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					Components of cellular network architecture

     recall:                               correspondent
                           wired public
                           telephone
                           network

                   MSC                          MSC

                          MSC
                                                       MSC
                                          MSC




               different cellular networks,
               operated by different providers

                                            6: Wireless and Mobile Networks   6-1
Handling mobility in cellular networks

   home network: network of cellular provider you subscribe to
    (e.g., Sprint PCS, Verizon)
        home location register (HLR): database in home network
      containing permanent cell phone #, profile information
      (services, preferences, billing), information about
      current location (could be in another network)
 visited network: network in which mobile currently resides
    visitor location register (VLR): database with entry for
      each user currently in network
    A VLR is co-located with the MSC
    could be home network




                                          6: Wireless and Mobile Networks   6-2
Mobile register with a visited network

   When a mobile enters some area covered by a new
    area
       Mobile registers with a visited network through
        exchanging signaling messages between the mobile and
        the VLR
       The visited VLR sends message to the mobile HLR
        stating the roaming number
       visitor location register (VLR) also obtains subscriber
        information from the HLR




                                          6: Wireless and Mobile Networks   6-3
  GSM: indirect routing to mobile
                                             home
                             HLR
                                             network                       correspondent
                                   2
                                         home
                                         Mobile
home MSC consults HLR,                  Switching
gets roaming number                      Center
(like COA in Mobile IP) of
mobile in visited network
                                                                    1               call routed
                                                                                    to home network
                                                            3           Public
                                       VLR                              switched
                                              Mobile
                                                                        telephone
                                             Switching
                                                                        network
                                              Center
                                   4
                                                            home MSC sets up 2nd leg of call
                                                            to MSC in visited network
               mobile
               user                                      MSC in visited network completes
                                        visited          call through base station to mobile
                                        network
                                                                6: Wireless and Mobile Networks   6-4
  GSM: handoff with common MSC

                                          Handoff goal: route call via
                                           new base station (without
                                           interruption) during a call
          VLR Mobile                      reasons for handoff:
               Switching                       stronger signal to/from new
                Center
                                                BSS (continuing connectivity,
                                                less battery drain)
          old           new
                     routing
                                               load balance: free up channel
          routing
old BSS
                                                in current BSS
                               new BSS
                                               GSM doesn’t mandate why to
                                                perform handoff (policy), only
                                                how (mechanism)
                                          handoff initiated by old BSS



                                                 6: Wireless and Mobile Networks   6-5
   GSM: handoff with common MSC
                                           0. a mobile periodically measures the strength of a
                                             beacon signal from its current base station as well
                                             as beacon signals from nearby base stations.
                                             These measurements are reported once or twice
                                             a second to the mobile’s current base station
              VLR Mobile                 1. old BSS informs MSC of impending handoff,
                   Switching                 provides list of 1+ new BSSs
                    Center 2             2. MSC sets up path (allocates resources) to new
                      4                      BSS
          1
                           7             3. new BSS allocates radio channel for use by mobile
               8
                                    3    4. new BSS signals MSC, old BSS: ready
old BSS   5                    6
                                 new BSS 5. old BSS tells mobile: perform handoff to new BSS
                                         6. mobile, new BSS signal to activate new channel
                                         7. mobile signals via new BSS to MSC: handoff
                                             complete. MSC reroutes call
                                         8 MSC-old-BSS resources released



                                                           6: Wireless and Mobile Networks   6-6
GSM: handoff between MSCs

                                          anchor MSC: first MSC
                                           visited during call
home network
                       correspondent          call remains routed
 Home
 MSC                                           through anchor MSC
                                        new MSCs add on to end
anchor MSC
                    PSTN                 of MSC chain as mobile
                                         moves to new MSC
        MSC

                               MSC
                                        IS-41 allows optional
                MSC

                                         path minimization step
                                         to shorten multi-MSC
                                         chain
               (a) before handoff


                                               6: Wireless and Mobile Networks   6-7
GSM: handoff between MSCs

                                        optional path
                                         minimization step to
home network
                       correspondent     shorten multi-MSC chain
 Home
 MSC


anchor MSC
                    PSTN
        MSC

                MSC            MSC




               (b) after handoff


                                           6: Wireless and Mobile Networks   6-8

				
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posted:12/12/2011
language:English
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