The signaling gateway (SGW) interconnects different signaling networks. For example, one of these signaling networks could use the Stream Control Transmission Protocol (SCTP)/IP-based SS7 signaling while the other uses the traditional Message Transfer Part-based SS7 signaling. The SGW can provide bidirectional gateway functions at the transport level (i.e., the SS7-based message transport would be converted to an IP-based message transport or vice versa). The gateway does not change the contents of the messages at the Application level.
The graphic illustrates a connection between the IMS, which uses SIP signaling, on the left and a circuit-switched network, which uses the ISDN User Part (ISUP), on the right. The media gateway control function (MGCF) converts the SIP signal to an ISUP signal. In the MGCF the Application Layer message is converted. Now in the MGCP we have the ISUP message being carried by the SS7 Message Transfer Part (MTP) 3 User adaptation (M3UA) layer, which is then carried by the SCTP layer over the IP backbone to the signaling gateway. Note that the SGW has a left-hand stack that matches the SCTP/IP stack and a right-hand stack that matches the traditional message transfer part of SS7. Note that the signaling gateway only converts the message transport protocols and not the application protocol. ISUP remains as ISUP from the MGCF to the circuit-switched.
Signaling Gateway Functions
The principal functions of SGWs connecting the SS7 network to the VoIP network are described below. (Other non-SS7 signaling gateways exist.)
- Transparency: The signaling gateway should be transparent to both the VoIP and the SS7 network. It should look like a pass-through device or a signaling endpoint for SS7 messages, supporting the proper application programming interfaces (API) and protocols. For the VoIP network the signaling gateway should look like either a VoIP device (supporting SIP or H.323) or an operating component of the distributed gateway (supporting SCTP and SS7oIP).
- Translation: The signaling gateway provides address translation between the addressing scheme of the SS7 (point codes) and the VoIP network (IP addresses). If the signaling gateway also acts as a signal transfer point for the SS7 network, it is required to perform global title translation functions (telephone number to point codes).
- Reliable Routing: The hallmark of the SS7 network is the reliability of its message delivery. The signaling gateway must ensure that the reliability is carried through the IP network. The reliability is ensured by SCTP’s multihoming capability. SS7oIP messages can use diverse paths through the IP network, by allowing the media gateway controller to reside on different IP subnets.