Video Calls Within The Walls (and outside too)
	An Inharmonious Tower Of Babble Videoconferencing Standards and Communications Problems "There are 50 ways to leave your lover," Paul Simon sang. In our testing of five products, we found at least five ways for videophone systems to use intrabuilding wiring. And none of these systems can talk to each other-you'll have to pick one vendor for each campus. However, these systems can make connections through one or more H.320 standard gateways to the outside public switched telephone network (PSTN). CorelVIDEO and Datapoint MINX can use an extra pair in a building's Category 3 or Category 5 unshielded twisted-pair (UTP) wiring. These separate circuits carry no competing network data traffic. Having effectively unlimited bandwidth at their disposal, low-cost converters simply baseband-modulate the NTSC or PAL camera output to send TV-quality picture and sound over the copper wire. However, signal attenuation can be a problem. Using its simple modulation technique, Corel Corp. can't plan on wire distances of more than 300 feet over Cat 3 cable, or 500 feet over Cat 5 to the nearest switching hub without coaxial cable line extenders. Datapoint Corp. uses a proprietary modulation technique that can operate up to 1,000 feet from the switch on Cat 5 wire. If all user stations are less than 1,000 wire-feet from the central hub/server location, all switching and media control can be performed by a super hub. No intermediate hubs are required, and the system is nonblocking. C-Phone transmits a bandwidth-trimmed, but uncompressed, NTSC audio/video signal over UTP or coaxial cable. Its system broadband-modulates the transmission medium much like a metropolitan cable TV system. User stations are assigned a channel number from 1 to 64. The bad news about this design is signal attenuation. C-Phone works well over coaxial cable, but in UTP installations, the higher channels degrade when transmission distances exceed 300 feet over Cat 5 cable. The good news is that video hub equipment is relatively simple and inexpensive. The C-Phone system is nonblocking in communities of 64 or fewer users. Call blocking is an important consideration in sizing and designing any telephone-like video distribution system that runs over UTP. Wire transmission distance limitations typically require one or more active switching hubs on each floor of a campus building. Users on a given hub can call any other local user. But if the call is to someone on another floor, the system must select a path from the caller's hub through a main switch to the called party's hub. If only one trunk path exists between hubs and the main switch, no more calls can be set up between other parties on those two floors. Call blocking can be reduced by adding more trunks. Switching hub designs usually provide a fixed number of ambidextrous ports. Some can be configured as trunk ports , the remainder as user line ports. CorelVIDEO's Media Control Unit hub design illustrates the possibilities. Each trunk requires two user port slots, so a given hub can support four trunks if there are 14 user ports-a 30 percent call blocking ratio. For totally nonblocked service, as many as 48 users and 48 trunks can be configured per hub. Incite's call setup system can be nonblocking in communities of 48 or fewer users. Incite's adoption of Iso-Ethernet technology enables a compressed audio/video signal to be carried within a campus over the same physical wire that the enterprise's data LAN uses. However, the 96 ISDN channels that bear the videophone calls use a bandwidth region that "sits above" the 10-Mbps data LAN space. Incite's system uses only the data LAN for short datagrams exchanged among a calling station, the Incite NT Server and the multimedia hubs during call setup. In the process, the hubs use a 96x96 crossbar scheme to assign the requested number of channels between caller and destination. A minimum of two channels are required for a video-and-audio call; six channels are typically requested so that 16 simultaneous 384-Kbps videoconferences can be set up within a user community. Intelect's LANscape is nonblocking to whatever extent an enterprise's data LAN will bear traffic. Intelect uses a packet-oriented IP-based transmission scheme; call setup requires the IP address of the destination station and establishment of a socket-a process analogous to setting up an FTP session or connecting to a Web server on an intranet. Once established, each videoconference exchanges packets at rates of about 3 Mbps to maintain the appearance of smooth 20- to 30- frame-per-second video. The possible number of simultaneous sessions depends entirely on the topology and capacity of the enterprise's data network. L ANscape will severely overload a 10-Mbps Ethernet. But as Fast Ethernet or ATM becomes available, the enterprise can easily scale up its videoconferencing activity without investing extra in speciali zed hubs and switches.

Back to Video Calls Within Walls (and Outside Too) SOHO to the Enterprise: End-to-End or Dead End? by Christopher Smith and Ron Bunal Updated May 23, 1997