). Today he has connected his new gateway to the small IP network (a LAN) that he has installed at Local Figure 7-9 exchange terminal Bob's old configuration with his ISDN phone. DSS-1 Figure 7-10 Bob's new configuration with his ISDN/SIP ISDN/SIP Local gateway. exchange gateway IPnetwork DSS-1 SIP phone Chapter 7 home. The payoff is that Bob can now receive and make calls from his com- puter or from his SIP phone when he is at home. He has upgraded his res- idential communications to bring them in line with the business communications he's come to rely on-all without much hassle at all. Let's look at another case. Laura's office has several phones connected to a Private Branch Exchange (PBX) (Figure 7-11). Her boss wants to switch telephone service providers because a new provider is offering more ser- vices for cheaper rates. The problem is that all of the employees in Laura's company are used to working with their traditional phones (which, inci- dentally, they like just fine) and don't want to migrate to SIP phones. To alleviate the need for users to change their behavior and management to invest in new technology, the new service provider installed a gateway, and now company staffers are using a SIP network without really noticing the change (Figure 7-12). They use the same phones, dial numbers in the same way, and receive calls just as before. Figure 7-11 Laura's office with its traditional phones. Building Applications with the SIP Toolkit Figure 7-12 Laura's office with its new PBX and still with its traditional phones. SIP-enabled IP network High-Capacity Gateways As opposed to single-box, low-capacity gateways, high-capacity gateways are usually distributed. That is, different parts of the gateway performing different functions are kept separate. Figure 7-13 shows the most common architecture for a high-capacity gateway: The Signalling Gateway (SG) receives signalling from the PSTN side and encapsulates it over IP (and vice versa). The transport protocol used for this purpose is usually the Stream Contr