What is ADSL?

Commonly simplified as DSL, Asymmetric Digital Subscriber Line (ADSL) is a technology for high-speed Internet access. It uses existing copper phone 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 browsing. In contrast, DSL is typically not as fast as cable Internet access. It is generally suitable for moderate gaming, computer-aided design, multimedia streaming, and large file downloading.

An ADSL (Asymmetric Digital Subscriber Line) modem.

ADSL speed

The fastest dial-up modems are rated at 56 kilobits per second (Kbps) and generally operate at around 53 Kbps under good conditions. In comparison, ADSL allows download speeds of 1.5 to 8 megabits per second ( Mbps ), depending on the degree of DSL service purchased. Cable internet is capable of supporting up to 30 Mbps.

Standard ADSL has considerably faster download speeds than upload speeds.

How does ADSL work?

ADSL uses standard phone lines to upload and download data on a digital frequency, which separates these data streams from the analog signals that phones and fax machines use. The phone can be used at the same time when browsing the web with DSL service because the signal is operating on a different frequency; this is not true for 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 may be generated by DSL signals.

A compatible Internet Service Provider (ISP) is required to receive DSL service, as is a DSL modem. The modem can be provided by the ISP or can be purchased separately by the end user. Most US-based ISPs that offer DSL service require at least one-year subscriber contracts. DSL is generally more expensive than dial-up service, but the latter is slowly becoming obsolete as user bandwidth requirements increase, due to things like video streaming.

See also  What is an external graphics card?

DSL is an “always on” service, which means that as long as the user’s computer is on, it will automatically remain connected to the Internet unless manually disconnected by software or hardware. Family members can share DSL accounts for a basic monthly fee. Unlike dial-up service, which stipulates that only one session is open at a time, multiple members can use DSL service at the same time on multiple computers in the home. A router can also be used with this type of ISP to provide wireless access throughout the house.

Asymmetric vs. symmetrical

The “asymmetric” in ADSL refers to the fact that the speed at which data is downloaded, the data arriving from the Internet to the end user’s computer, is faster than the rate of data being loaded, the data traveling from the computer from the user to the Internet. Data upload speed is slower because web page requests are relatively small data strings that do not require a lot of bandwidth to be handled efficiently. Consequently, more speed can be dedicated to downloading 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 on the physical distance from local hubs, as well as the number of people using the service at one time in the same area.

See also  What is a DVR or Digital Video Recorder?

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 it 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.

Leave a Comment