DIGITAL SUBSCRIBER LINE

xDSL

xDSL is a modem technology, enabling high-speed delivery of data, audio, and video in a digital form over the existing telephone. There are several types of DSL (Digital Subscriber Line) technology, namely DSL, HDSL, SDSL, ADSL, VDSL and RDSL. Various DSL technologies differ from each other in data rate, operating distance, ratio between downstream and upstream speeds, as well as concerning their applications. 

What's special about xDSL? 

The cables connecting most households to the phone network are mainly simple twisted pair copper wires, which have only been able to carry analogue traffic. Modem speeds have gradually increased through the use of various compression and other techniques,but at today's fastest (56 kilobits per second (kbps) they are approaching the theoretical limit for this technology. 

DSL technology enables much higher speeds across the twisted pair lines from the exchange to the home. Speeds up to 2 Megabits per second are readily achievable - 30 or more times faster than today's fastest modems. This means that consumers and teleworkers can use applications that need these higher speeds even if their town or village doesn't have new, high performance cable networks. DSL has its own kind of "modems". Also, the technology allows the network to manage traffic rather than allocating complete end-to-end circuits, so that from the user perspective it can appear that the data connection is "always on". xDSL deployment to homes will make it easier for Internet connections can be sold on a fixed rate "per month" basis rather than per minute, encouraging wider and more intensive use of Internet, e-commerce, teleworking etc. 

DSL stands for Digital Subscriber Line: 

• Digital - means a line able to carry data traffic in its original form, as opposed to analogue.
• Subscriber Line - the line connecting the individual subscriber to the local exchange 
• Analogue - the phone lines we have used for voice phone calls until today have been analogue lines and we have used MoDems (Modulator-Demodulator) to convert the digital output of the computer to analogue form for transmission and back into digital form at the other end for use by the computer. The performance of conventional modems and analogue lines has restricted bandwidth - the speeds at which computers can be connected across the phone network and the capacity of the networks to handle traffic. 

The use of digital lines makes transmission of computer information faster and more reliable. It also allows much faster connect and disconnect, eliminating the slow process required for modems to establish a connection and start handling traffic. Over time its expected that all future telephony will be digital. 

DSL -Digital Subscriber Line 

DSL modems create altogether three channels, two B channels and one D channel. The data rate of the D channel is 16 kbps and it is mainly used for signalling. Both B channels can carry up to 64 kbps duplex (downstream speed equals the upstream speed). The operating distance of DSL is 18,000 feet. 

DSL Applications 

DSL is a modem used for narrowband ISDN. The target customers of ISDN include not only businesses but also residential subscribers. ISDN can be used for voice and data communications and its speed is sufficient to support also desktop videoconferencing. Several experts do not, however, see ISDN as a major end-to-end service to support multimedia applications. They rather predict that its main use will be to provide home access to the Internet. On the other hand, the increased use of real-time video and audio over the Internet necessitates even higher-speed access to the Internet. 

HDSL - High data rate Digital Subscriber Line 

HDSL is the most mature of the xDSL technologies This has been in use for some years in some countries to provide broad band facilities to business premises (leased line), providing 2 Mbps across up to three miles of wire. 

HDSL requires two twisted pair lines to carry data at a rate of 1.544 Mbps both upstream (from the subscriber to the network) and downstream (from the network to the subscriber). Another alternative of HDSL technology uses three twisted-pair copper lines to carry 2.048 Mbps. These data rates are provided as far as 12,000 feet. 

HDSL Applications 

HDSL technology is mainly used by early adopters in a restricted area (such as student living in a campus area) as well as by private organizations, as it demands two or three twisted pair copper lines. Typical applications of HDSL include access to the following systems: PBX network, cellural antenna stations, digital loop carrier systems, inter-exchange POPs, Internet servers and private data networks. 

VDSL - Very high speed Digital Subscriber Line 

VDSL is a new technology not expected to be in use in public networks for some years. Its expected to provide speeds as high as 52 Mbps downstream and between 1.5 and 2.3 Mbps upstream, but over shorter distances than ADSL - 1.3 km at 13 Mbps and 0.3 km at 52 Mps. Note that by deploying higher performance lines from exchanges to street cabinets, these speeds could be delivered to more homes. 

ADSL - Asymmetric Digital Subscriber Line 

Asymmetric means that a higher speed is available from the exchange to the user (downstream) and a slower speed from the user to the exchange (upstream) . For example the user can download web pages or videos at high speed, but can only send at a significantly lower speed. Capability varies with distance - up to 9 Mbps downstream can be supported up to one mile from an exchange and 2 Mbps up to three miles. 64 kbps is the usual upstream speed. Note that the distance is the length of the cable, because of the way networks are routed a 3 mile cable may mean only 2 miles physical distance. ADSL can operate in parallel with conventional analogue voice telephony over the same line. 

SDSL - Single line Digital Subscriber Line 

SDSL has the ability of transferring data at the same speed as HDSL, but it requires only one twisted-pair copper wire. Additionally, POTS (Plain Old Telephone System) and digital data transfer can occur simultaneously on SDSL. However, the operating distance of SDSL is only 10,000 feet. 

SDSL Applications 

Typical SDSL applications are the same than supported by HDSL. However, SDSL has one important advantage over HDSL: it is also suitable for residental users, as they usually have only a single telephone line. 

RDSL - Rate adaptive Digital Subscriber Line 

RDSL is a rate adaptive xDSL technology, which means that it can work at different data rates depending on the length of the line and thus, it can overcome the distance limitations for xDSL to a certain extent. 

RDSL Applications 

RDSL allows speed adjustment according to the line quality. It enables carriers to serve users who reside a long way from the nearest central office. A remote user may not get the fastest speed because of the distance of the line, he will, however, get the best possible rate. Thus, RSDL can basically support all the same services supported by DSL, HDSL and ADSL depending on the line distance. 

Sosurces: 

http://www.webopedia.com/TERM/X/xDSL.html

http://www.eto.org.uk/faq/faqtterm.htm

http://www.tayvaughan.com/multimedia/hkkk/itp97/reports/report1.html