Protocol standoff! Version 4 of the H.323 call control protocol for IP telephony is just about ready to hit the streets, but SIP has been stealing the spotlight as an IP-centric apps enabler. Many carriers want both - Online - Session Initiation Protocol

Telecom Asia,  Jan, 2002  by John C. Tanner

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Voice over IP has come a long way since its days as a PC hobbyist toy and a neat way to make poor quality but cheap long-distance calls. The quality has improved so much that even international carriers use it to save money on costly backbone routes. International VoIP wholesaler ITXC claims that up to 30% of its IP telephony traffic comes from Tier-1 carriers. TeleGeography estimates that 6% of IDD traffic last year was VoIP.

Behind the scenes, however, there has been an ongoing and frequently passionate battle over the signaling protocols that add call control to IP telephony. On one side is H.323, the global umbrella standard from the International Telecommunication Union. On the other is SIP (Session Initiation Protocol), a signaling protocol adopted in early 1999 by the Internet Engineering Task Force.

Thanks largely to initial deployment and interoperability problems, H.323 has taken something of a PR beating over the years, while IP telephony proponents and Net-heads in general have been increasingly enthusiastic about SIP as the only signaling protocol that makes sense in a world that, one day, will run on nothing but IP. SIP has some strong backing from players like Microsoft, which is including SIP capabilities in the instant messaging application of its new Windows XP operating system. Even the wireless world is behind SIP, with the 3GPP group responsible for UMTS/W-CDMA standards saying it will incorporate SIP into 3G mobile systems.

However, H.323 is far from a lame duck. Version 4 of the standard, which was finalized in November 2000, is ready for commercialization. In late 2001, RADVision, which manufactures the H.323 protocol stacks used by the majority of vendors who make H.323-compliant products, released its Version 4, meaning that Version 4 products are on the way, probably later this year.

How much demand there is for either technology, as always, depends on who you ask, but with each technology possessing its strengths and weaknesses, and with H.323 having a legacy base that SIP does not, some vendors and market analysts see enough room in the next-generation network for both H.323 and SIR

New and improved H.323

The rivalry between H.323 and SIP is a manifestation of the overall rivalry between the circuit-switched telephony world and the (mostly, if not completely) IP-based packet-switched world.

H.323 originated in the circuit-switched telephony world as a group of protocols to allow better managed, carrier-quality IP telephony on enterprise LANs via ATM or ISDN. However, explains Christian Biller, Siemens Information and Communication Networks' product line manager for multiservice switching, as VoIP gained popularity as a cheap long-distance tool, "many service providers responded to the growing customer demand for cheap Internet calls by deploying H.323 equipment to deliver `carrier-grade' VoIP with dedicated quality of service".

H.323 has developed over the years to support an impressive set of telephony features similar to circuit-switched services. Version 4 adds a few more key features, perhaps the most notable of which is IP trunking.

"In a carrier network, you'll typically have a gatekeeper that sits in front of a bank of gateways, so that when calls are coming in over the VoIP network and they're outbound for the PSTN, you want to make sure that call terminates correctly," explains Paul Jones, voice architecture software engineer for Cisco systems, and chairman of ITU Study Group 16, which oversees H.323 standardization, and the editor of the H.323 Version 4 standard. "Version 4 gives the gateway the ability to signal to the gatekeeper exactly how many DSos it has available, and also to be able to do that on a trunk group basis. You can logically group trunks of DSos together, so you might have trunk group A for termination to AT&T's network, and trunk group B that goes off to MCI's network."

Other features in H.323 Version 4 are actually enhancements or better defined specs for existing H.323 features, such as fax/voice switchover, and in-band tones and announcements (such as "The number you have dialed is no longer in service") that allow the caller to hear the announcement without being billed for a connected call. H.323 Version 4 also better defines gatekeeper redundancy.

H.323 Version 4 also allows for two different types of call-set-up -- Fast Connect and H.245 tunneling -- to run simultaneously rather than concurrently during the call-set up process.

"That means we can set up an audio call in both directions as before, but also fully exchange terminal capabilities [such as exchanging DTMF digits] with the end points in two messages," says Jones -- "two messages" being one from the caller terminal and one from the receiver terminal. "So you can set up two-way audio and video without any further signaling, which improves performance by cutting down on signaling time."

Something strange

Jones says that Version 4 brings "a lot of stability and maturity to the protocol". Even so, it has been H.323's history of vaguely defined specs that has prompted criticism of the standard, particularly from SIP proponents who dismiss H.323 as so faulty and needlessly complex that interoperability problems between H.32.3 problems are inevitable.