OpenBSC software supports various BTSs.
It exposes A-over-IP interface from ip.access, while a legacy MSC expects GSM A-interface, that is BSSAP SS7 signaling and G.711-coded voice over E1.
The are several advantages of OpenBSC for an operator:
While the goal is to interface this network segment to a legacy GSM network.
A picture below is aimed to provide some ideas how it works.
In the content of the pilot, Telscale SS7 card is connected to Alcatel S-12 in the middle of Russia, while OpenBSC and BTS are running more than 1000 km away, being interfaced over OpenVPN.