Voice over ATM Technology


This article is excerpted from ATM Angles for IP Edge, in Network World, 02/15/99, written by Denise Pappalardo.

At the ATM Forum meeting in the beginning of February, members outlined plans to make ATM the technology of choice for transporting IP traffic over WANs. The forum’s latest efforts include defining new methods for running IP data, video and voice over ATM. In addition, the forum is teaming with the Internet Engineering Task Force to ensure quality of service (QoS) for IP traffic crossing ATM backbones.

IP traffic flows over ATM backbones today, but the forum wants to go beyond simple transport. The group last week approved its Guaranteed Frame Rate (GFR) specification, a new ATM class of service that lets users more effectively mix Ethernet and IP traffic on an ATM network. The forum’s pending Realtime Multimedia over ATM (RMOA) specification is aimed at bringing reliability to IP voice and video applications.

The forum’s efforts are especially important to business users because sending traffic over the Internet isn’t always the most reliable transport method. That’s where the forum’s GFR comes in. Part of the forum’s traffic management specification, GFR will let ISPs offer business users guaranteed minimum bandwidth over the Internet and allow users to borrow from unused bandwidth when it’s available. GFR will also let customers send traffic that exceeds their guaranteed rate and does not run the risk of losing frames. This is similar to the way carriers provision frame relay service to offer users a committed information rate.

While ISPs offer customers network availability guarantees today, those guarantees are typically not based on a specific technology, and they often don’t guarantee bandwidth.

To foster further QoS developments through the Internet, the ATM Forum and the IETF are working closely to map the IETF’s Differentiated Services (Diff-Serv) to ATM. The ATM Forum’s and the IETF’s work in this arena is new, with technical contributions from the Diff-Serv working group reaching the forum for the first time last week. While Diff-Serv defines how to identify a variety of IP QoS traffic classes, the IETF does not define what makes up these classes, says George Dobrowski, president of the ATM Forum and director of broadband networks at Bellcore. The goal is to define these classes and map Diff-Serv to ATM to ensure they can work together.

The ATM Forum and IETF are working together to establish how the groups’ separate QoS developments can foster a standard that will ultimately bring end-to-end QoS to the Internet.

RMOA is another new ATM Forum specification that is expected to optimize IP video, audio and telephony traffic traveling over an ATM network, Dobrowski says. RMOA defines how to tie together ATM’s variable bit rate (VBR) class of service and the International Telecommunication Union’s (ITU) H.323 voice-over-IP standard. VBR lets users send traffic at a predefined, guaranteed rate over an ATM network. Any traffic that exceeds that rate isn’t guaranteed.

The ATM Forum, the ITU and the IETF are trying to reach a consensus on which version of H.323 they will support. Dobrowski says today there are three versions of H.323, and if the various standards organizations settle on one, users should have less trouble with interoperability and feature support.

Originally the ATM Forum touted ATM as the end-all network solution. Cost and complexity destroyed this claim. The market also turned to Fast Ethernet and Gigabit Ethernet as less costly, simpler alternatives. Now it appears that the forum has changed its tune to one of coexistence and convergence, says Marlis Humphrey, chairman of the board at the ATM Forum. Instead of touting ATM as the only network technology users need, the forum is saying ATM is “a superb tool to deliver IP-based services,” she says.

See this and other interesting articles at Network World Fusion.