ADSL (Asymetric Digital Subscriber Line) is the service provided by the BT network to every exchange in the country. There are just over 1,000 BT exchanges and three current varieties of ADSL.
At the lowest end of ADSL is Exchange Activate which is a cut down version supplied to a small number of exchanges in Scotland, mostly in the Highlands and Islands. It is characterised by only providing 512kbps (0.5Mbps) download speeds and from a limited number of internet service providers (ISPs), usually only 3-4 depending on exchange.
Next is standard ADSL Max which provides up to 8Mbps download speeds from a wide variety of ISPs. (ADSL Max is the term for BT's range of commercial ADSL services that are rate-adaptive and are deployed over BT phone lines.)
The fastest version is ADSL2+ which provides up to 24Mbps download speeds and is being rolled out to many BT exchanges as part of their current network upgrade plans.
DSL in the form of VDSL (Very-High-Speed Digital Subscriber Line) is also the basis of Fibre to the Cabinet (FTTC) services which are being rolled out by BT as their Next Generation Broadband platform. This replaces the existing copper connection between the exchange and the green street cabinet with fibre and installs VDSL equipment in an upgraded street cabinet into which the existing customer copper lines connect. This means there is a shorter length of copper between the customer and the high speed fibre network which means faster connections can be provided. Headline speeds from FTTC are 76Mbps download and near 20Mbps upload.
Local Loop Unbundling is where an alternative provider to BT, such as Talk Talk or Sky for example, install equipment into the local exchange and provide their own ADSL services. There are a relatively small number of these in Scotland due to the economics of supply and demand.
Sub-loop unbundling is where an alternative provider installs broadband equipment into the local BT green street cabinet. They then backhaul the connection from this cabinet by high capacity fibre or radio to their own network and use the BT copper phone line to provide higher speed services to the customers.
ADSL or FTTC can be used as a relatively low cost backhaul link into broadband not-spots. If a nearby location can be found with a fast DSL connection, this can be extended by wireless into a not-spot area, for example. This can be a useful low cost solution where no other feasible options are available. Note that for a community broadband project an ISP (Internet Service Provider) able to offer a flexible and responsive service will need to be identified - some ISPs do not allow sharing or redistribution of their service.
BET (Broadband Enabling Technology) is a system that allows a copper line to transmit broadband signals further than with conventional ADSL, however, speeds are limited to 2Mbps.