ver two-way data transmission over a cable network, one unused 6 MHz television channel, in the 50 - 750 MHz range is typically allocated for downstream data traffic. Another unused 6 MHz channel, in the 5 - 42 MHz range, is used to carry upstream data. Whenever someone clicks on a hyperlink, sends e-mail, or uploads files, they are sending data upstream. Unfortunately, the upstream band is subject to all sorts of interference that can garble data. This shortcoming makes it close to impossible to use a coax-only cable system for two-way high-speed data traffic. Coaxial cable picks up noise from motors, CB radios, microwave ovens, and other appliances. Ham radio and VCRs can interfere tremendously with upstream data. Only CATV systems that have been upgraded to HFC plant are capable of high-speed two-way data transfer. The use of optical fiber reduces noise and increases the upstream bandwidth, facilitating upstream data transmission. Optical fiber can also transmit signals over much longer distances before requiring amplification. To send the data over the HFC network, laser transmitters convert signals sent from the head-end into optical signals. At each cable node, a laser receiver reconverts the signals so they can again be transmitted over tree-and branch configured coaxial cable plant, which goes into each individual house. The most important factor in the deployment of two-way cable data services is the availability of high-quality two-way HFC plant. But upgrading to HFC is very expensive. It costs a cable company $200 - $250 per home to upgrade to HFC plant (Clark, 1999). Some cable companies that have not upgraded to HFC are offering cable modems that use the RF coaxial cable spectrum for fast downstream transmission and a traditional dial-up modem to handle upstream communications over the public telephone network. However, telephone-return modems do not provide some key benefits available with two-way cable mo...
