Commonly simplified as DSL, Asymmetric Digital Subscriber Line (ADSL) is technology for high-speed Internet access. It uses existing copper telephone lines to send and receive data at speeds that far exceed conventional dial-up modems, while still allowing users to talk on the phone while they surf. By contrast, DSL is typically not as fast as cable Internet access. It is generally well-suited for moderate gaming, computer-aided design, streaming multimedia, and downloading large files.
The fastest dial-up modems are rated at 56 kilobits per second (Kbps), and usually operate at about 53 Kbps under good conditions. By comparison, ADSL allows download speeds from 1.5 to 8 megabits per second (Mbps), depending on the grade of DSL service purchased. Cable Internet is capable of supporting up to 30 Mbps.
How Does ADSL Work?
ADSL uses standard telephone lines to upload and download data on a digital frequency, which sets these datastreams apart from the analog signals that telephones and fax machines use. The telephone can be used at the same time when surfing the Web with DSL service because the signal is operating on a different frequency; this is not true of conventional dial-up Internet access. It may be necessary to install inexpensive filters on each phone or fax line to remove any "white noise" on the line that might be generated from the DSL signals.
A compatible Internet service provider (ISP) is necessary to receive DSL service, as is a DSL modem. The modem may be provided by the ISP, or it may be purchased separately by the end-user. Most US-based ISPs that offer DSL service require subscriber contracts of at least one year. DSL is usually more expensive than dial-up service, but the latter is slowly becoming obsolete as user bandwidth requirements rise, due to things like streaming video.
DSL is an "always on" service, meaning that as long as the user's computer is powered on, it will automatically stay connected to the Internet unless it is manually disconnected via software or hardware. Family members can share DSL accounts, with a basic monthly fee. Unlike dial-up service, which stipulates that only one session be open at a time, multiple members can be using DSL service at the same time on various computers in the house. A router may also be used with this type of ISP to provide wireless access throughout a home.
Asymmetric vs. Symmetric
The "asymmetric" in ADSL refers to the fact that the speed at which data is downloaded, the data coming to the end-user's computer from the Internet, is faster than the rate for uploaded data, the data traveling from the user's computer to the Internet. The speed of uploading data is slower because Web page requests are fairly small data strings that do not require much bandwidth to handle efficiently. Consequently, more speed can be dedicated to downloading more bandwidth-intensive data.
Some businesses may require matching rates for uploading large files. For them, Symmetric Digital Subscriber Line (SDSL) is an option. "Symmetric" indicates that both datastreams are operating at the same speed of 1.5 to 7 Mbps, depending on the grade of service purchased. SDSL service requires a dedicated telephone line, however, because unlike ADSL, telephone and fax services cannot share a line with this service.
ADSL is not accessible to all communities, and coverage is often especially spotty in rural areas. Dedicated DSL providers, or even the local phone company, can verify if service is available in a specific locale. Speeds will vary depending upon the physical distance from local hubs, as well as the number of people using the service at one time in the same area.
Some customers who live close to an ISP hub may be able to take advantage of newer varieties of ADSL, called ADSL2 and ADSL2+, which have even greater throughput rates, from 12 to 24 Mbps for downloading and 1 to 3.5 Mbps for uploading. In addition, there are other types of DSL that offer customers other benefits. Rate Adaptive DSL (RADSL) uses a special modem that can adapt to changing line conditions, changing the speed as needed. Very high bit-rate DSL (VDSL) offers download speeds of up to 52 Mbps, but is not as widely available and is only able to achieve such high speeds very close to a hub.
Is DSL Ever as Fast as Cable?
Under identical conditions, cable has a strong speed advantage over ADSL; however, identical conditions rarely exist. In a given locale, cable speeds can suffer if they are too far from the nearest hub, or may suffer a bottle-neck if too many users are online at once. Artificial bandwidth caps placed on service at times of heaviest usage are not uncommon either. While the same factors are true of DSL, it does mean that in certain markets, some DSL providers may actually be able to provide faster service than some cable providers — especially if newer technologies like VDSL are available. As a result, whether DSL is as fast as cable is not always obvious, and it is a good idea for people looking for an ISP to research the local market and read customer reviews before committing to a particular provider.