How Does DSL Work and What Determines Its Speed?

A digital subscriber line (DSL) is the original high-speed broadband solution. Cable, fiber, and satellite have gained a lot of popularity over the past several years, but over 20 percent of Americans still use DSL.

Let’s dig in and answer the big questions like How does DSL work? and What determines the speed of your DSL connection?

DSL piggybacks on existing copper phone lines, using them to transmit digital data — your internet uploads and downloads — alongside traditional phone signals.

Whereas dial-up internet blocks phone signals from using the line, DSL operates at higher frequencies than phone signals do. Because it doesn’t interfere with phone service, DSL can be an always-on solution.

Many internet service providers (ISP) offer DSL networks. (This doesn’t mean that DSL operators offer only DSL; in many cases, the same provider may operate multiple types of internet networks, including DSL, cable, and fiber. A provider’s offerings may vary depending on where you live.)

DSL has gone through several major changes in recent years, with constant updates in order to compete with technologies like cable and fiber, which offer overall faster download and upload speeds. This has led to the popularity of VDSL2, ADSL, and other DSL varieties you’ve probably seen while shopping for home or business internet service.

All types of DSL fall into one of two categories:

• Symmetrical: Upload and download speeds are the same.
• Asymmetrical: Download speeds are faster than upload speeds, taking advantage of the fact that most users download more content than they upload.

ACRONYM	DESCRIPTION	TYPE	DOWNLOAD SPEED	UPLOAD SPEED
ADSL	Asymmetrical Digital Subscriber Line	Asymmetrical	8 Mbps	384 Kbps
ADSL 2+	Asymmetrical Digital Subscriber Line 2+	Asymmetrical	20 Mbps	850 Kbps
VDSL	Very High Bit-Rate Digital Subscriber Line	Asymmetrical	52 Mbps	2.3 Mbps
VDSL2	Very High Bit-Rate Digital Subscriber Line 2	Symmetrical	100 Mbps	100 Mbps
SDSL	Symmetric Digital Subscriber Line	Symmetrical	1.54 Mbps	1.54 Mbps

Most customers have access to at least two broadband technology options. DSL is one of the most common, available to more than 90 percent of the U.S.

Cable and fiber are the most popular alternatives to DSL, although options like satellite internet and fixed wireless are becoming increasingly popular for rural internet users. You can check the specific options in your neighborhood by using our ZIP code coverage.

Sadly, not everyone can get the speeds listed on their DSL provider’s website. Your actual internet speeds will depend on how close you live to the nearest DSL access multiplexer (DSLAM) and how your home network is set up.

1. Distance From DSLAM or “Local Office”

   Unlike with cable, DSL speeds are strongly impacted by the distance between the provider and the subscriber.

   • Within one mile: Speeds should be close to those advertised in the DSL provider’s marketing materials.
   • Between one and three miles: Speeds may likely be considerably slower, and you may encounter issues with packet loss.
   • Beyond three miles: You’ll likely have problems with both the speed and consistency of your connection.

   Unfortunately, determining your distance from the local DSLAM isn’t simple. Generally, DSL providers promise that they can deliver speeds close to those advertised, although customers who live too far away to get comparable speeds will simply not be serviced.

2. Your Home Network Setup

   Even if you are getting high-speed DSL in your home, the signal still has to pass through your modem and your router before it reaches your computer. Bad Wi-Fi can slow down your internet connection just as easily as living too far from the DSLAM.

   If you are trying to speed up your internet connection, it may be better to hardwire PCs, gaming consoles, and other bandwidth-intensive devices to your router using an Ethernet cable — even if it means sacrificing the convenience of Wi-Fi. Ethernet connections offer faster speeds and lower latency than Wi-Fi, especially if you haven’t upgraded to a new router in a while.

   Older modems may not be able to support the most recent speed upgrades by your provider. Modems don’t have to be upgraded nearly as often as routers, but modems are still packed with technology, and that technology may no longer be able to function in the modern world. If you are renting a modem from your ISP, they can probably tell you if it’s time for an upgrade.

   If you have a combination modem/router — also called a gateway — you should keep in mind that either the modem or router functionality could be outdated, although usually the router functionality demands an upgrade first. If you’re using a single box to run your network, it’s probably a gateway.

DSL gets a bad rap in the broadband world since the top speeds it offers are roughly half the speed of cable and a fraction of the speed of fiber. But those are top speeds, and there are some cases where DSL is actually faster than cable — even for suburban and urban customers who may have several broadband providers to choose from.

Cable service is very susceptible to peak-time slowdown. This is a common problem in neighborhoods serviced by coaxial cable internet connections, as the network can get congested when everyone gets home from work and starts watching Netflix.

While DSL is a direct line to the internet provider, cable connections are shared across dozens or even hundreds of residences. Cable providers assume that everyone won’t use their maximum bandwidth at the same time, and as a result, many neighborhoods wind up oversubscribed.

DSL can be an attractive alternative in these neighborhoods since the speed, even if it’s a modest 35 Mbps or so, will always be 35 Mbps, regardless of what the neighbors are up to. Sometimes, it pays not to follow the crowd.

Because it doesn’t require a phone line, satellite internet is one of the few high-speed internet options that is more widely available than DSL. Satellite internet continues to be less popular than DSL, though, due to two factors: price and latency.

Satellite is typically one of the most expensive internet options, with a price tag much higher than that of DSL. Although satellite offers more bandwidth than DSL, it tends to suffer from high latency, making it difficult to play online games over a satellite internet connection.

There is hope on the horizon. Starlink, the upcoming satellite internet service from Elon Musk, claims that it will bring lower-priced, lower-latency satellite internet to the world in the near future.

Ultimately, DSL will always be perceived as the budget option since it provides raw speeds slower than those of cable or fiber internet and is cheaper than satellite and some other widely available broadband options.

For rural customers, DSL may be the only practical option, but thankfully, advances in technology have improved it to the point where it can more than handle modern use cases like streaming Hulu, chatting on Skype, and gaming.