E). For example an ACM (acknowledgement) message is equal to SIP 183. There are different ways to arrange the MGW, the MGC and the signalling gateway (SGW). See Figure 14.20. The SGW and the MGC can be combined as shown in Figure 14.21. See the MGC-SGW combined version in Figure 14.22. This approach allows the installation of one SGW/MGC that controls several distributed MGWs. The SGW role is to establish and join one or more IP-SS7 links and to maintain the state of the connection between the two networks. Maintaining the state implies sequence Understanding Changing Telecommunications - Building a Successful Telecom Business SS7 ISUP ISUP/IP SIGTRAN Softswitch PSTN/ISDN switch network H.248 E1 speech Voice channels gateway RTP/UDP/IP Figure 14.19 A typical interface between VoIP and PSTN/ISDN Service network Featuer Policy Appl. Appl. Appl. server server server server server server server Open API Service platform Media Server Server ISUP/IP (SIGTRAN) ISUP/MT H.248 IP network Figure 14.20 A 'softswitch' part connected to an all SIP/IP network. Support servers at the top numbering, acknowledgements, retransmissions and notifications if anything is out of sequence. IP congestion control, the detection of session failures and security are other important functions performed by the SGW. An SGW also contains signal transfer point (STP) functionality, in order to convey SS7 messages over the message transfer part (MTP3 and MTP3) in TDM and ATM networks as well as over the stream control transport protocol (SCTP) in IP networks. The SGW may terminate SS7 signalling or translate and relay messages 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 gat