February 1998

BY MICHELLE BLANK

In May 1996, the International Telecommunications Union (ITU) finalized the first version of a new standard for multimedia communications over packet-based networks. The H.323 recommendation, as it is known in the engineering community, is the nucleus of the burgeoning H.323 market. The first version of the recommendation has quickly been reduced to practice by software developers — first, in the form of the H.323 protocol stack, and next, in the form of the H.323 gatekeeper (which contains an embedded H.323 stack). These are the core software building blocks with which H.323 developers create H.323-compliant hardware products and applications. Developers of these enabling software are evolving their protocol stacks and gatekeeper software to implement version 2 of the H.323 standard which was due for final approval in late January of this year.

H.323 ENTITIES
H.323 “entities,” as they are called in the recommendation, include: terminals, gateways, multipoint control units, and gatekeepers.

Terminals: Voice-only and/or multimedia (voice/data/video) “conferenc-ingready” terminals such as PCs and IP (Internet Protocol) phones with voice and/or video capabilities.

Gateways: Voice-only and/or multimedia (voice/data/video) internetworking gateways for connecting IP packetbased networks and circuitswitched networks.

Multipoint Control Units (MCUs): For conferences between 3 or more users.

Gatekeepers: For “must have” network management and virtual PBX like capabilities. When such products are strategically combined and integrated, a whole new generation of H.323-based IP telephony and multimedia conferencing applications are possible.

FINANCIAL BENEFITS
Computer-telephony application deployers are being driven to adopt H.323 solutions rapidly. Why? Because when it comes to managing data/voice/ video simultaneously, the allure of a ubiquitous IP network capable of handling data/voice/video is money. The current situation, where voice and video travel on a circuit-switched network while data and applications reside on IP networks (LAN/intranet/Internet), assures appropriate QoS and security controls for mixed media. But this state of affairs requires the continuous and costly management of both infrastructures.

If an IP network can provide the same quality of service (QoS) for voice and video as does the circuitswitched network, while also carrying data traffic and supporting data applications, the total cost of network operations for an enterprise will be significantly lower than the cost of operating multiple networks.

In addition to the possibility of dramatic cost savings resulting from the integration of multimedia in today’s computer telephony applications, many companies see a positive cash flow emerging from new revenue generation opportunities. Imagine a business traveler using an automated airline ticket-ing kiosk to change some aspect of his or her travel plans. If the transaction requires the assistance of an airline agent, (e.g., flight change between airlines) the appropriate agent could be brought on-screen. Both the customer and the agent would have access to the travel itinerary, credit card information, and other pertinent options.

In a business help desk scenario, a user in difficulty could access live help on the screen when other options seem futile. An H.323 call center application would establish an IP telephony connection between the user and an expert with specific application knowledge. The application might even be capable of identifying and routing the call to an agent that has supported the user in the past. The notified technical support agent would have a complete record of the user’s history and be able to share the same screen with the user while providing interactive assistance. CTI implementations with multimedia will rely heavily on the H.323 gatekeeper because it is the industry standard mechanism that is used for:

• Call control and call routing.
• Basic telephony services such as directory services and PBX functions (e.g., call transfer, call forwarding).
• Controlling H.323 bandwidth usage to provide QoS and protect other critical network applications from H.323 traffic.
• Total network usage control.
• System administration and security policies.

Although the H.323 recommendation describes a gatekeeper as an optional component, without the benefits of a gatekeeper, CTI developers and deployers will not be able to take advantage of the full power and spectrum of services envisioned by the creators of the H.323 standard for gate-keeperenabled IP telephony and multi-media conferencing applications. To understand the true potential of H.323compliant solutions in business, CTI developers and deployers should familiarize themselves with the fundamentals of H.323 gatekeepers.

ZONES — THE DOMAIN OF GATEKEEPERS
The H.323 standard is implemented in production networks through zones. An H.323 zone is the collection of all H.323 terminals, gateways, and MCUs managed by a single gatekeeper. A zone must contain at least one endpoint, and may or may not have any gateways or MCUs. A zone must have one and only one gatekeeper. Zones are independent of network topology and may be comprised of multiple network segments connected using routers or other internetworking devices.

