Summary

DSL and cable modem network access are two alternate ways to connect to an NSP without the use of more expensive dedicated service. DSL technology is a modem technology using existing twisted-pair telephone lines capable of carrying high-bandwidth applications.

There are several forms of xDSL, each designed around specific goals and needs of the marketplace. Each of these is summarized in Table 7-2.

Cable systems originally were designed to deliver broadcast television signals efficiently to subscribers' homes. Downstream video programming signals begin around 50 MHz, the equivalent of channel 2 for over-the-air television signals. The 5 MHz to 42 MHz portion of the spectrum is usually reserved for upstream communications from subscribers' homes.

Each standard television channel occupies 6 MHz of the Radio Frequency (RF) spectrum. Traditional cable systems have 400 MHz of downstream bandwidth, capable of carrying the equivalent of 60 analog TV channels. Modern hybrid fiber/coax (HFC) systems have 700 MHz of downstream bandwidth, with the capacity for approximately 110 channels.

The MCNS defined the DOCSIS 1.0 standard, which in turn was accepted as the North American standard.

Table 7-2. DSL Service Summary

DSL Type	Description	Data Rate Downstream; Upstream	Distance Limit	Application
ADSL	Asymmetric digital subscriber line	1.544 to 6.1 Mbps downstream; 16 to 640 Kbps upstream	1.544 Mbps at 18,000 feet; 2.048 Mbps at 16,000 feet; 6.312 Mbps at 12,000 feet; 8.448 Mbps at 9,000 feet	Used for Internet and web access, motion video, video on demand, remote LAN access.
HDSL	High-data-rate digital subscriber line	1.544 Mbps duplex on two twisted-pair lines; 2.048 Mbps duplex on three twisted-pair lines	12,000 feet on 24-gauge wire	T1/E1 service between server and phone company or within a company; WAN, LAN, server access.
SDSL	Single-line digital subscriber line	1.544 Mbps duplex (U.S. and Canada); 2.048 Mbps (Europe) on a single duplex line downstream and upstream	12,000 feet on 24-gauge wire	Same as for HDSL but requiring only one line of twisted-pair.
VDSL	Very-high digital subscriber line	12.9 to 52.8 Mbps downstream; 1.5 to 2.3 Mbps upstream; 1.6 Mbps to 2.3 Mbps downstream	4500 feet at 12.96 Mbps; 3000 feet at 25.82 Mbps; 1000 feet at 51.84 Mbps	ATM networks; Fiber to the Neighborhood.

DSL and cable modem network access is not available in all parts of the country or even to every house and business within a city. Before planning on deploying either of these services, it is imperative to discuss these plans with the local DSL/Cable NSP. In the event these services are not available for connectivity, you need to consider the more traditional Frac-T1/T3, ISDN, or dial-up services.