SIP Is Grabbing Spotlight, But Older Sibling Still Has Sizzle

H.323 remains a very powerful and wellstandardized protocol. It is so stable and resilient that the rumors of its death only a year or two ago have proven to be unfounded. In fact, H.323-based services continue to show growth in service provider usage and profit.

Today H.323 is much more than a legacy IP telephony protocol. The new H.323 version 4 enables full-featured and well-managed real time voice and video over IP with a rich set of add-on user services and the combination of H.323 and HTTP, which introduces a new level of user experience and enables the creation of a new breed of services. H.323 is well-suited for multiparty, multimedia sessions with advanced conference setup and control capabilities.

A Little History

The H.323 protocol is best known as the original call signaling protocol that made real time multimedia communication over IP possible. H.323 was defined as the first VoIP protocol in May 1996 and is now found in almost every VoIP product and at every point in the LAN and WAN network.

In the enterprise there is a steady usage growth of H.323, with the protocol powering video conferencing terminals, IP voice phones, IP PBXs, wireless IP phones and H.323-based Wi-Fi phones.

H.323 also operates in most VoIP backbones (Class 4 switches). Today Internet telephony service providers operate many billions of VoIP minutes every month and the numbers keep growing. VoIP accounts for more than 10 percent of all international long-distance minutes.

The total share of VoIP minutes in proportion to total PSTN voice minutes continues to increase.

A good example is ITXC, whose voice traffic is pure H.323. The company, whose revenue has increased from quarter to quarter since its establishment in 1997, surpassed $80 million in VoIP service revenue in the first quarter of this year. And China Unicom, eDial, Genuity, iBasis, ITXC, MCI, Net2Phone, Ntera and PhoneOpia combined have serviced billions of VoIP minutes to date. Cisco’s largest VoIP carrier, China Unicom, has transported many billions of H.323-based VoIP minutes and is now approaching one billion H.323 VoIP minutes each month.

H.323 Services

The suite of VoIP services delivered today can be divided into several categories:

• End-user communication services, which include voice calling cards, voice/video conferencing, data collaboration, call centers and distance learning;
• VoIP supplementary services, which are becoming more popular and include multiline, call diversion, park and pick-up, consultation, follow-me and many other IN-type services;
• Communication between various terminal types, including PC-to-phone, fax-to-fax, phone-to-phone and Web call; and
• Internet telephony service provider central office equipment to support IP PBX services, gatekeeper services, H.323/SIP gateway services, voice VPNs, H.323/SIP softswitches and customer premises equipment, H.323/SIP wireless/mobility services and wholesale transit.

Most products are based on, and most service providers are using, version 2 or 3 of the H.323 protocol. However, a new breed of H.323 version 4-enabled products is starting to emerge.

H.323 version 1 was ratified in May 1996 and in 1997 new products started to be released. While H.323 version 1 tried to “do it all” using H.323, the more advanced versions allowed for interworking with complementary protocols such as HTTP, SCTP, SRTP, Megaco and well-known algorithms such as AES, DES and 3DES. Users suggested refinements and the protocol was improved with version 2, which was adopted by many more vendors. Later, version 3 was ratified with additional enhancements.

However, the major change to the H.323 standard was in version 4, ratified in November 2000. Normally it takes a year or two between a standard ratification and real product deployment, so H.323 version 4-enabled products are only now hitting the market. Version 4 provides powerful new IP telephony enhancements, including additions in service management; Web-based and advanced supplementary services; improved PSTN signaling interworking; advanced service capabilities for features such as privacy, security, and ease of billing; carrier-class network optimization features to enable required scalability and ensure service availability; better scalability; advanced security for both service providers and users; add-on services; and support for the general extensibility framework (GEF) to enable development of new services.

Security services are growing for various aspects of voice and video over IP communications. These include accurate user authentication, billing verification information, enabling user privacy to avoid auditing of a call session and largescale key management. H.323 version 4 defines a sub protocol H.235 version 2 with security subprocedures named Annex D, E and F.

H.235 Version 2, as part of H.323 version 4, enables the following security services for the benefit of the end user and the service provider:

• User authentication, which ensures accurate authentication of the end user. This is an important feature to enforce usage and service usage profile policies and for usage accounting;
• Media confidentiality, which enables the end user to establish secured media communication channels with other parties, which cannot be audited by a third party. This is a very important service for users to assure that their calls are not audited;
• Nonrepudiation, which uses a digital signature as a proof of service usage. The digital signature can be stored as part of a call service data record, which contains accounting information for billing processing and billing verification services, for billing verification purposes in case of service usage denial;
• Integrity, which is the ability to assure that every message received from a given declared IP address is indeed coming from that source; and
• Scalability, or the ability to deploy the solution in an unlimited number of H.323 zones.

In H.323, the central device serving all other H.323 endpoints is a gatekeeper. Each endpoint should be registered with one gatekeeper so that all registered endpoints with a certain gatekeeper are referred to as the gatekeeper zone. The enhanced security feature of H.235 Annex E and F can be deployed for an unlimited number of zones to provide a scalable solution for an entire VoIP network.

Increased Scalability

Version 4 of the H.323 standard also delivers a scalable gateway architecture. Specifically, H.323 4 describes various designs to decompose the gateway into the separate media gateway controller and media gateway setups.

The newly ratified H.323 standard also enables service providers to introduce a server farm such as gatekeepers and gateways that will appear to the served endpoint as if they were one entity. This method enables them to include multiple optional IP addresses in a message to be handled by any one of those optional IP addresses to improve scalability and robustness.

Scalable user authentication is outlined in H.235 Annex E and F. This standard uses public key infrastructure-based methods with certificates and provides digital signatures. The user authentication methods defined in the spec enable support for an unlimited number of gatekeepers.When combined with the LDAP scheme (H.350), which was recently ratified, scalable key management and personalization is also possible.

In another new benefit to Version 4 of H.323, the specification of alternate gatekeepers, which enhance service availability and traffic shaping for H.323 endpoints, is added. With this procedure, a gatekeeper can redirect an endpoint to register to an alternate gatekeeper if load balancing is needed. Also, the gateway can communicate with the endpoint and guide it to register with another gatekeeper in the event of service failure.

H.323 defines “robustness” for H.323 entities in H.323/Annex R. This annex defines a set of procedures and infrastructures for improving service availability and recovery, and defines methods for recovery of call signaling and call control in the event of failure. For example, version 4 defines virtual entities, where two or more closely coupled peer entities collectively appear as a single entity to the rest of an H.323 system while providing fault recovery. Also, in version 4 of the H.323 standard, signaling used in a PSTN network can be transmitted through H.323 and then passed on to the connecting PSTN remote end element/network as if the two PSTN endpoints were in the same PSTN network. Using this method, an international long-distance call will cost almost as little as a local call.

Version 4 also specifies similar user services and signaling to that found on the PSTN.

Eli Orr is H.323 product manager at RADVISION Ltd. He can be reached at eliorr@radvision.com .

Share this article: Email, Slashdot, Digg, Del.icio.us, Yahoo!MyWeb, Windows Live Favorites, Furl
Add this article feed to: RSS, My Yahoo, Newsgator, Bloglines