By definition, the gatekeeper is instrumental in the creation, control, and management of H.323 zones. To architect an H.323 network zone in a meaningful way, the network planner can choose to use different criteria. For example, zones can be defined according to geographic locations (such as different branch locations) or in accordance with overlap of a physical network connection (such as a subnet on the floor of a building or a range of IP addresses), or by a functional (organizational) paradigm.

A zone’s gatekeeper is logically separate from network endpoints, however, the physical location of the gatekeeper can be within a terminal, MCU, or gateway (or even in a non-H.323 network device). By virtue of the widespread adoption of this international standard in commercial H.323 entities, gatekeepers can support the terminals, gateways, and MCUs of diverse manufacturers. However, the standard does not dictate precisely how gatekeepers are implemented by developers. The result is that many different gatekeeper implementations will be available. (The ITU standards committee and working groups have not yet worked through all the details associated with “intergatekeeper” issues.)

H.323 GATEKEEPER BASICS
Whenever an H.323 entity goes live, it will send out a query to the network, asking if any (and which) gatekeepers are present and accepting a registration request from this particular type of entity. This discovery process and the subsequent registration with one (and only one) gatekeeper is the first step in zone management.

When a gatekeeper is present, it provides all registered users in its zone with:

• Address Translation -Users typically do not know the IP addresses of other terminals. Thus, when a user makes a call, the gatekeeper translates an alias address (name or e-mail address) to a transport address. A translation table may be automatically updated each time the gatekeeper receives and accepts a registration request; other methods of updating the translation table are also allowed.
• Admissions Control - Users will not all be able to access the network at the same time because of limited shared resources. Gatekeepers inject policies that protect the integrity of the network for all users. For example, gatekeepers will authorize network access along the policy guidelines a network administrator has selected when setting up a gatekeeper and/or zone. Network access and special services (use of a gateway, for example) may be based on call authorization, bandwidth usage, or some other criteria. The network manager may also choose to admit all requests, under certain low-use circumstances. It is important to note that admissions control is a way to preserve the integrity of the calls/sessions (that is, provide QoS guarantees) that are already up and operating when a user requests access.
• Bandwidth Control - Besides network access control, the gatekeeper offers network managers the ability to restrict or assign bandwidth to different applications along certain protocol conventions. This is another place network managers will enforce QoS guarantees and other enterprise-wide usage policies. The gatekeeper may also be configured to accept all requests for bandwidth changes where no policies are enforced.

The H.323 standard states that, in addition to the required capabilities listed above, a gatekeeper may perform other optional functions such as:

• Call Control Signaling - The gatekeeper may choose to complete the call signaling with the endpoints and may process the call signaling itself. Alternatively, the gatekeeper may direct the endpoints to connect the call signaling channel directly to each other. In this manner, the gatekeeper can avoid handling the H.225.0 call control signals. The gatekeeper may have to act as the network as defined in Q.931 in order to support supplementary services (e.g., call forward, call transfer). H.225.0, Q.931, and H.245 are a few of the several protocols encompassed under the H.323 umbrella standard.
• Call Authorization/Access - The gatekeeper may reject calls from a terminal due to authorization failure. The reasons for rejection may include, but are not limited to, “restricted access to/from particular terminals or gateways,” and “restricted access during certain periods of time.” The criteria for determining if authorization passes or fails are considered the domain of specific company management and, like other policies, outside the scope of any recommendation.
• Bandwidth Management - As eluded to above, the gatekeeper can control the number of H.323 terminals simultaneously using the network. Through H.225.0 signaling, the gatekeeper may reject calls from a terminal due to bandwidth limitations. This may occur if the gatekeeper determines that there is not sufficient bandwidth available on the network to support the call. The criteria for determining if bandwidth is available are considered the domain of specific company policy. This function can also operate during an active call when a terminal user requests additional bandwidth.
• Call Management - The gatekeeper may maintain a list of ongoing H.323 calls that is similar to PBX logs. This information may be necessary to indicate that a called terminal is busy, and to provide information for the bandwidth management function.

