SS7 "Clasic" vs SS7 over IP

SS7 "Classic"

The term SS7 classic differentiates between SS7 over IP and narrowband 64-kilobit SS7. SS7 classic is signaling for call delivery that follows a separate physical path from the bearer channel to set up calls. A Service Switching Point (SSP) communicates to another SSP, with media traffic fl owing through on a separate channel from the signaling. Service Control Points (SCP) provide services that can be delivered via signaling alone (e.g., 800 service). Service Nodes (SN) and Intelligent Peripherals can be used for delivering services that require both signaling interaction and interaction with the bearer. Voice mail, follow-me services, and prepaid service are typical SN applications. Figure 1 represents the classical Intelligent Network (IN) environment, with the ability to deploy service on SCPs and SNs/ Intelligent Peripherals.

Figure 1: SS7 “Classic”

Evolution to SS7 over IP

The Internet Engineering Task Force (IETF) is driving much of the activity to develop protocols for the evolution of SS7 to SS7 over IP. The Signaling Transport (SIGTRAN) working group is focusing on how the existing SS7 protocol will run over IP.

One approach is to attempt to enable the SS7 service levels to run over IP. The fi rst step is creating components - such as Simple Control Transport Protocol (SCTP) - to run directly over IP, thus replacing Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) to provide a reliable transport for signaling in the telephony networks.

In addition, adaptation layers adapt the generic SCTP transport capability to meet the needs of various SS7 protocols. In the case of Message Transport Protocol 2 (MTP2) peer-to-peer adaptation (M2PA), the adaptation layer adapts the SCTP generic transport to enable Message Transfer Part 3 (MTP3) to run over it. As a result, an existing SS7 link can run directly over IP with all the existing services remaining the same.

Message Transfer Protocol 3-User Adaptation layer (M3UA) is an adaptation layer that adapts the SCTP to the MTP3 boundary. Consequently, Integrated Services Digital Network User Part (ISUP) and Signaling Connection Control Part (SCCP) can run directly over IP without having to adopt an SS7 link-based topology. While M2PA requires the maintenance of the topology of SS7 and SS7 links, M3UA requires only SS7 endpoints, thus participating at only the services level rather than needing the SS7 topology.

Another evolving adaptation layer is SCCP User Adaptation Layer (SUA). Although not as mature as some of the other adaptation layers, SUA is a protocol evolving to allow Transactional Capabilities Application Part (TCAP) to run on top of SCTP. TCAP can run on top of M3UA as well, but SUA is slightly lighter weight than M3UA and SCCP as an endpoint (see Figure 2).