Script: Cable-DSL audio primer

If you've been thinking of high-speed Internet connections for a small business or remote office, you may be wondering whether to go with DSL or cable modems. In this primer, we'll explain and compare the two technologies.

DSL stands for digital subscriber line. The foundation of the technology is the four-wire telephone cable that is standard throughout North America. Until recently, only two wires were needed for voice calls. The other two were largely unused until the mid-90s, when explosive demand for Internet bandwidth led several firms to look beyond the 56K bit/sec limit on analog dial-up connections.

Early DSL standards entailed installing a signal splitter at the customer premises to separate the voice and data as well as a "DSL modem" to connect the user's PC to the data-stream. The two extra wires in standard telephone cable were employed to carry digital data to and from the local telephone office, which had to be located no more than 2 1/2 miles away. At the telco office, a digital subscriber line access multiplexer, or DSLAM, served as the connection between the individual DSL streams and the backbone network. Top speeds were 1.5M bit/sec, but at a much lower cost than a T-1 or ISDN connection.

As the technology has matured, the DSL playing field has gotten a lot more crowded, and a lot more complex. In fact, DSL is often known as "xDSL" to account for the many flavors of DSL that have arrived on the scene. A common variant is asymmetric DSL, or ADSL. Asymmetric refers to the unequal upstream and downstream speeds -- typically, you have much higher bandwidth for downloading than uploading. ADSL technology is aimed at home users and small offices that are mostly interested in high-speed access to Web content, and even streaming media. Download speeds can top 8M bit/sec, depending on the distance to the local telco office. Upload speeds are much slower - typically maxing out at 640 kilobits per second. As for cost, the faster the downstream DSL connection, the higher the monthly price.

Symmetric DSL, or SDSL, has equal upstream and downstream speeds of up to 1.5M bits/sec in both directions. Then there is VDSL, or very high bit rate DSL, a developing asymmetric flavor that can reach downstream speeds of 55M bit/sec, but only at a distance of up to 1000 feet from the local telco office. Beyond that, VDSL transmission rates drop dramatically, which severely limits its customer base.

Also gaining in popularity is DSL Lite, or G.Lite. This is yet another asymmetric variant, but what makes it different is that it does not require a splitter at the customer premises. The line is instead split at the telco office, which cuts down installation costs. Speeds range from 1.5M bit/sec to 6M bit/sec downstream, and about one-tenth those rates for upstream.

Cable modems are based on a different foundation, the existing cable TV infrastructure in most U.S. cities and towns. It works like this: The coaxial or fiber-optic cable used to carry HBO and CNN to a house has enough extra bandwidth that can used for bidirectional network traffic. At the cable company's head end controller, IP traffic is modulated, or converted to an analog signal, for delivery over a spare downstream television channel. When the signal reaches the user site, it hits a splitter. The splitter sends normal cable TV traffic to the user's set-top box. The Internet traffic is shunted off to the user's "cable modem" where it is demodulated, or converted back into Ethernet packets, and fed into the user's computer using a 10Base-T Ethernet card. The process is reversed for upstream traffic, but on a channel with less bandwidth, resulting in slower upload speeds. Users also have to share bandwidth with other cable modems in their neighborhood.

As for which is better, DSL or cable, there is no clear answer. DSL is affordable, good for business installations and usually consistent in terms of available bandwidth. On the other hand, most customers don't have much choice -- not all telephone companies offer DSL services, and those that do usually offer only one DSL flavor. There have been complaints about months-long waits for installations from certain telephone companies and ISPs, problems getting installations to work, poor service, billing problems and logjams at central offices when the product has been oversold. The situation is made more complex when multiple companies are involved: The phone company provides the physical line, a local ISP may supply the service, but it may be reselling a third-party national carrier. Another problem: DSL speeds tend to deteriorate over distance - installations located more than 3 miles from the local telephone office are impossible with current technology.

Also, recent failures among DSL service providers have left customers without any high-speed service.

Cable modems are a good choice in a residential area that already has a cable TV infrastructure. This makes it a popular choice for telecommuters or people who have home offices. Cable modem services also are inexpensive, and are frequently bundled with cable TV packages. But there are disadvantages, too. The Ethernet card limits download speeds to just 4M bit/sec. Also, if many people are online at the same time, speeds drop dramatically because the bandwidth is shared with others in the neighborhood. The shared network can lead to security holes, which may require so-called "personal firewalls" or VPN software to plug the gaps. And finally, while the 'cable guy' may be a pro at installing cable TV, he may not be so knowledgeable about attaching the modem and other Ethernet gear to your computer.

But while it's not clear whether DSL or cable is king, there's one thing for certain: Broadband is a lot better than dial-up, and will remain so for the foreseeable future.

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