Certain items, such as gatekeeper management information data structure, bandwidth reservation for terminals not capable of this function, and directory services remain open for further study. In the interim, each gatekeeper developer must provide their own unique solutions, assuring that they are compliant with any H.323 endpoint. As pointed out above, the gatekeeper is a critical policy injection tool. The policies a gatekeeper can enforce include:

• Bandwidth management/QoS guarantee.
• Guaranteed authentication.
• ccess to services.
• Security.

ADVANCED GATEKEEPER CONSIDERATIONS
In order for CTI applications to become successful, they must provide users with the same telephony services that users are used to having with current PBX systems. The PBX-like functions that the H.323 standard supports are call transfer and call forwarding. These features are referred to as supplementary services.

Version 1 of the standard includes a mechanism for call forward, however, this mechanism has been replaced by a different mechanism in Version 2. Version 1 did not provide for transfer. The H.450 protocol will be used for supplementary services in Version 2, which will include both call transfer and call forwarding. Line hunting and terminal group hunting are two other critical PBX functions. Neither of these are provided for in the H.323 standard.

Recognizing the essential nature of these CTI functions for virtual PBX type applications, RADVision implemented a set of gatekeeper APIs (Applications Programmer Interface) to enable CTI application designers to offer H.323 compliant endpoints critical supplementary services. This includes providing a solution for line hunting and terminal group hunting that can be used by H.323 compliant endpoints — even Microsoft’s popular NetMeeting which does not currently support the standard H.323 RAS (Registration Administration Status) protocol that is used for communicating with gatekeepers.

In the future, we will see an expansion of these virtual PBX CTI functions. Some services will be independent of the gatekeeper, others supplied directly by the gatekeeper. Work is going on in the ITU and TIPHON (Telephony and Internet Protocol Harmonization on the Network) initiative to define a comprehensive set of telephony-like supplementary services.

LICENSING GATEKEEPER TECHNOLOGY
To help streamline the implementation of H.323 applications, companies are licensing gatekeeper application development software. When comparing gatekeepers for potential licensing, remember these points:

• Time To Market - H.323 networks are under development today. H.323 development time must be rapid. To shorten the time to market, a developer should evaluate APIs carefully before licensing. Assure yourself that the interfaces are designed specifically with third-party application developers in mind, and are available immediately. Development software deployed and proven in real-world situations is always preferable.
• Platform Independence - An H.323 gatekeeper can reside in any endpoint. If it resides on a terminal, the operating system is probably some flavor of Windows or UNIX. The gatekeeper may also reside on an internetworking gateway (where a realtime operating system like VxWorks or pSOS is desirable), a server (where Windows NT is the most popular operating system), and an MCU (Windows NT, UNIX, or a real-time operating system are possible). Look for a plat-form, operating system-, and terminalindependent gatekeeper package.
• Involvement In Standards - Choosing to partner with a company that has core competency in H.323 extends the lifetime of a strategic investment such as the H.323 gatekeeper. The ITU-T H.323 recommendation is an evolving standard. Version 2, ratified by the voting members of the ITUT Experts Group in January 1998, differs substantially from Version 1. Look for a partner with experts that focus on interpreting and mastering all the nuances and technical aspects of the H.323 standard.
• Robust APIs - Developers need to be able to create unique value-added extensions for products. All H.323 gatekeepers must comply with the mandatory functions of the ITU recommendation, however, there are many optional functions as well. Try to combine a high level of compliance to both the mandatory and optional functions of H.323 with a robust and rich set of APIs that is powerful and flexible enough to support your specific application needs.

By choosing to license these core enabling H.323 software technologies, CTI developers and network managers will be able to spend their time and expertise on unique application-specific challenges and opportunities, and value-added points of distinction for the emerging IP telephony and multimedia conferencing market.

Michelle Blank is president of RADVision, Inc. As a recognized leader in the H.323 arena, RADVision is an active participant in the ITU-T Forum and continues to serve on the Experts Group that defines the H.323 standard. RADVision is a leading provider of standards-based internetworking gateways for videoconferencing and telephony applications. For more information, visit the company’s Web site at www.radvision.com.