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A-Interface

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.

Transport

TDM

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.

SCCPLite

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.

AoIP

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

To capture protocol traffic with Wireshark when connected via OsmoSTP use port 2905 capture filter and bssap display filter.

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