Digital Subscriber Line (DSL): technology, variants, and uses

DSL differs from an Internet connection via an analogue telephone connection (POTS) or ISDN in that a much larger frequency range is used for data transmission, which enables a much higher data rate; however, the range of the signal is severely limited by this large frequency band, so that the signal must already be processed ((de)modulated) in the local exchange.

In the case of DSL variants such as ADSL, which are usually marketed to residential customers, the frequency range used for fixed network telephony is cut out by means of a high/low pass (splitter), which means that DSL can be used in parallel with the normal telephone. Fax, analogue telephone or ISDN are thus also available during DSL operation. This opens up new applications, because Internet access is now always available, just like with a dedicated line.

Between the customer's DSL modem and the exchange, which is only a few kilometres away, the digital DSL signal is transmitted via the telephone line to the provider's DSL multiplexer DSLAM. The signal is then transmitted via a broadband fiber optic connection from the DSLAM to a broadband access server as a concentrator and from there to the provider's backbone.

The high capacity of the backbone connection means that the subscriber line (local loop) can be better utilised than with analogue or ISDN data transmission, as the data no longer has to be transmitted via the conventional telephone network. With DSL, improved modulation methods and the use of a larger bandwidth have an effect (details below).

While ISDN is primarily used for telephony with several user channels over the same exchange line, and secondarily for simultaneous telephony with an existing narrowband Internet connection, ADSL (asymmetric DSL: high data rate in the direction of the user, low data rate in the direction of the Internet) is the first technology that network operators have installed for high-speed Internet access for residential customers.

ISDN has thus gained a competitor in the residential customer segment in the form of DSL, because with ADSL it is possible to use the Internet and make telephone calls simultaneously via the same subscriber line, even in conjunction with an analog fixed-link connection - as is the case with ISDN - and the Internet connection is considerably faster than with narrowband ISDN Internet access.

SDSL (symmetrical DSL with the same DSL data rate in the send and receive directions; colloquially referred to as upstream and downstream) is primarily used for business customers who also require a fast connection for sending data, but was also marketed for residential customers for a time by QSC subsidiary Q-DSL home. Due to its high range, SDSL is also suitable for supplying customers with long access lines who cannot be supplied, or only inadequately supplied, by means of the low-reach ADSL-over-ISDN used in Germany.

DSL as the basis for migration to the Next Generation Network

Since mid/late 2006, most providers in Germany have been attempting to increase customer loyalty with so-called triple-play complete connection packages. This involves using the local loop to transmit three services, typically telephony (often by means of DSL telephony via unbundled DSL), Internet access and video/TV (see also VDSL, ADSL2+ and bitstream access). The favourable price - when all services are fully utilised - may be offset by a lack of flexibility, especially if individually available offers are thereby forced off the market.

The traditional fixed network providers are increasingly migrating their circuit-switched services to a next-generation network platform, whereby the network connection is converted from a fixed network connection with a bundled DSL connection to an unbundled data connection that can be implemented more cost-effectively, which means that the local switching technology can be dismantled and its locations converted to pure DSLAM locations.