Cable Modem: A device that allows digital data to be received/transmitted by a user of a Cable TV system. As of 2001, many but not all cable companies support cable modem use. A large number of different, and mutually incompatible, cable modems are in use on those cable systems that do. The industry standardized on a specification known as IEEE 802.14 in 1998, but older systems may not comply.

Cable TV companies that do provide digital service primarily act as Internet Service Providers. The major ventures in this area is known as @HOME and RoadRunner. The primary limitations on Cable TV digital communications are the failure of older cable TV networks to provide uplink (user-to-cable provider) communication and a lack of bandwidth on older systems to allocate to cable communications use.

Download speeds over Cable TV are difficult to determine. Claims of bandwidth of many MHz are claimed, but practical devices appear to deliver speeds between 100 and 700Kbits/sec (which is 15 to 30 times the speed of typical phone line connections). The determination is complicated by the fact that cable bandwidth is often shared by groups of 500-2000 users -- most of whom currently to not have modems. Both opponents and proponents of Cable modems agree that current users are probably getting more bandwidth than future users will get, but no one knows how much more bandwidth and how much additional bandwidth technology will be able to deliver without affecting other services on the cable.

A common arrangement for a two way system is to allocate one unused TV channel to downlink and some bandwidth below channel 2 to uplink. The 6MHz wide NTSC TV channel used for downlink can offer a bandwidth of 27 to 36 mbps depending on the modulation scheme used. The uplink channel typically has .5 to 10 Mbps capacity. However, because of collisions, noise, and other overhead, the effective bandwidth under load is more like 5 to 14mbps downlink and much less uplink. That is roughly 0.5 to 1.5 mByte per second. Noise is a major problem with cable modems. Moderate noise has minimal affect on television but is quite disruptive to digital communications so many cable systems were designed without regard to noise and must deal with considerable noise. The configuration of 500 to 2000 users per cable segment predates data applications and is dictated by the geometry of the wiring from users to a facility called a distribution hub. Altering the configuration significantly requires laying new wires and is neither easy nor inexpensive.

The Internet itself appears often to be unable to deliver material at speeds consistent with cable modems. Maximum speeds are probably seen only on material cached at the cable company computer. Cable ISPs provide for massive caching of commonly referenced Internet material.

Cable "modems" are usually implemented as ethernet hubs with the user communicating through a Network Interface Card. Installing a cable connection to a computer on a local network can be complicated and often is not supported by cable companies. If a firewall is not present, files on the local network may well be accessible to users from the Internet.

updated 2/24/2001

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