Takeup of broadband in the UK is growing fast, with a 500% increase in subscriber numbers in 2001. In this issue we bring you a guide to the technology behind this new generation of services.

Broadband can refer to a number of technologies that allow consumers to make use of services which require a lot of data to be transferred, such as video-on-demand and interactive gaming, as well as high-speed Internet access.

There are various different definitions for broadband: Oftel defines broadband as any higher bandwidth, always-on service offering data rates of 128 kbps (thousand bits per second) and above.

Bandwidth is the amount of information that can be carried, measured in bits per second (bps). The greater the bandwidth or data rate, the faster you can send information from one point to another. Typical broadband data rates are 512 kbps (thousand bits per second) downstream and 128 kbps upstream (see below).

This compares with dial-up narrowband services over a standard domestic modem that offer rates of up to 56 kbps. The majority of people using the Internet at home use a narrowband connection.

In the UK, there are two main technologies delivering broadband to residential users: cable modem and ADSL (asymmetric digital subscriber line).

Cable modem offers high-speed Internet access over cable networks for customers in cable franchise areas. Data rates are 512kbps downstream and 128kbps upstream. Cable modem currently accounts for the majority of home broadband users in the UK.

ADSL is one of the family of DSL technologies which transform a normal telephone line into a high-speed digital line that enables access to telephony services and the Internet at the same time. ADSL provides always-on access to the Internet at speeds that are 10 to 40 times faster than a standard 56k modem.

With ADSL, downstream data rates (the amount of data going from the network to the customer) are usually higher than upstream rates (from the customer to the network) as consumers will be downloading lots of data, for example video images and sound, but only sending back small responses such as text.

With the related SDSL (symmetric digital subscriber line) technology, the maximum upstream and downstream rates are the same, so it is more useful for businesses sending larger amounts of data over the Internet.

The industry is developing other variants such as HDSL (high data rate DSL) and VDSL (very high data rate DSL).

The performance of DSL depends on the distance between the customer's home and the exchange. Using a technology called rate adaption the reach of DSL is now 5.5km.

Local loop unbundling, which is often mentioned in the context of broadband, is not a technology but a process to enable operators other than BT to offer their own DSL products direct to consumers by installing their own equipment in BT's exchanges.

Broadband fixed wireless access (BFWA) allows high-speed data connections using radio links between an aerial located on the user's premises and a base station, rather than using a telephone line or a cable television network. BFWA is available to residential and business users in major urban areas in the UK.

Developing mobile and satellite technologies should in the future enable customers in more remote areas to use broadband. A number of service providers have now launched broadband satellite services in the UK and further rollout of services is expected later this year. Broadband satellite services are currently more expensive than DSL or cable modem.

Finally, an important technology for delivering broadband to business customers is leased lines: always-on dedicated connections which can offer varying bandwidth depending on the company's requirements. If you are connecting from your desktop at work, it is likely to be through a leased line.

Further information about developments in broadband, including the latest statistics, is available in Oftel's Internet and Broadband Access Brief.