How DSL Works

When you connect your computer, tablet, or other device to the Internet, you can go through a standard modem, a local area network (a LAN) that is connected to the Internet through a cable modem delivering Internet access via the coaxial wire that also delivers cable television, or through a Digital Subscriber Line (DSL). DSL is a technology that allows high-speed data transmission through the same copper wires that are used to deliver landline phone service.

DSL Internet service has many advantages, including the ability to surf the Internet while still being able to use your landline phone at the same time for voice calls. DSL also offers much faster connectivity to the Web than a phone modem does. DSL can often be added to your phone service without requiring any new wiring be added to your existing phone line, saving installation costs. The modem you need to take advantage of DSL service is often supplied by the same company that supplies you with phone service, usually free of charge.

There are downsides to DSL service, too. The quality and the availability of your connection depend on how close you are to the local provider’s central office. Upload speeds are very slow, so if you regularly transfer large files onto the Web, your connection will seem slow.

How DSL Uses Standard Telephone Lines

Telephone lines that serve homes and businesses are nothing more than two copper wires that are strung on poles and enter your house to serve your phones. Copper wires are excellent conductors, and they’re capable of handling much more data than the analog signals that are required to make and receive voice calls. The range of frequencies they can support, also known as bandwith, allows DSL providers to send data for your Internet connection on different frequencies than those required for you to talk and hear on your phone. By matching bandwith to different tasks, it’s possible for the same two wires that carry your conversations to connect to the Internet at the same time without interfering with one another.

ADSL Is Optimized For Looking At The Internet

DSL service is segregated into bandwith that uploads and downloads data. If you want to see things on the Web, that’s downloaded information. If you want to show things to the Web, that’s uploaded information. Since most people use the vast majority of their available bandwith to download data, the makers of DSL equipment have unbalanced the ratio of data that is carried by a DSL line. This is where the term Asymmetrical DSL, or ADSL comes from. Your ADSL uses three to four times more bandwith for downloads than uploads.

DSL Service Can’t Be Offered Everywhere

DSL and ADSL is a distance sensitive technology. That means that there are limits to how far away from your service provider you can be before the degradation of the signal makes it unusable. Your telephone company connects everyone’s landline phones to central offices where the analog signals from your phone are digitized and then sent over fiber-optic or satellite lines for the remainder of their journey to the central office near the phone you are calling. Voice phone calls can be boosted to reach a central office if it’s too far away, but that method doesn’t work for DSL data.

When you have ADSL service installed in your home or office, you are provided with low-pass filters that block frequencies above 4 Kilohertz. These filters are added to all phone jacks that do not serve your DSL modem. Since all analog voice communications take place in frequencies below 4KHz, the filters block out all data transmissions that might interfere with your regular landline telephone calls.

DSL Service Doesn’t Degrade With More Users

On the customer’s end, DSL services provide what you would call a modem, but are more technically referred to as a transceiver. The transceiver is the point where your computer or network device is connected to the DSL line. On the other end, the provider has a device called a DSLAM. The DSLAM connects many customers together and provides a high-capacity Internet connection to all of them simultaneously. DSLAMs have a big advantage over cable modems. Cable modems share a whole neighborhood’s cable connections through what’s termed a node. A node is like a shared cable box for many users. As more users sign on and use bandwith, your cable service can bog down because everyone is sharing the node. A DSLAM is connected directly to the Internet, and each user is connected directly to the DSLAM, so other users don’t affect your bandwith.

For more DSL connection information, see DSL Vergleich.