Digital Subscriber Line (DSL)

DSL is a family of technologies that provides data transmission over the wires of a local telephone network.

Asymmetric Digital Subscriber Line (ADSL)

ADSL is the technology used to provide the first-generation of broadband connections over existing copper telephone lines, and has been deployed on a mass scale around the world.

Data is transmitted over the telephone line at frequencies that are too high for the human ear to hear.  A DSL filter, known as a “splitter”, fitted to the telephone socket inside the house breaks out the frequencies for voice from those used for data, and sends them to the correct piece of hardware (telephone or computer).  At the other end of the line in the telephone exchange, a so-called a DSL Access Multiplexer (DSLAM) separates the voice and data traffic so that it can be carried over the phone company’s separate voice and data networks.

ADSL, which is available in all but a handful of UK telephone exchanges, offers headline speeds of 8 Mbps, depending on what version of technology is available.  However, the speed a user actually receives depends on a number of factors related to the characteristics of copper phone lines.  ADSL works best the shorter the distance from the telephone exchange to the customer premises.  Other factors like the quality of the copper and connectors, aluminium cables in the network and line-sharing devices (DACS) also affect the service.  Hence it is estimated that around 10% of homes and businesses cannot get a 2 Mbps service from their connection and around 166,000 cannot get any sort of ADSL broadband.

21CN and ADSL2+

BT is in the process of rolling out 21CN (an abbreviation for 21st Century Network), which is long-term project to upgrade the core of the network so that it can carry both voice and data – for the simple reason that it is more efficient to manage one network rather than two.  As part of this programme, BT is replacing DSLAMs in the exchanges with new equipment than can support ADSL2+.

ADSL2+ has a headline speed of 24 Mbps, which can represent a significant bandwidth boost for some.  But, like all copper technologies, the speed of ADSL2+ depends on line quality and distance; beyond 3 km from the exchange there is no real speed advantage over ordinary ADSL.  An estimated 50% of telephone lines are capable of speeds above 8 Mbps, with the majority remaining in the 8–12 Mbps bracket.

Very high speed Digital Subscriber Line (VDSL)

VDSL is usually deployed in combination with fibre-to-the-cabinet (FTTC).

FTTC boosts broadband speeds by shortening the distance from the electronic equipment to the customer. This involves laying fibre-optic cables from telephone exchanges to green street cabinets or their equivalent, and installing faster VDSL2 equipment in the street cabinet to provide broadband over the remaining few hundred meters of telephone line.

The speed offered by VDSL depends on its “profile” which is essentially the set of frequencies used. The most common configuration in the UK today offers up to 40 Mbps download. As with other copper-based technologies, top speeds are only available for users located next to the cabinet. Speed decreases rapidly with distance from the cabinet, and at distances beyond 1 km VDSL2 offers ADSL-like performance. The average distance from the street cabinet to customer is around 300 m, so the majority of end users can expect to see broadband speeds in the region of 25 Mbps with this approach.

This article originally appeared in Beyond Broadband.