InterMSC Handover

An Inter-MSC hand-over occurs if a subscriber with an active/ongoing call moves between two cells that are served by different MSCs. Each MSC serves a number of BSCs which in turn serve a number of BTS. The geographic area served by one MSC is considered the "MSC coverage area". Inter-MSC hand-over hence happens at mobility of subscribers between MSC coverage areas.

From the MSC point of view, this involves transactions on both the A interface (towards the BSCs) as well as the E interface (towards other MSCs).

As inter-MSC happens between different MSCs, a MSC can assume different roles in such a hand-over scenario:

  1. The initial MSC which is serving the subscriber before any inter-MSC hand-over occurs. It is called MSC A in the relevant specs
  2. The first MSC to which the call/subscriber is first handed over (so-called basic Inter-MSC hand-over). It is generally called MSC B in the relevant specs.
  3. Any subsequent MSC to which the call/subscriber is handed over (so-called subsequent Inter-MSC hand-over). It is called MSC B' in the relevant specs.

MSC A is the anchor MSC which was serving the subscriber/call before the first hand-over. It continues to handle the Layer3 messages such as CC and MM from the MS, even after any number of hand-overs. Also, it continues to handle the termination of the voice call towards the PSTN (via MNCC in OsmoMSC). Any other MSC which may currently be serving the subscriber after a successful inter-MSC hand-over merely acts as a proxy for the BSSMAP messages, as well as for the voice communication.

Relevant specs

  • TS 23.009 -- Handover Procedures with ladder diagrams and tons of SDL diagrams
  • TS 29.002 -- MAP message encoding; can be used as guidance when specifying GSUP equivalent
  • TS 48.008 -- A[oIP] interface protocols BSSAP/BSSMAP
  • TS 29.010 -- Interworking betwen A (BSSAP) and E (MAP) interface
  • TS 49.008 -- BSSAP features supported during/after Inter-MSC HO
  • TS 23.284 -- LCLS implications of Inter-MSC HO
  • Misc: TS 24.237, TS 23.237, TS 23.231
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