Signaling Transport Working Group
The IETF Signaling Transport Working Group (sigtran) was formed to address the transport of packet-based PSTN signaling—specifically SS7 signaling—over IP networks. The group’s principal challenge was to preserve the functional and performance requirements of PSTN signaling, while using an IP infrastructure. The working group has completed its assigned tasks and has published several RFCs and drafts.
The principal RFCs generated by the working group are described below.
- RFC 2719, Architectural Framework for Signaling Transport: Provides the overall design of the signaling gateways and the protocols
- RFC 2960, Stream Control Transmission Protocol (SCTP): Details this transport protocol and provides scenarios for implementations
- RFC 3331, Signaling System 7 (SS7) MTP2 User Adaptation (MTUA) Layer: Details the operation of the adaptation protocol with scenarios and packet formats
- RFC 3332, SS7 MTP3 User Adaptation (M3UA) Layer: Details the operation of the adaptation protocol with scenarios and packet formats
Sigtran Protocol Stack
The diagram attempts to represent the protocol stack defined by the sigtran working group. It shows a dual stack with a common Application Layer section. The native stack includes the MTP and SCCP protocols. The lower layers of the SS7oIP stack include the IP and Network Interface Layer from a standard TCP/IP stack. At the Transport Layer is a new protocol, the SCTP, a modified version of TCP. Above SCTP is a series of adaptation protocols.
Above SCTP is where the confusion is created; the nice lines of the stack do not reflect the complexity of the actual implementation. The various adaptation layers actually emulate MTP2, MTP3, SCCP and ISDN’s Q.921 functions. So, for an accurate functional representation, MTP3 should be found on top of the M2PA and M2UA protocols, SCCP or ISUP should be found on top of the M3UA, TCAP should be found on top of the SUA, and ISDN’s Q.931 messages should be found on top of the IUA protocol. Doing so in a single diagram would cause more confusion than clarification.
Using multiple layers of adaptation allow multiple implementations within the gateway vendor community. One vendor could implement only IP/SCTP/M2UA, while maintaining SS7’s MTP3 and above protocols. Another could implement a combination of M3UA and SUA for transport of TCAP and ISUP messaging.