DSL Technology

DSL (Eng. Digital Subscriber Loop) - a digital subscriber loop, the family of broadband access to the Internet technology. The standard data reception speed fluctuates between128 Kbps up to 24,000 Kbps, depending on the applied DSL technology and its level. For ADSL technology, the speed of dispatching data is lower than its speed of data reception, but symmetrical to SDLS. Joseph W.Lechleitter, inventor of DSL modem and employer of Bellcore, demonstrated the construction projects of such devices in the 80s.

The local terminal of the Public Switched Telephone Network was primarily designed for voice communication and signaling - for the oldest, basic POTS services; the notion of data communication was unknown at that time. Due to economical reasons, the telephone system sends the audio signal nominally between the frequencies of 300 and 3,400 Hz, which are consistent with the range required for human voice so that it can be clearly understood. Services using Dial-up modems are limited by POTS channel capacity.

Within the local telephone exchanges, the speech is discretized to 64 kbit/s of the data stream in the form of an 8-bit signal using sampling frequency 8,000 Hz, because according to Nyquist`s theory, each signal over 4,000 Hz is not let through a telephone net.

The local terminal connecting the telephone exchange to majority of the subscribers, is capable of sending frequencies higher than POTS`s limit of 3.4 kHz. The limit cannot be higher than that depending on the quality and distance of the terminal. DSL technology uses the higher, unused band of the terminal by creating 4312.5 Hz, broad channels starting at - 10 and 100 kHz - depending on the configuration of the system. The channel allocation is continued upon higher and higher frequencies (up to 1.1 MHz for ADSL) until the new channels are recognized as unfit for usage. Each channel is extended so that it is can be used on more than one route, as in case of POTS connections. More of the usable channels is compared to greater available bands and that is why the distance and quality of the line are crucial factors. The number of channels fit for usage is divided into two groups of movement streams - of reception and of dispatch - based on the initially configured proportions. In the given group individual channels are joined into a couple of cycles, each in different direction. Just like analogue modems, DSL transmitters constantly monitor quality of the channels and, depending on whether the given channel is useful or not, they are added or removed.

The commercial success of DSL and similar technologies is reflected in the fact that in the last decades, as electronics is becoming faster and cheaper, the costs of digging out pits for the new wiring have remained unchangeably high. All of the DSL technologies use highly complex algorithms for converting the digital signal, overcoming the inseparable limitations of wires` strands. Recently costs of a similar installation would have been incredibly high but thanks to VLSI technology, costs of installation of DSL in the already existing local terminals, with DSLAM multiplexer on one end and a DSL modem on the other, requires a smaller amount of expenditures that could arise at the same route and distance, when installing a new optic fiber.

DSL technology is used in majority of apartments and minor offices, the proper filters enable simultaneous operation of voice services and of DSL. DSL modem can use the same subscriber`s line that the communication devices based on POTS technology use, turning faxes and analogue modems on. At the same time, only one DSL modem can use the subscriber`s line. The standard method of making DSL available to many computers in the same premises is using a router, which connects the DSL modem and local Ethernet or Wi-Fi net. The channels of dispatch and receipt streams are used to obtain connection between the subscriber and Internet services supplier.

See also: Types of DSL Technology (ADSL, VDSL, RADSL)