Broadband is the common name given to the Asymmetric Digital Subscriber Line (ADSL) service, an always-on link between you and your Internet Service Provider (ISP).

The "asymmetric" part of the name alludes to the difference in connection speed between uploads and downloads, where the upload speed is usually 256 kb/s (kilo-bits per second) and the dowload speed can be anywhere from 512 kb/s up to 8Mb/s (megabits per second, where 1 megabit ≈ 1000 kilobits), although this will increase as newer technology is rolled out. The download connection speed will vary depending upon your distance from the telephone exchange, the quality of your in-house telephone wiring, your "modem" and restrictions imposed by your ISP. The actual speed you experience on-line may also be further degraded by bottlenecks at the exchange, your ISP or on the "route" your network activity takes across the digital landscape known as the internet. For domestic installations the lower upload speed is more than adequate for carrying keystrokes and file uploads one at a time.

There is another service available called SDSL (Symmetric Digital Subscriber Line) where the upstream and downstream speeds are the same (hence the "symmetry"), but this is generally used by businesses and costs a lot more than ADSL. Most domestic users would see no benefit from using this service.

Telephone operators install equipment in the exchange to share a high-speed Asynchronous Transfer Mode (ATM) connection between multiple subscribers, typically using a 50:1 "contention ratio". This means that up to 50 people could be using the physical connection at the same time, although the impact of this is rarely seen outside peak periods. The ATM equipment can connect to any UK-based ISP. This allows you to choose which of them provides you with your Broadband service, however a new provider will not commence your service until the old one has released you from their contract, by issuing you a PAC code. This protects their commercial interests, but can be abused, causing frustration in the user community.

At your end of the connection, ADSL shares the physical wires used to connect your telephone to the exchange. It does this without any interference by way of the "splitter" you are requred to install on all telephone sockets in the building. These devices contain electronic filters to ensure that the always-on digital connection and the analogue voice service of the normal telephone do not interfere with one another. There is a similar piece of equipment at the exchange for each telephone connection, which splits the signals back into two, one digital and one analogue.

The digital side of the exchange "splitter" is connected to the ATM, which operates in a similar fashion to the standard telephone exchange, but with restrictions placed on who you can "dial". The "number" you dial is defined by the UserID you pass to the router, which looks like an email address, such as user@service.provider. In this example the "service.provider" is used to route the connection to your ISP, and the "user" is used, along with the password you supply, to authenticate the connection. Once connected to your ISP in this way, they are responsible for routing your requests to the appropriate services on the internet and returning any replies.