Types of DSL Technology (ADSL, VDSL, RADSL)

Asymmetric Digital Subscriber Loop, ADSL is a technology that makes asymmetrical access to the Internet possible and one which is a type of DSL. The asymmetry consists in the fact that the dispatch of data to the user (the Internet) is faster than the other way around. The technology in-question was created with view to users who more often receive data (ex. from Internet websites) than dispatch them (ex. those who run an Internet server). This standard uses ordinary, copper telephone wires. ADSL allows for far faster communication than the telephone modem technology, which has to convert all the signals to the analogue signal (at the user`s end) and then usually again to the digital signal (on the recipient`s end). In ADSL technology the signal is digital at both ends and thanks to this the exchange of data is possible` both the subscriber and the operator (ISP) need to place ADSL modems at both ends of the telephone line. In some technical solutions, the Internet recipient also has to use a splitter which separates the ADSL and the telephone signal. ADSL allows for transmission with speed at the rate of16 kb/s to 8 Mb/s. The speed with which data can be sent is usually considerably lower (usually by a half or more). The transmission band is presented below.

For data transmission HDSL uses the copper telephone wires, known far and wide since the times of Graham Bell. However, it uses, unlike the ordinary telephones 300 - 3400Hz band, a much broader one of 6 to 259kHz. It is possible thanks to natural bands of transmission of couples of copper wires. In the beginning, for transmission of 2Mbites/s, HDSL technology used symmetrically both ways 3 pairs of copper wires (SDSL). Technological advancement, demonstrating itself in refinement of signal coding method, made it possible to reduce number of them at first to two pairs and then to one pair necessary for full capacity of 2Mbits/s symmetrically that was presumed by HDSL. Unfortunately, apart from evident advantages of HDSL, namely using the traditional telephone infrastructure to reach the subscriber (the last mile), also has certain drawbacks. The major one is high dependence of the capacity on length of the cables and relatively small range often limiting reach of the technology to big cities or villages that have their own telephone exchanges.

RADSL (Rate Adaptive DSL) - a version of broadband access to the Internet technology, Digital Subscriber Loop, which automatically adapts the speed depending on the quality of the signal. Modern ADSL technologies are usually RADSL technologies. The Adaptive version of the asymmetrical access, called RADSL (Rate Adaptive DSL), allows for automatic adjustment of co-operating modems to the capacity available in the transmission route at the given moment. It is the most effective form of transmission via existing information channels with dynamically changing capacity, even during usage of a given telecommunication service.

VDSL (Eng. Very High Speed DSL) - VDSL technology can be recognized as development of ADSL standard and, at the same time, compatible with services based on SDH or ATM technologies. Greater transmission possibilities of the systems complaisant with this standard are obtained through application of frequency band of 10 to 30 MHz. Due to this, reach of the transmitting system is considerably limited to 1 km, because damping of high-frequency signals is very strong in the subscriber`s loop. VDSL permits transmission in both asymmetrical and symmetrical mode, guaranteeing throughput up to 56 Mbit/s in the asymmetrical mode and 26 Mbit/s in each direction in the symmetrical mode. The frequency band used in transmission spreads from 138 kHz to 30 MHz. Remaining compatible with services of higher layer protocols, such as ATM or SDH, the data transmission system VDSL guarantees transmission of two data paths: channel with the interlace, designed for data requiring particularly strong protection from disturbances and a fast channel used for dispatching data requiring minor delays. Application of a wide frequency band in VDSL transmission results in the dispatched data signal being threatened with strong disturbances of various origin. The strongest type of disturbances in the subscriber`s loop are the near-end and remote crosstalks, of both foreign origin (crosstalks of the signal of other types of systems in operation within the same link bunch), as well as of its own origin (crosstalks of VDSL signal from other systems in operation within the same link bunch). In particular, disturbances originating in HDSL transmission can be distinguished, within the frequency range from 138 kHz to 300 kHz, ADSL, with band up to 1,1 MHz, as well as in other VDSL systems using the same wire. Apart from the crosstalks, what is also a very serious disturbance`s source are the noises of radio origin. They can be divided into, first and foremost, disturbances caused by interference with signals of commercial radio stations and disturbances coming from signals of amateur transmitters. They cause further limitations of transmission possibilities of VDSL system in particular frequency ranges. Presence of such a wide range of disturbance signals with relatively high power (in case of signals from an amateur radio, power of the disturbances can reach even 0 dBm) makes it necessary to apply complex solutions in transceiver system of VDSL, such as advanced signal correction systems or systems of radio signals equalization.

See also: DSL Technology