What is ADSL (Asymmetric Digital Subscriber Line)?

This internet connection type leverages existing phone lines

Asymmetric Digital Subscriber Line, or ADSL, is a communication technology that offers faster connection speeds over traditional telephone lines than dial-up internet provides. It powers many internet connections worldwide and enabled the broadband internet speeds that drove Web 2.0 and beyond.

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

Ethernet cable
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What is an ADSL Line?

Newer technologies like fiber require bespoke fiber optic cables to take advantage of the faster speeds offered by such advances. However, ADSL is more flexible. It operates over the same copper telephone lines that have powered 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. Due to prohibitive deployment costs, it would be many years before the technology caught on. 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 specialized equipment.

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

How Does ADSL Work?

For asymmetric digital subscriber line connections to work correctly, several technologies must operate in tandem. A 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 others also from the local area are connected to a digital subscriber line access multiplexer (DSLAM).

Ethernet ports on a modem
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, connecting the 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 that form the basis of the connection. However, there's a lot that can affect the speed of an ADSL connection, and much of it is not in the hands of the consumer.

Connections described or marketed as uncapped typically max out at around 10 Mbps. These tend to bundle more local connections into the same bandwidth pool, meaning more people are 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 higher speeds. In some areas, speeds can reach 40 Mbps, because there isn't competition from as many users.

Another factor impacting ADSL speed is the distance from the telephone exchange. As the distance 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, alleviate the issue of great distances between a modem and the exchange. Still, there's no supplanting a shorter line when it comes to improving ADSL speed.

Modern ADSL lines are more capable of delivering high-speed internet access across longer distances. However, any connection that's ten miles or more from an exchange will 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. It's a must if you want to enjoy the complexity and broad feature set of today's internet.

Fiber is recommended if it's cost-effective. It's faster and less susceptible to environmental factors like exchange distance and weather. However, if fiber is more expensive, ADSL is an alternative that's more than quick enough for most web functions.

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