Subsystems are a collection of modules, protocol stacks, and/or APIs that make up a more complete entity than a protocol stack.
An interworking between TeleSoft's Compact SIP software and PSTN protocols like ISDN PRI and BRI, QSIG, and T1 RBS and E1 R2 CAS. This allows you to add a SIP interface to your ISDN PBX or an ISDN interface to your SIP device. This software development kit is also a good start on a standalone gateway.