The BSC NAT/MUX is the Base Station Controller Network Adress Translation / Multiplexer.

The BSC NAT/MUX makes NAT traversal of IP-based A interfaces (between BSC and MSC) easy.

The real BSC is opening its A interface through one TCP (or SCTP) connection to the multiplexer.
The multiplexer is opening its uplink A interface to the MSC.

The multipexer is rewriting SCCP source local references (as multiple BSCs could be connected) and
thus makes all of the real BSC's connected to the multiplexer appear as one BSC to your MSC.

Furthermore, it is listening on UDP port 2427 (MGCP Gateway) and is forwarding the MGCP messages via TCP (or SCTP)
to the BSC that should handle the endpoint.