ct LAN- based multimedia systems to network-based multimedia systems. It defined a network architecture that included gatekeepers, which per- formed zone management and address conversion; endpoints, which were terminals and gateway devices; and multimedia control units, which served as bridges between multimedia types. H.323 has been rolled out in four phases. Phase one defined a three- stage call setup process: a precall step, which performed user registra- tion, connection admission, and exchange of status messages required for call setup; the actual call setup process, which used messages similar to ISDN's Q.931; and finally, a capability exchange stage, which established a logical communications channel between the communicating devices and identified conference management details. Phase two allowed for the use of Real-Time Transport Protocol (RTP) over ATM, which eliminated the added redundancy of IP and also pro- vided for privacy and authentication, as well as greatly demanded tele- phony features such as call transfer and call forwarding. RTP has an 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 Telephony added advantage: When errors result in packet loss, RTP does not request resends of those packets, thus providing for real-time processing of application-related content. No delays result from errors. Phase three added the ability to transmit real-time fax after estab- lishing a voice connection, and phase four, released in May 1999, added call connection over UDP, which significantly reduced call setup time; inter-zone communications; call hold, park, and pickup; and call and message waiting features. This last phase bridged the considerable gap between "IP voice" and "IP telephony." Several Internet telephony interoperability concerns are addressed by H.323. These include gateway-to-gateway interoperability, which ensu