Videoconferencing and Multimedia Delivery
February 8, 1999
Early in H.320's evolution, H.261 was the only video codec available, and G.711 was the only audio codec. G.711 is a simple system that doesn't really compress audio. It consumes between 56-Kbps and 64-Kbps of bandwidth and reconstitutes telephone-quality audio at 50 Hz to 3,600 Hz. Given a 128-Kbps connection, 64 Kbps is assigned to the G.711 audio stream; the remaining 64 Kbps is allocated to the H.261 video codec. This strangles the codec to video frame transmission rates on the order of 5 fps.
H.320 quality now can be improved by using the G.728 audio codec, which needs only a 16-Kbps transmission path to reconstitute 50 Hz to 3,600 Hz. H.261 video can subsequently use the remaining bandwidth to achieve 10-fps to 15-fps performance.
In this example of typical call setup between H.320 room systems, three BRI lines are inverse multiplexed to a combined bandwidth of 384 Kbps in each direction. The G.722 high-quality codec can be used. (It reconstitutes audio in the 50-Hz to 6,000-Hz range.) Although G.722 needs 48 Kbps, 336 Kbps will be left for H.261 video-enough to achieve full motion 30-fps performance.
Similar audio/video tradeoff strategies are possible in H.323 visual telephony. To avoid swamping a LAN, you need to support acceptable communications quality while using as little bandwidth as possible for each call. In this illustration, G.723 audio and H.263 video are constrained to consume no more than 200 Kbps in each direction. T.120 collaboration data packets are transmitted separately. We'll explore H.323 call setup in more detail below.
H.323 is the evolutionary product of more than ten years of protocol development by the ITU. As a result, it accommodates or defines methods for interoperating with equipment covered by older standards, such as H.320. New H.323 desktop terminals or small-group systems easily can be introduced to an enterprise that already has a significant investment in traditional room systems through the use of gateway equipment. This chart shows the connection possibilities already defined by the ITU for communication between H.323 and terminal types associated with other network transports.
Before describing how the more complicated interprotocol connections work, we'll look at a simple situation: a call between two H.323 terminals on a single LAN. Note that H.323 is an architectural definition. It encompasses a complicated set of specific protocols that govern operations in various layers of the communication process.
Providing details about each of these protocols is beyond the scope of this chapter. However, we'll identify the protocols that operate at each layer in case you wish to research further. You also can refer to the "Generic Visual Telephone System" block diagram in the Primer section of this chapter.
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