xDSL is a generic term used to refer to the family of DSL technologies. There are several different forms of DSL with different capabilities suited to different markets including:

ADSL (Asymmetric DSL)
Provided over an existing telephone line (one copper pair), ADSL converts it to a high speed digital line up to 40 times faster than a traditional modem. It is asymmetric because it provides more bandwidth in one direction than the other.

SDSL (Symmetric DSL)
Similar to ADSL, but it offers the same bandwidth in both directions. It is suited to situations where the user needs to send and receive large amounts of data.

VDSL (Very High Speed Digital Subscriber Line)
VDSL can operate in symmetric or asymmetric mode, providing speeds of between 6 Mbps and 25 Mbps. VDSL is typically provided from a street cabinet which is directly linked by fibre to an exchange, which can be difficult and expensive to install.

Our first commercial DSL services are based on ADSL technology, but we are exploring both SDSL and VDSL.

Faster speed
e.g. A peak bandwidth of at least 500Kbps, which is 10 times the speed of a 'normal' connection.

Always on connection
i.e. You do not need to 'dial up'. Instead, the broadband line is always connected, like your water supply or electricity.

Flat rate charge
i.e. You pays a monthly fee and can be online all day, every day for no extra charge.

For home users and small/medium businesses

A splitter/microfilter is added to the phone socket of the ADSL enabled phone line. The splitter/microfilter has two sockets, one for the phone cable connection and one for the cable that connects to the ADSL modem or router that you would need to use for this service.

If you have more than one phone socket connected to the ADSL enabled phone line then you will need a microfilter for each one. The extra phones can be plugged into the correct socket of the microfilter, the other socket of the extra microfilters, where you would normally attach the modem/router cable, can remain empty. If you run an extension cable from the microfilter that is attached to the master socket to another phone then you do not need a microfilter at the end of this extension.

Internet access
The access speeds available with ADSL make it practical for web sites to carry more feature-rich information such as embedded video clips, animations and good quality audio. The virtually instant downloading of internet pages significantly enhances the online experience.

Intranet access
Fast access to a corporate Intranet (internal communications network) makes teleworking a realistic proposition for businesses as well as their employees, bringing benefits to both the organisation and the individual.

TV based services
ADSL can be used to deliver interactive applications to the TV, such as email and net surfing, movies and TV on demand, music and internet radio and home shopping and banking.

An ADSL line is dedicated up to the local exchange and we have one ADSL modem per end user in the local exchange. ADSL is circuit oriented rather than broadcast, so each connection is independent of all others. Commentators and technical analysts tend to feel that ADSL is a more secure technology because of this.

An always on connection - so no need to waste time 'dialing up'.

Flat rate tariff - no call charges for the time spent online, so you have a predictable price for unlimited access.

Simultaneous phone usage - you can surf and talk at the same time. The ADSL service runs over your existing BT telephone line, so there is no need to pay for an extra phone line to be put in. (Note that normal telephone rental and call charges will still apply though)

Because of the technical limitations of ADSL and lack of customer demand in some areas, some areas are not commercially viable for ADSL. BT is exploring a number of options for these areas.

Sometimes, although you have an ADSL enabled exchange, your phone line may be of too low quality to enable ADSL to run effectively. This is either due to the length of your cable from the exchange (which must be 5.5km for 512k connections and 3.5km for the 1024k and 2048k connections) or it may be that there is interference from surrounding cables that lay with your cable underground. Sometimes your cable will be part fibre-optic and ADSL cannot run on a fibre-optic cable.

The line quality of your BT cable that runs from your house to the exchange is paramount. A line test is carried out and a reading is taken in decibels. The first test must read under 55 decibels. Once this test has been passed a test must be carried out directly from your property. The reading here must be under 60 decibels. This second test is carried out once you have connected your equipment - an engineer at the exchange will send signals down the line to take a reading.

These are just some of the reasons ADSL may not be able to run on your phone line. All these reasons will give high decibel readings that inhibit the ADSL service from working at the optimum level.