itself and the number of simultaneous users seeking access to a shared line. ADSL. Asymmetric Digital Subscriber Lines locate modems on either end of existing copper telephone lines. As the name suggests, they realize downstream speeds up to 9 Mbps, but upstream speeds up to 640 Kbps. As ADSL operates point-to-point, it does not need media access control, and each user gets the full rate available continuously. However, ADSL modem speeds do depend upon line distance, and the longer lines found today may support speeds no greater than 1.5 Mbps. The average line, however, will support speeds up to 6 Mbps. Variable rate ADSL modems will adapt to line length, offering high-speed service to almost all telephone subscribers Cable Modems and ADSL have comparable capabilities and both can be built into broadband IP-based infrastructures. However, other issues remain, for example security, reliability, stability, and home wiring, etc. It is likely that all of them will pale before the commercial benefits of ubiquitous access enjoyed by telephone companies and ease of network deployment enjoyed by CATV companies, but they must be considered, by operators and users alike, as the information superhighway begins, finally, to take some shape. Security All signals go to all cable modem users on a single coaxial line, creating serious prospects of intended or inadvertent wiretapping. ADSL, on the other hand, is inherently secure. Intended wire tapping requires invading the line itself (often underground) and knowing the modem settings established during initialization -- not impossible, but very difficult. Encryption and authentication will be important parts of both systems, but necessary for cable modems. (Several cable modem vendors have put encryption into their modems.) Reliability Cutting a CATV line in the street or losing above ground cable in a storm will bring down all users on that line. A single streaming transmitter on a CATV line will br...
