The A interface exists between the BSC and MSC inside a GSM network. It uses SS7 as transport, particularly connection-oriented SCCP.

The main specifications for A are 3GPP TS 48.006 and 48.008.



Traditionally, it is spoken oder TDM links such as E1/T1. Voice channels are transcoded at the BSC and transmitted like ISDN voice calls in G.711 on 64kBps TDM slots.

Osmocom doesn't support this out of the box, a media gateway would be required.


Long before 3GPP had sang standard for transporting A over IP, some companies implemented what is called SCCPLite. This uses the IPA multiplex over TCP as transport for SCCP, together with MGCP for configuring RTP media flow IP/Port information.

Osmocom implements this so far only in osmo-bsc-sccplite and osmo-bsc_nat.


Later, 3GPP standardized a method on how to transport A interfaces over IP and called it AoIP. It uses real SCCP over M3UA/SCTP/IP for the signalling plane and replaces the G.711 TDM user plane with using untranscoded air interface codec payload in RTP/UDP/IP. See 3GPP TS 48.103 for the user plane (RTP) handling.

The BSSMAP protocol was extended to be able to negotiate per-call IP/Port information between BSC and MSC.

Osmocom OsmoBSC works with 3GPP AoIP and has been tested to interoperate with several MSCs, not just OsmoMSC

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