The cellmgr_ng project
The cellmgr_ng was initially written to allow to hook a BTS/BSC behind a traditional E1 link with MTPL1, MTPL2, MTPL3 to a soft switch supporting the the ipaccess protocol. The MTPL1 and MTPL2 is handled by a vendor solution right now, everything starting from MTPL3 is available in source. The code supports interfacing with the hardware encoders and can send and receive RTP with AMR and is controlled with the MGCP protocol.
This application implements forwarding of SCCP from the MTPL3 linkset to the IPA protocol, it supports connection tracking to work around implementation problems of various BSCs and to handle the event of network disconnection without taking the MTP Linkset down.
A simple version of the cellmgr_ng that just relays between the MTPL3 linkset and the connection to a MSC or similiar system.
This a Signalling Transfer Point or a Media Gateway. It can handle a MTP L3 linkset and a M2UA connection. Right now it does not support fail over in M2UA but this can be added easily. It has basic support for ISUP blocking/unblocking/reset.
Use the OpenBSC MGCP Protocol and Network implementation and interface with the hardware to provide the gateway from E1/T1 to IP/RTP.