bjd7p@calico.cs.Virginia.EDU (Bert J. Dempsey) (04/14/91)
Below are the main points turned up in my investigations of the concept of using Cable TV networks for medium distance (e.g city-wide) data transmissions. Thanks to everyone who responded to my original posting. (I exhausted all the leads that I had time to follow.) I welcome further comment on or discussion about CATV networking. ---------------------------------------------------------------- (1) Applitek Corp. has developed a system which allows data transmission over broadband CATV networks. Applitek Corp. has a "work-at-home" package, which involves data transmission over cable TV coaxial lines at 10 Mbits/s. The system consists of a headend translator installed at the headend of the CATV network and bridges that transparently link Ethernet subnets over the CATV lines. In a private home, for example, the Ethernet subnets would be 1-node subnets (a home computer with an Ethernet card in it). On the CATV broadband side of the bridge, transmission is 10 Mbits/s and can range up to 35 miles. At the headend of the CATV network, the single-channel RF-agile translator uses a 6MHz channel in both the forward and reverse directions. The translator is a $3600 box. The bridges currently cost around $10,000, but their next-generation versions are due out within a year at an anticipated $4,000 price tag. Applitek has a couple of sites operational, including a part of the Portland, Ore. city government. (2) The city of Springfield, Massachusetts is renting space from their local cable company, using chipcom marathon bridges (ethernet to token bus) to connect their municipal buildings. (3) The CATV concept requires that signal quality over the CATV network be tested for acceptability since data transmissions are much fussier than video signals. This parameter is likely to vary significantly from site to site. (4) It is somewhat difficult to assess exactly how well CATV networks will scale as the node populations grow larger. I am not aware of anyone having hands-on experience with very large node populations (over a hundred and up to a few thousand), though the demand for such networks can be anticipated. (5) Legal issues concerning CATV data transmission are somewhat fuzzy. However, there are signals that the mood in Congress is, if anything, in favor of encouraging CATV networks as an alternative to telephone company data services. The number of years till ISDN services become widespread, a number between 0.5 and infinity, is another wildcard in the growth and success of CATV networks. (6) Alas, in my own local picture, an immediate problem is that many of the local cable operator's lines are equipped with single-directional amplifiers. While upgrades to enable bi-directional transmission are in the works, this process is projected to take years to complete. However, some portion of their network might have reverse channel capability within a relatively short time frame. -------------------------------------------------------------------- Bert Dempsey (bjd7p@uvacs.cs.Virginia.EDU) Computer Science, University of Virginia "Be careful when interacting with the PC"