pter compares SIP to another IP telephony signaling protocol: the ITU recommendation H.323, entitled "Packet-Based Multimedia Communication." H.323 is introduced and explained using a simple call flow example. H.323 and SIP are then compared. Introduction to H.323 H.323 [1] is an umbrella recommendation that covers all aspects of multimedia communication over packet networks. It is part of the H.32x series1 of protocols that describes multimedia communication over ISDN, broadband (ATM), 2 telephone (PSTN), and packet (IP) networks, as shown in Table 8.1. Originally developed for video conferencing over a single LAN segment, the protocol has been extended to cover the general problem of telephony over the Internet. The first version was approved by the ITU in 1996 and was adopted by early IP telephony networks because there were no other standards. Version 2 was adopted in 1998 to fix some of the problems and limitations in version 1. Ver- sion 3 was adopted in 1999 and includes modifications and extensions to enable 1. In this chapter, the use of an X instead of a digit does not imply that all digits (0-9) in the range are included. In this case H.32x does not include H.325 to H.329, which have yet to be defined. 2. In this context, broadband means transported over an asynchronous transfer mode network. In anticipation of the universal deployment of ATM networks by carriers, the ITU developed a suite of protocols to support conventional telephony over ATM networks. For example, Q.2931 is the extension of Q.931 ISDN over ATM. Today, the term is used to mean high bandwidth connections-faster than modem speeds. SIP: Understanding the Session Initiation Protocol Table 8.1 ITU H.32x Family of Standards Protocol Title H.320 Communication over ISDN networks H.321 Communication over broadband ISDN (ATM) networks H.322 Communication over LANs with guaranteed QoS H.323 Communication over LANs with nonguaranteed QoS (IP) H.324 Communication over PSTN (V.34 modems) communications over a larger network. Version 4 was adopted 