Dsl Vs Cable Essays - Modems, Internet Access,

Dsl Vs Cable Cable Modems and ADSL Two modem technologies have emerged over the past year for switched data communications services. Cable Modems operate over two-way hybrid fiber/coax and provide user rates as high as 10 Mbps. ADSL Modems (Asymmetric Digital Subscriber Lines) operate over existing copper telephone lines and provide rates as high as 9 Mbps. Both technologies address the large markets for Internet access, remote LAN access for work at home and telecommuting, and network access for the hundreds of millions of personal computers in place today and to be sold over the next ten years. Cable modems may offer more raw speed than ADSL, but that advantage is compromised by inevitable reductions in available cable modem speed. Cable modems share a line with tens of other users; as more users join a line, the capacity available to any one inevitably drops. The top speeds of both technologies will not be usable for years anyway. Internet server speeds, network delays, and personal computer limitations will hold usable rates at or below 2 Mbps for some time. ADSL offers higher security and reliability profiles. Both technologies are at about the same state of maturity and integration. Cable modems may offer a less expensive network solution because of its shared architecture, but that differential is more than offset by infrastructure costs required to upgrade existing networks. The largest advantage of ADSL, and it is a significant one, is the number of telephone lines already installed that can support ADSL, or prospectively available with network upgrades. Today the global ratio is in the order of 400 million to 6 million, or about 60 to 1. Aggressive upgrades will not improve the ratio to better than 10 to 1 in the next five or six years. Even in the United States the ratio today is in the order of 20 to 1, and will not likely get better for CATV suppliers than 3 to 1 over the next five or six years. The end of 1997 will have sold two hundred million personal computers. 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 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