8.8 Connecting to the Internet
8.8.5 Digital Subscriber Line (DSL)
Digital Subscriber Line (DSL) is an always-on technology. This means there is no need to dial up each time to connect to the Internet. It is a relatively new technology currently being offered by phone companies as an add-on service over existing copper wire or phone lines.

DSL comes in several varieties:

  • Asymmetric DSL (ADSL) currently is the most common implementation. It has speeds that vary from 384 Kbps to more than 6 Mbps downstream. The upstream speed is typically lower.
  • High Data Rate DSL (HDSL) provides bandwidth of 768 Kbps in both directions.
  • Symmetric DSL (SDSL) provides the same speed, up to 3 Mbps, for uploads and downloads.
  • Very High Data Rate DSL (VDSL) is capable of bandwidths between 13 Mbps to 52 Mbps.

IDSL has a top speed of 144 Kbps, but is available in areas that do not qualify for other DSL implementations. IDSL is actually DSL over ISDN lines.

The figure summarizes useful information on the different varieties of DSL. The generic term for DSL, including all its implementations, is xDSL.

Transfer rates are often broken down into upstream and downstream rates. Upstream is the process of transferring data from the end user to the server. Downstream is the process of transferring data from the server to the end user. For instance, when a user submits their username and password to gain access to e-mail, they are uploading, or transferring that data upstream, to the e-mail server. When the contents of the mailbox are displayed on the web browser, that data is downloaded, or transferred downstream, to the computer.

ADSL is currently the most commonly used DSL technology. Its fast downstream speed, typically 1.5 Mbps, appears to work in its favor because most Internet users spend the majority of their time doing tasks that require a lot of downloading, like checking e-mail and surfing the web. The slower upload rate does not work that well when hosting a web server or FTP server, both of which involve upload-intensive Internet activities.

ADSL uses a technology called Frequency-Division Multiplexing (FDM) to split bandwidth in order to create multiple channels. Other DSL implementations use another technique known as echo cancellation, which is more efficient, but also more expensive. Either way, this ability to create multiple channels is the reason a DSL user can be surfing the Internet while at the same time using the telephone to call a friend. The advantages and disadvantages of cable modems are discussed in the next section.