omputers. At present run rates, another 240 million will be added by 2001 as PCs start to approach the global population of televisions. Small offices and residences will absorb at least 25% of them, or 100 million. Forester has projected 6 million cable modems will be installed by the year 2000. With suitable pricing, telephone company connections could be triple that number, yielding an altogether reasonable figure of 25 million personal computer users operating at megabit rates as the century turns. BASIC MODEM TECHNOLOGIES Cable Modems. While cable modems come in many forms, the most typical create a downstream data stream out of one of the 6 MHz TV channels that occupy range above 50 MHz (and more likely 550 MHz). An upstream channel carved out of the currently unused band between 5 and 50 MHz. Using 64 QAM, a downstream channel can realize about 30 Mbps (the speed of 10 Mbps refers to PC rates associated with Ethernet connections). Upstream rates vary considerably from vendor to vendor. The downstream channel is continuous, but divided into cells or packets, with addresses in each packet determining who actually receives a particular packet. The upstream channel has a media access control that slot user packets or cells into a single channel. To avoid collisions, the system gates each upstream packet onto the network with control signals embedded in the downstream information stream. (Some cable modem configurations divide the upstream into frequency channels and allocate a channel to each ! user. Others combine the two multiplexing methods. A few modem companies are proposing techniques like spread spectrum or code division multiplexing to provide more robustness in the presence of ingress noise.) Cable modem rates do not depend upon coaxial cable distance, as amplifiers in the cable network boost signal power sufficiently to give every user enough. Variation in cable modem capacity will depend rather on ingress noise in the line...
