What is ADSL (Asymmetric Digital Subscriber Line)?

This internet connection type leverages existing phone lines

Ethernet cable

Max Pixel

The Asymmetric Digital Subscriber Line, or ADSL, is communication technology designed to offer faster connection speeds over traditional telephone lines than dial-up internet can provide. It's what powers many internet connections around the world, and it's the technology that enabled the broadband internet speeds which drove Web 2.0 and beyond.

Although today ADSL is supplanted in many well-connected areas by fiber internet connections, ADSL is still an important technology for many. Understanding what ADSL is can be an important step in picking the right connection for your home or business.

What is an ADSL Line?

Where newer technologies like fiber require bespoke fiber optic cables to take advantage of the faster speeds offered by such advances, ADSL is much more flexible. It operates over the same copper telephone lines that have been powering voice calls between landlines for decades. ADSL transmits digital data over those same lines at high speed, letting you do everything from sending emails to watching YouTube videos.

ADSL was originally patented in 1988, but it would be many years before the technology would catch on due to the prohibitive costs involved in its deployment. To reach the kind of speeds that would make ADSL a viable alternative to dial-up internet, clever signal processing was required, which in turn required specialist equipment.

ADSL began to roll out nationwide in the late 1990s and was popularized throughout the 2000s. While rural telephone exchanges may still rely on older technologies, today, most have been upgraded to support ADSL, though they also support more contemporary technologies like fiber as well.

How Does ADSL Work?

For asymmetric digital subscriber line connections to work correctly, a number of technologies have to operate in tandem. An end user needs an ADSL modem, which is connected by a traditional twisted pair of copper telephone wires and connects to a local telephone exchange. At that end, the copper wire and a number of others also from the local area, are connected to a digital subscriber line access multiplexer (DSLAM).

Asim18/Wikimedia

The user's ADSL modem transmits and receives data to and from the DSLAM. It uses frequencies that differentiate the digital data from potential voice calls taking place on the same phone line. Those two signals are split at the exchange and handed off to different networking technologies further up the chain, helping to connect the end user to the telephone or web server at the other end.

What is ADSL Speed?

ADSL has a hard speed ceiling of around 50 Mbps due to the available bandwidth of the copper wires forming the basis of the connection. However, there’s a lot that can affect the speed of an ADSL connection and much of it isn't in the hands of the consumer.

Connections described or marketed as "uncapped" typically max out at around 10 Mbps. They tend to bundle together more local connections into the same bandwidth pool, meaning there are simply more people attempting to use the available bandwidth.

Capped ADSL connections, which have stricter limits on the number of simultaneous connections in the local area, can reach much higher speeds. In some areas, speeds can reach as much as 40 Mbps, because there simply isn't the competition from as many other users.

Another factor impacting ADSL speed is distance from the telephone exchange. As it increases, the signal strength degrades, leading to a slower and less reliable connection. Loop extenders, which boost the strength of the signal on longer lines, can go some way to alleviating the issue of great distances between a modem and the exchange, but there’s no supplanting a shorter line when it comes to improving ADSL speed.

Modern ADSL lines are far more capable of delivering high-speed internet access across longer distances, but any connection that's 10 miles or more from an exchange is still going to suffer negative consequences associated with being that far removed.

Should You Get ADSL?

If you can't get fiber internet access — whether that's fiber to the cabinet, or fiber to the premises — ADSL is the next best thing and a must if you want to enjoy the complexity and broad feature set of the modern internet experience.

Fiber is recommended if it's cost effective in your area, as it's faster and much less susceptible to environmental factors like exchange distance and the weather. However, if Fiber's more expensive, then ADSL is a fantastic alternative that’s more than quick enough for most web functions.

Wireless alternatives are another option for those with coverage, and new standards like 5G promise higher speeds that are well in excess of what's possible with ADSL. However, wireless internet access can be much more limited in terms of the data you use. It can also be expensive, so make sure to determine the costs involved before opting for such a connection over a more traditional ADSL solution.