res that telephony can be accomplished between different vendors' gateways; gatekeeper-to-gatekeeper interoperability, which does the same thing for different vendors' gatekeeper devices; and finally, gateway-to-gatekeeper interoperability, which completes the interoper- ability picture. Session Initialization Protocol (SIP) While H.323 has its share of supporters, it is slowly being edged out of the limelight by the IETF's Session Initialization Protocol (SIP). SIP sup- porters claim that H.323 is far too complex and rigid to serve as a stan- dard for basic telephony setup requirements, arguing that SIP, which is architecturally simpler and imminently extensible, is a better choice. In reality H.323 is an "umbrella standard" that includes (among others) H.225 for call handling, H.245 for call control, G.711 and G.721 for CODEC definitions, and T.120 for data conferencing. Originally created as a technique for transporting multimedia traffic over a local area net- work, gatekeeper functions have been added that allow LAN traffic and LAN capacity to be monitored so that calls are established only if ade- quate capacity is available on the network. Later, the Gatekeeper Routed Model was added, which allowed the gatekeeper to play an active role in the actual call setup process. This meant that H.323 had migrated from being purely a peer-to-peer protocol to having a more traditional, hierar- chical design. The greatest advantage that H.323 offers is maturity. It has been avail- able for some time now, and while robust and full-featured, was not orig- inally designed to serve as a peer-to-peer protocol. Its maturity, therefore, is not enough to carry it. It currently lacks a network-to-network Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Chapter 3 interface, nor does it adequately support congestion control. This is not generally a 