to handle calls set
  up or received by MSs registered with it and
  controls certain services (primarily those
  associated with outgoing calls). This includes
  their International Mobile Subscriber Identities
  (IMSI), current Location Area Identities (LAIs)
  and supplementary service entitlements.
  Responsibilities of the VLR include
• executing supplementary service programs
  (outgoing calls barred)
• initiating authentication and ciphering
• initiating paging
• mapping of various identities (MSISDN, IMSI,
  TMSI, MSRN)
• passing location information to HLR

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8. Interrogating Node

• (IN) is the target for calls bound for GSM users.
  It is responsible for determining the location of
  a called subscriber and for routeing calls
  accordingly. The IN is commonly combined with an
  MSC forming what is known as a Gateway MSC
  (GMSC).

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9. Home Location Register

• (HLR) is an intelligent database and service
  control function responsible for management of
  each subscribers records and control of certain
  services (primarily those associated with
  incoming calls). It carries subscription details
  for a subscriber and location information
  enabling the routeing of incoming calls towards
  the subscriber, i.e. the MSC/VLR currently
  serving the MS. Responsibilities of the HLR
  include
• management of service profiles
• mapping of subscriber identities (MISDN, IMSI)
• supplementary service control and profile updates
  
• execution of supplementary service logic e.g.
  incoming calls barred.
• passing subscription records to VLR

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10. Authentication Centre

• (AuC) is an intelligent database concerned with
  the regulation of access to the network ensuring
  that services can be used only by those who are
  entitled to do so and that the access is achieved
  in a secure way.
• The principle is that the AuC and the SIM have a
  unique key for every subscriber (Ki) which is
  used as the basis for generating a response
  (SRES) to a random number (RAND) generated by the
  AuC. Only the true SIM will be able to generate
  the correct response and thus gain access to the
  network.
• The AuC is generally integrated with the HLR.
• Also EIR the Equipment Identity Register -
  another database that holds a list of allowed
  equipment identities, i.e. ME numbers white /
  grey / black lists

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GSM Interfaces

• The component parts of the GSM system
  interconnect using standard interfaces. These
  allows an operator to purchase different parts of
  the system competitively, i.e. from different
  manufacturers.
• The more important interfaces are
• Um  the air interface
• A interface  between the BSC and MSC

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GSM Characteristics
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GSM Processes

• handoff
• Paging
• Location Area
• Location Update
• Location Area Message Sequence as example
• Incoming Call
• Outgoing Call

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Handoff

• Handoff is the means of maintaining a call when a
  user moves outside the coverage area of the
  serving cell.
• The call must be switched to an alternative cell
  to provide service, automatically and without
  loss of service.
• handoff is a complex process requiring
  synchronisation of events between the mobile
  station and the network.
• In particular, there is the need to route the
  call to the new cell before handoff can be
  effected whilst maintaining the old connection
  until the new connection is known to have
  succeeded.
• handoff is a time critical process requiring
  action to be taken before the existing radio link
  degrades to such an extent that the call is lost.

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Paging

• Paging is a process of broadcasting a message
  which alerts a specific mobile to take some
  action, for example if there is an incoming call
  to be received.
• If the system does not know the precise cell in
  which a mobile is located it must perform paging
  in a number of cells.
• An extreme approach would be to undertake paging
  throughout the entire coverage area of a cellular
  system whenever a mobile is to be alerted
  however, in anything but the smallest system this
  would be wasteful of valuable signalling
  capacity, particularly over the air interface.
• The problem is addressed by the use of location
  areas and location updating.

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Location Updating

• Location updating is used to reduce the area over
  which paging must be undertaken in a cellular
  system.
• Coverage area is divided up into many location
  areas.
• All cells broadcast the identity of the Location
  Area (LAI).
• Each time a mobile station observes that it has
  moved into a new location area it informs the
  network by performing a location update this
  enables the network to perform paging over a
  smaller area than would otherwise be necessary.
• In the extreme case each cell could be a location
  area, the system would know very precisely where
  a mobile was but at the expense of a very high
  level of location update signalling. As a
  compromise location areas are generally defined
  as a group of cells.

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Location Area
Public Land Mobile Network (PLMN)
MSC / VLR Area
Location Area

• GSM Network Areas...

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GSM Network Areas

• Public Land Mobile Network
• the area served by one operator, e.g. Vodafone /
  Orange etc.
• MSC / VLR Area
• a group of Location Areas served by one MSC /
  VLR, i.e. all the cells connected to that MSC /
  VLR
• Location Area
• a Group of Cells within one MSC/VLR Area
  identified by its Location Area Identity (LAI)
• Cell
• identified by its Cell Global Identity number
  (CGI) corresponds to the radio coverage are of
  one base transceiver station

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Location Update

• The MS detects that it has entered a new location
  area by comparing the last known LA (stored on
  the SIM) with the information broadcast by the
  local cell.
• The MS gains access to a radio channel and
  requests a location update.
• If the serving MSC/VLR is unchanged the network
  can immediately authenticate the MS and note the
  change of LA.
• If the MS has moved MSC/VLR, the MSC/VLR
  addresses a message to the HLR.
• The HLR notes the new location (VLR) and
  downloads security parameters to allow the
  network to authenticate the mobile, it also
  passes on subscription details of the user to the
  new VLR and informs the old VLR to delete its
  records.

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LA Update Message Sequence

• MS to NEW MSC (via BSS) location updating
  request
• New MSC to NEW VLR update location area
• New VLR to HLR update location
• HLR to OLD VLR cancel location
• OLD VLR to HLR cancel location ACK
• HLR to NEW VLR  insert subscriber data
• NEW VLR to HLR insert subscriber data ACK
• HLR to NEW VLR update location ACK
• NEW VLR to NEW MSC  update location area ACK
• NEW VLR to MS location updating accept
• ACK acknowledge

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