InformationWeek
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May 14, 2001 |
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Settling On Standards
By Candee Wilde
lthough IP PBX vendors seem to agree that standard protocols will help pave the way for network convergence in the enterprise, they can't quite settle on which protocols they should all use.
Most early IP PBX systems support H.323 and some argue this set of protocols is best-suited for voice-over-IP systems. But critics say H.323--initially approved by the International Telecommunications Union (ITU) for multimedia communications over packet-switched networks--is more complex than necessary. H.323 helped standardize IP-based videoconferencing, which requires encoding, decoding, and packetizing both audio and video signals. IP telephony only involves sound.
H.323 defines four major pieces of a communication system for packet networks: terminals, gateways, gatekeepers, and multipoint control units. Terminals are user endpoints on IP-based networks that support signaling and compress and decompress transmissions. Gateways provide a connection path between the packet-and circuit-switched networks; they perform call set-up and control on both networks, in addition to translating between transmission formats. Gatekeepers control address translations (changing phone numbers to IP addresses, and back), admission, and bandwidth and zone management on the network. Finally, multipoint control units support conferencing between three or more endpoints.
Late last year, the ITU and IETF proved that agreement on voice-over-IP standards is possible. The two standards bodies agreed on a single standard for gateway devices--the ITU-T H.248. The standard will be published as Megaco (Media Gateway Control Protocol) by the IETF. H.248/Megaco facilitates the exchange of voice, video, and data traffic between the public switched telephone network and an IP network. It works in conjunction with the Gateway Location Protocol to enable a caller with a public switched telephone network phone number to locate the destination device and establish a session. It provides the gateway-to-gateway interface for the Session Initialization Protocol (SIP).
H.248/Megaco also enables a media gateway--the network element that provides conversion between the information carried on telephone circuits and the data packets carried over IP networks--to control and manage communications equipment on the network. By controlling IP terminal devices, such as telephones, from the media gateway, the devices themselves require less intelligence and processing power. On circuit-switched networks, relatively "dumb" (and inexpensive) telephones receive instructions from an intelligent central switch. Because IP devices are not wired to a specific switch, without a protocol like H.248/Megaco they would have to function on their own using internal processors. Adding processors adds cost to IP devices.
Another IP telephony standard, the ITU's H.450, defines how to place different PBX functions, such as call hold, transfer, forwarding, waiting, and park and pickup, on distributed servers.
The IETF's text-based SIP was a relative latecomer to the list of voice-over-IP protocols, but it has gained momentum quickly. SIP is a particular favorite of engineers with data-networking experience, in part because it's an open IP standard and a relative of Internet stalwart HTTP. Like H.323, SIP initiates, controls, and terminates interactive IP sessions, including voice calls. But unlike all other protocols used in voice-over-IP systems, SIP works from network endpoints and eliminates the need for an intelligent gateway to mediate calls.
By using the Session Initialization Protocol, some developers argue that new services and applications would be easier to develop for various IP devices, including phones and wireless handhelds, because the intelligence would not reside in a network switch. And since it is an application-layer control protocol, SIP champions say it's the easiest protocol to use to write native Web applications for voice over IP.
Other protocols used in voice over IP are: Lightweight Directory Access Protocol, which is also used in browsers and E-mail programs to provide a common method for searching for addresses in an Internet directory; and Computer Supported Telephony Application, which is an international standard interface between network servers and PBXs.
Gartner analyst Rich Costello says once the industry settles on the appropriate voice-over-IP standards, IP PBX systems will have a much better chance at penetrating the mass market. For now, he says, what these standards will ultimately be is still in doubt. In the meantime, vendors are supporting multiple protocols.
For that reason, choosing an IP PBX that supports these industry standards can help users protect their investment and avoid getting stuck with a proprietary solution, says Joan Vandermate, director of product-line management for Siemens Enterprise Networks LLC, in Reston, Va. By purchasing standards-based IP equipment today, users are more likely to be able to reuse some of their equipment if, for example, they were to decide to purchase IP Centrex from a telecom service provider in the future.
Similarly, Vandermate recommends that customers purchase IP products that support open application programming interfaces, such as telephony API, J-TAPI, and Computer Supported Telephony Application, so that they're free to buy off-the-shelf, third-party applications rather than becoming locked in to a single vendor. Siemens' HiPath 5500 IP-PBX system supports many industry standards and open interfaces.
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