Digital Subscriber Line (DSL) gives you broadband access over your existing copper telephone wires. A DSL connection to the Internet is a high-speed, ‘always on’ (you don’t need to dial up your ISP each time you want to connect) connection that lets you use your telephone lines for making and receiving calls and for Internet access simultaneously. In this article, we’ll take a closer look at different types of DSL and how DSL works.

DSL is also known as xDSL, with the ‘x’ standing for various kinds of DSL technologies. These technologies differ in the connect speed and connection (asymmetric or symmetric) they provide. A point to note in DSL technology, whatever the flavor, is that there’s a trade-off between speed and distance. That is, the more the distance between your premises and those of the service provider, the lower the speed you’re likely to get. So, DSL works best if you’re closer to the premises of your service provider.

DSL flavors

Some popular kinds of DSL are:
ADSL Asymmetric Digital Subscriber Line, as the name suggests, is an asymmetric connection, that is, it provides higher speeds for downstream (from the Internet to the user) data than for upstream (from the user to the Internet) data. This kind of Net usage pattern is seen most in homes and among individual users, where downstream data usually includes graphics, audio, and video, while there isn’t much data to transfer upstream. Downstream speeds for ADSL range from 1.5–9 Mbps, while upstream speeds are up to 1.5 Mbps, for a distance of 18,000 feet from the service provider’s premises.

ADSL Lite (or G.lite) This is a lower speed version of ADSL and provides downstream speeds of up to 1Mbps and upstream speeds of 512 kbps, at a distance of 18,000 feet from the service provider’s premises. It is intended to simplify DSL installation at the user’s end.

R-ADSL The Rate-Adaptive Digital Subscriber Line provides the same transmission rates as ADSL, but an R-ADSL modem can dynamically adjust the speed of the connection depending on the length and quality of the line.

HDSL The High Bit-Rate Digital Subscriber Line provides a symmetric connection, that is, upstream speeds and downstream speeds are the same, and range from 1.544 Mbps to 2.048 Mbps at a distance of 12,000–15,000 feet. Symmetric connections are more useful in applications like videoconferencing, where data sent upstream is as heavy as data sent downstream. HDSL-II, which will provide the same transmission rates but over a single copper-pair wire, is also round the block.

IDSL The ISDN Digital Subscriber Line provides up to 144 kbps transmission speeds at a distance of 18,000 feet (can be extended), and uses the same techniques to transfer data as ISDN lines. The advantage is that, unlike ISDN, this is an ‘always on’ connection.

SDSL The Single-line Digital Subscriber Line provides symmetric transmissions at rates similar to HDSL. The difference is that it uses a single copper-pair wire to do so (while HDSL uses two or three), and operates at a maximum distance of 10,000 feet from the service provider’s premises.

VDSL The Very High Bit-rate Digital Subscriber Line is the fastest of all xDSL flavors and provides transmission rates of 13–52 Mbps downstream and 1.5–2.3 Mbps upstream over a single copper-pair wire, at a distance of 1,000–4,500 feet from the service provider’s premises.

Of these, ADSL and HDSL have found the widest implementation, the former being more popular for home usage.