Mapping H.323 Multimedia to ATM QoS

BOSTON (05/23/2000) - Until recently, corporations heavily invested in ATM backbones have been unable to utilize the inherent quality-of-service (QoS) features in ATM. With the advent of less expensive Ethernet switching and trunking technologies, network managers have been wondering if the promise of end-to-end QoS will ever be seen.

Even with the development of H.323 standards for multimedia transport over LANs, there has been no way to integrate this technology with ATM. But with the use of media gateways, firms can merge the two technologies for end-to-end QoS.

The H.323 protocol suite uses several protocols, notably the Real-Time Transport Protocol (RTP) and the Real-Time Control Protocol (RTCP) over LAN-based media to provide for multimedia data streams.

RTP was developed by the Internet Engineering Task Force to handle streaming audio and video. Its duties include sequencing, buffer control and time synchronization.

RTCP is used to monitor QoS and control RTP sessions. H.323 media streams can be transported over ATM using three methods: direct IP over ATM, RTP over ATM and compressed RTP over ATM. Direct ATM has always been an option, but it can hinder performance for media streams such as voice over IP because of the overhead associated with RFC 1483 (Multiprotocol Encapsulation over ATM) and LAN Emulation headers and call setup procedures. Real-time video is not as susceptible to such overhead because it uses larger data transfer blocks.

Because of the delays associated with voice, such as speech-encoding and distance latency, it's critical to keep overhead low. RTP over ATM solves this problem by terminating the IP/User Datagram Protocol at the media gateway while mapping the H.323 QoS to ATM's QoS.

For admission procedures and call setups between endpoints, H.323 terminals use a modified version of Q.931, a protocol used in ISDN signaling. A software or hardware implementation of a device known as the gatekeeper provides call control services to H.323 endpoints. These call setup procedures are performed using direct IP over ATM or LAN emulation.

Next, a logical channel is opened between H.323 terminals for the H.323 media stream. Then the media gateways use OpenLogicalChannel messages, which contain the ATM addresses for the RTP and RTCP channels. At this point, the ATM switched virtual circuits are positioned between media gateways across the ATM backbone. ATM virtual circuits are configured using the standard ATM User Network Interface (UNI) and Private Network-to-Network Interface (PNNI) signaling protocols. The ATM Forum's UNI 3.1/4.0 and PNNI specifications describe the call setup procedures within an ATM network. The ATM UNI SETUP messages carry the port numbers for the RTP media stream in a Generic Identifier Transport information element.

Neither the H.323 endpoints nor the gatekeeper have any knowledge of the ATM backbone. The media gateway is responsible for translating the bandwidth and QoS parameters from the H.323 connection requests to ATM signaling traffic descriptors. Because RTP headers are transmitted across the ATM backbone, 12 bytes of overhead are added to each non-ATM frame that must be segmented into ATM cells. ATM cells carry 48 bytes of user data with an overhead of 5 bytes.

In order to reduce the effects of the RTP on the 10% ATM cell tax, media gateways have the option of performing RTP header compression.

Because many vendors are pushing ATM and H.323 as their solution for QoS, it is important for network managers to ask vendors what level of support for H.323 and ATM they offer. Technology decisions are often made based on the level of support offered, rather than on what is currently available today.

Bottom line: The standards for QoS service integration are slowly becoming reality. However, the success of their implementation remains to be seen.

Burns is a senior consultant at IQ Group, an IT consulting company in Philadelphia. He can be reached at

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