essages over an IP network to an MGC or another SGW. Because of its critical role in integrated voice networks, SGWs are often deployed in groups of two or more to ensure high availability. The MGC uses the signalling information from the SGW (e.g. ISUP, SIP, H.323, BICC) to control one or more edge nodes. Control Network Signalling module GSTN side Media gateway controller IP side Control protocol Voice stream Voice stream Media gateway Figure 14.21 Gateway architecture 14.9.2 TRANSPORT PART MAPPING When using IP as a transport technology there is a need to transfer SS7 signalling (BICC/ISUP, MAP, etc.) over IP. VoIP networks carry SS7-over-IP using protocols defined by the Signalling Transport (Sigtran) working group of the IETF. The SS7 over IP stack is shown in Figures 14.22 and 14.23. See also Figure 14.13. The stream control transmission protocol (SCTP) is designed to transport PSTN/ISDN/PLMN signalling messages over IP networks, but is capable of broader applications. SCTP is an application-level datagram transfer protocol. Appl. Figure 14.22 Evolution of SS7 over IP Understanding Changing Telecommunications - Building a Successful Telecom Business MAP/CAP/ N-ISUP MAP/CAP N-ISUP Figure 14.23 CS Protocol stacks for signalling over IP M3UA Protocol Layer The message transfer protocol 3-user adaptation (M3UA) protocol layer implements the SS7-like signalling network. It provides the same connectionless service to its users as MTP3. M3UA establishes connectivity between adjacent signalling nodes. M3UA adapts the SCTP to the MTP level 3. Consequently ISUP protocol SCCP can run directly over IP without having to adopt an SS7 topology. While M2PA requires the maintenance of the topology of SS7 and SS7 links, M3UA doesn't. M3UA only requires SS7 endpoints, thus participating only at the services level. The message transport protocol 2, peer-to-peer adaptation (M2PA) layer matches the SCTP generic transport to a primitive level that allows MTP level 3 to run over it. Another evolving adaptation layer is the SCCP us