Standards evolve for IP telephony

Dec. 1, 1999

Many see the rapid growth of the Internet as an important driver for future fiber sales. This growth in the Internet implies an increasing use of Internet protocol (IP) in the network. One of the interesting applications, IP Telephony, is now drawing considerable attention from a variety of standards bodies.

The International Telecommunication Union-Telecommunication (ITU-T) and the Internet Engineering Task Force (IETF) were the first standards bodies to develop the basic standards for IP Telephony (as part of multimedia conferencing). The two groups, using significantly different approaches, each came up with a suite of standards.

The ITU-T suite is represented by H.323, an umbrella ITU-T recommendation that describes packet-based multimedia communications systems. The recommendation refers extensively to other ITU-T recommendations for certain specifics, while defining the relevant system components (e.g., multipoint conference units, gateways, terminals, and gatekeepers) and how these components interact with each other. Most important among the referred recommendations are H.225.0 and H.245, which are used between H.323 components for call and media control. Version 2 of H.323 was published in 1998 and Version 3 is planned for publication in March 2000.

Most notable in the IETF suite of standards is RFC 2543, the session initiation protocol (SIP). SIP is often perceived as a newcomer because it is not as widely implemented as H.323 to date. Depending on how it is used, SIP can either complement, or compete with, the H.323 suite. With the increasing acceptance of SIP in the industry, a new working group has been formed in the IETF to continue its development.

The public-switched telephone network is vastly different from IP networks in many aspects, such as bandwidth allocation, numbering and addressing, signaling, and voice encoding. Interworking of the two gives rise to new issues. The industry has come to an agreement that the disparities should be largely handled by a gateway device (connecting to the networks involved) that speaks and interprets all the languages (signaling and media) used in the networks. Further, the gateway device should be partitioned into subcomponents to support scalable distributed implementation: media gateway, media gateway controller, and signaling gateway. The specifics of the relevant interfaces between the subcomponents, however, are still under intense debate. In particular, the media-control interface and the signaling interface are being discussed simultaneously in the ITU-T, IETF, the Telecommunications and IP Harmonization over Networks Project of the European Telecommunication Standardization Institute (ETSI TIPHON), and the Multiservices Switching Forum (MSF).

To ensure that there is just a single standard for the media-gateway control interface in the end, the ITU-T and the IETF have agreed to develop cooperatively the protocol for the interface. When completed, the protocol will be published as an ITU-T recommendation (H.248) and an IETF RFC concurrently.

Hui-Lan Lu is a multimedia standards manager at Lucent Technologies and can be reached at (732) 949-0321 or [email protected]. She has been active in the ITU-T and IETF on the integration of the PSTN and the Internet since 1997.

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