July 1999

H.323:  Forward, March!

BY DALE SKRAN

There has been a lot of milling about in the H.323 standards camp - preparation for a foray into the realm of multimedia communications over packet networks. All of this activity underscores the burgeoning interest in packet-based multimedia communications. There is a global proliferation of packet networks and a growing demand for multimedia applications. What the world has been waiting for is for the ultimate command, the "Forward, march!" so to speak.

With increasing talk of integrating the PSTN and IP networks, and with new recommendations being added to the H.323 standards to address the needs of multimedia communications across varied networks, the time for that command to be issued has finally come.

Since H.323 standards efforts began, the standards body has produced numerous recommendations that will benefit service providers and customers alike. The H.323 suite of recommendations provides VoIP equipment vendors with a single standard, which in turn allows service providers to add equipment without worrying about interoperability issues. This ease of interoperability then will attract new service providers to the market, increasing competition and driving down prices for customers.

WHAT IS COVERED BY H.323?
The H.323 standard provides for multimedia broadcasting over the Internet, audiovisual communications over ATM networks, and the provisioning of various services. It also includes recommendations for conversational voice, video, and fax transmission as well as standards for video conferencing, network gateways, voice services, and security.

H.323 grew out of efforts to standardize real-time multimedia communications across additional transport media, including packet networks. H.323 expands on an earlier standard, H.320, which addresses multimedia on ISDN transports only. This earlier standard has been widely used for video conferencing by a number of leading vendors. Now, as the basis for standardized VoIP, H.323 is gaining support from most major VoIP gateway, gatekeeper, and PC client vendors, as well as from the video conferencing industry.

SOME HISTORY
H.323 standardization has happened in a series of waves beginning in June 1996. Most recently, a major push in December 1998 resulted in numerous recommendations that now have materialized into a large number of H.323-related standards. H.323 standards efforts are ongoing, and the next wave of activity is expected to continue through February 2000.

The initial aim of H.323 was to connect LAN-based multimedia systems to network-based H.320 systems. A key requirement was minimal transcoding of audio, video, and data, which led to the adoption of a common subset of coders and protocols. Features and functions focused on supporting business conversational services rather than broadcast or uncontrolled multicast operations.
Although initially targeted for LAN use, the H.323 standard does not preclude its being used for Internet applications. The goal is support for both distributed and hierarchical call models at every level. This makes it highly flexible and suitable for a wide range of applications.

H.323, which uses ISDN-based call signaling, is recognized as a natural choice for use in public switched telephone networks (PSTNs) as well as in IP gateways. Because of this, H.323 systems not only must be able to operate on the global Internet but also must be scalable to the size of the current telephone network.

The primary elements in any H.323 system include the gatekeeper, the endpoints, and the Multipoint Control Units (MCUs). The gatekeeper is responsible for “zone management.” All endpoints in the gatekeeper’s zone must register with it. The information gathered is then used to direct calls to those devices on the packet network. This function is essential since IP addresses are typically dynamic and change with each session. The gatekeeper is also responsible for converting telephone numbers to IP addresses.

Endpoints in the H.323 system include terminals and gateways. A terminal is a client PC or Internet phone. The gatekeeper becomes aware of these endpoints via registration. A gateway endpoint is one that connects to another network, such as the PSTN. The MCU in the H.323 system provides a multimedia bridge that can mix audio, video, and data traffic and also switch video traffic.

VISIONS AND REVISIONS
In the first version of H.323, call setup included three phases. The first phase is the pre-call, in which registration, admissions, and status (RAS) messages are used to request admission. The second step is call setup using ISDN-like messages, and the final step is capability exchange, which opens logical channels for media and conference management.

A subsequent version of the standard (H.323 V2) in January 1998, added “fast connect” features to the standard. It was recognized that the highly flexible call setup procedures of the first version, while very appropriate for negotiating connections between multimedia conferencing sessions, were too slow for simple audio calls. An optional, backwards-compatible procedure was added to allow complete call setup with a single message exchange. This version also made possible “pre-granted” admissions, in which pre-authorized customers can bypass the RAS messages when making calls. For example, customers connected to a PBX in a physically secure building might be given this privilege.

The second version of the standard also allowed Real Time Protocol (RTP) to be placed directly on an ATM adaptation layer without IP overhead, provided privacy and authentication at the application level for customers unable to use transport level security such as IP Security (IPSec), and provided call transfer and call forwarding. Although it was adopted in January 1998, implementations of H.323 V2 have just begun shipping.

The third version of the H.323 standard, adopted in September 1998, added recommendations for real-time fax and the use of H.323 systems in a broadcast fashion for large announcements. They specify the building of an H.323 IP fax machine that allows initial voice conversations followed by fax exchanges. Such machines can make use of an existing H.323 call processing infrastructure (including gatekeepers) for address translation and H.323 gateways for connecting to fax machines on the PSTN. This version also contains recommendations for enabling much higher quality video in H.323 systems.

The next group of H.323 related standards consists of all the recommendations adopted in May 1999. Some of these include call connection over UDP, inter-domain communications, call hold, call park/call pickup, call waiting, and message waiting, among others. By enabling H.323 systems to use UDP as well as TCP for call signaling, these recommendations reduce call setup time, and provide less variation in call setup time due to network congestion. They also include specifications for doing a so-called “rollup” to hide employee names and private information while still allowing calls to be completed via an exchange of dial-plan information between “domains,” or groups of zones.

With its flexibility and scalability, plus its design to handle real-time, conversational voice communications, the H.323 standard is highly suited for integrating disparate networks. Service providers, carriers, and equipment vendors all stand to benefit from interoperability that adds functionality while keeping the details of transport transparent to the end user. This is exactly what H.323 is intended to do. Now: “Forward…. March!”

Dale Skran is VP of engineering, enterprise networking at Ascend Communications. Ascend Communications is a leading provider of wide area network (WAN) and Intelligent Network (IN) technologies for the next-generation public network — a packet-based infrastructure that integrates data, fax, video, and voice communications. For more information, please visit their Web site at www.ascend.com.