re generated from T.38 gateway, and all mes- sages are sent to the SIP proxy server on its network. The path between the T.38 gateway and the SIP proxy server contains an IP router. Although this is not necessary, it is however done to maintain compati- bility with the next two network models where IP routers play an impor- tant role. The SIP proxy server of the sender's network communicates with that of the recipient's network and initially sends SIP messages to set up a call. Once that is done, the T.38 gateway starts fax transmission. The originating T.38 gateway starts sending fax packets to the SIP proxy server. That is, the SIP proxy server routes and interprets SIP messages. It also routes IFP packets without actually interpreting them. This sce- nario is likely if the SIP proxy server is implemented on a router within the network. In such a case, it interprets SIP messages, translating them and maintaining the state for them. All other messages are not inter- preted by it but merely routed. This model is shown in Figure 22.11 [11]. 2. The Network Model 2 is the same network as model 1 but a different setup. It not only includes the components and links of the first model, but also has a direct link between the IP routers (the so-called proxy- bypass approach). That is, all the SIP signaling is carried out on the path that traverses the two SIP proxy servers. However, once the call is setup, all the fax data packet transmission is done through IP routers and the proxy-bypass links only. This mimics a network where signaling travels on separate links, and data is sent across another set of links. This is possibly because data does not need to go through SIP proxy servers or other network entities. This frees up resources at such enti- ties and segregates signaling from data transmission, much like PSTN © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security T.38 G/W IP Router SIP Proxy SIP and SIP Proxy IP Router T.38 G/W messages FIGURE 22.11 SIP proxies routing SIP and fax