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Video networking can be accomplished over a number of facilities and service offerings, depending on the application and the amount of bandwidth required (Figure 14.2). Analog circuits will support videoconferencing at low speeds, although the results are less than completely pleasing. The failed videophones offered by AT&T, BT and others make use of dialup analog circuits. As is always the case, digital circuits offer better performance than do analog circuits.
Figure 14.2 Videoconferencing across the Wide Area Network (WAN).
ISDN circuits are preferable to analog circuits, because they provide more bandwidth and better transmission quality. However, the greater cost and lesser availability of ISDN has slowed the acceptance of videophones based on ISDN technology. Switched 56/64 Kbps circuits can be used for videoconferencing, generally grouped or bonded to provide multiple channels. Switched 384-Kbps connectivity can be provided on the basis of Fractional DS1 or through ISDN PRI channels in a channel group known as HO.
DS-1 facilities support full-motion, high-quality videoconferencing over dedicated networks at rates up to 2.048 Mbps for E1 and 1.544 Mbps for T1. However, such facilities are costly and not widely deployed in such applications. Large user organizations with dedicated DS-1 backbone networks make highly effective use of videoconferencing, with the video communications contending with voice and data for network access through intelligent MUXs.
Broadband networks are much more capable of supporting the demands of videoconferencing. Frame Relay will support video, although that clearly is not the primary reason for its existence. While Frame Relay will perform well under normal circumstances, it is likely to yield herky-jerky video should the network suffer severe congestion. SMDS provides much better video performance than Frame Relay, due to improved congestion management and lesser latency. However, and as discussed in Chapter 11, SMDS is a MAN technology of limited availability. ATM undoubtedly will be the network technology of choice in a convergence scenario. ATM will offer tremendous bandwidth over fiber optic or hybrid fiber/coax networks and in support of voice, image, facsimile, and data traffic, as well as video.
LANs were not designed with video in mind, but neither were existing WANs for that matter. In any event, LAN technology has developed to the point that it will support video, although such implementations are still unusual and problematic. By way of example, a 10 Mbps Ethernet LAN may provide only 4 Mbps throughput; sharing that bandwidth among 20 or 25 users reduces the available bandwidth to unacceptable proportions. Additionally, LANs generally impose delay on the signal. This is especially true of Ethernet, as explained in Chapter 9.
The basic problem is that video is isochronous, or stream-oriented, traffic. LANs were not developed to support continuously streaming data. Additionally, video is highly bandwidth-intensive. Even 100 Mbps LANs have trouble supporting high-quality video intermingled with more traditional and legitimate LAN data traffic. Traditional (relatively speaking) LAN options for videoconferencing include Fast Ethernet, Switched Ethernet and FDDI. More recent technologies and standards include ATM and IsoEthernet.
Videoconferencing, at the time of this writing, clearly is the primary application for video networking. According to the MultiMedia Telecommunications Association (MMTA) and the International Teleconferencing Association, revenues for videoconferencing products and services grew from $2.3 billion in 1993 to $4.4 billion in 1995. Revenues are expected to reach $8 billion in 1997, for a compound annual growth rate of 33% since 1990. Computer Intelligence Infocorp estimates that in 1994 some 14,000 U.S. establishments had installed videoconferencing systems, with the education market leading the way at 21% of the total [14-13].
Videoconferencing systems consist of cameras, monitors, video boards, microphones, and speakers. Videoconferencing can be accomplished in the workplace over a LAN, although such applications are unusualit generally is more effective, easier, and less costly to walk across the hall and hold a face-to-face meeting. Videoconferencing over the WAN, however, offers great benefits in terms of reduced cost and increased availability for meetings. Systems for videoconferencing can be quite substantial or can be PC-based. Videoconferencing has increased significantly over the past few years, as the cost of equipment and bandwidth have decreased. Videoconferencing systems comprise room systems, rollabout systems, and PC-based systems.
Great strides are being made in the support of PC-based videoconferencing systems that can be supported over LANs and, in the near future, through PBXs. As noted by Bob Metcalfe (of Ethernet fame), five out of six calls handled by a PBX never reach the WAN; rather, they are station-to-station. Although videoconferencing originally was touted as a means of reducing travel expense, it eventually may save more shoe leather than gasoline [14-20].
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