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.