Digital Subscriber Line

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DSL Modem

Digital Subscriber Line, or DSL, is a family of technologies that provide a digital connection over the copper wires of the local telephone network. Its origin dates back to 1988, when an engineer at Bell Labs devised a way to carry a digital signal over the unused frequency spectrum. This allows an ordinary phone line to provide digital communication without blocking access to voice services. Bell's management, however, were not enthusiastic about it, since it was not as profitable as renting out a second phone line for consumers who preferred to have access to the phone when dialing out. This changed in the late 1990s when cable companies started marketing broadband Internet access. Realising that most consumers would prefer broadband Internet to a second dial out line, Bell companies rushed out the DSL technology that they had been sitting on for the past decade as an attempt to slow broadband Internet access uptake, to win market share against the cable companies.

As of 2005, DSL provides the principal competition to cable modems for providing high speed Internet access to home consumers in Europe and North America; although on average, cable is much faster than DSL in most commercial situations. Older ADSL standards could deliver 8 Mbit/s over about 2 km (one mile) of copper wire. The latest standard ADSL2+ can deliver over 20 Mbit/s per user over similar distances. Many copper lines, however, are longer than one mile (2 km) reducing the amount of bandwidth that can be transmitted. Modern cable systems, on the other hand, can provide 30 Mbit/s downstream, but this bandwidth is shared between all the users on the cable segment (which could be 100 to 200 households).

1 How it works
2 Equipment
3 Protocols and configurations
4 DSL technologies
5 Transmission methods
6 See also
7 External links

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How it works

The Public Switched Telephone Network was initially designed to carry POTS calls, as the concept of data communications as we know it today did not exist. For reasons of economy, the system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as commercial bandwidth. Dial-up services using modems are constrained by the POTS channel's Shannon capacity, which indicates the maximum data rate which can be supported by a given amount of bandwidth.

At the Local Exchange (UK terminology) or Central Office (US terminology) the speech is generally digitised into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects). The bandwidth between the commercial bandwidth limit and the channel limit can be utilised in a fully digital end to end connection to achieve a full 64 kbit/s on an ISDN line.

The local loop connecting the central office to most subscribers is capable of carrying frequencies well beyond the 3.5 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be as high as the tens of megahertz. DSL takes advantage of this unused part of the circuit by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor. The pool of usable channels is then split into two groups for upstream and downstream traffic based on a preconfigured ratio. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether or not they are usable.

The commercial success of DSL and similar technologies largely reflects the fact that in recent decades, while microchips and disk drives have been getting faster and cheaper, the cost of digging holes in the ground remains very high. All flavors of DSL employ very complex digital signal processing algorithms to overcome the inherent limitations of existing POTS wires. Not long ago, the cost of such signal-processing power would have been prohibitive, but today the cost of installing DSL for an existing local loop, with a DSLAM at one end and a DSL modem at the other end, is orders of magnitude less than would be the cost of installing a fiber-optic cable over the same route and distance.

Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including FAX machines, can share the wires with DSL. However, in most cases only one DSL modem can use a local loop at a time; it is generally not possible for a customer to have multiple DSL connections over a single local loop. As of 2005, the standard way to let multiple computers share a broadband connection is to purchase an inexpensive router that establishes a local Ethernet or Wi-Fi network on the customer's premises.

Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as Internet access.

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Equipment

The subscriber end of the connection consists of a DSL modem. This converts data from the digital electronic pulses used by computers into a digital audio stream of a suitable frequency range for the particular DSL variant in use.

In addition the subscriber may need to install a passive electronic filter (known variously as a "filter", "micro-filter", or "splitter") if using the POTS service on the same line. This ensures that the DSL modem and the telephone only receive the frequencies they are designed to handle. Subscribers can plug a filter into an existing telephone socket when using a "wires-only" service, or alternatively the DSL provider may install it. Some POTS devices, such as "tapeless" digital answering machines, are especially sensitive to small amounts of high-frequency signals leaking across the simple passive filters provided in the installation kit from the DSL supplier; the customer may therefore need to purchase higher-quality "active" filters from a third-party supplier or move some POTS devices to a room farther away from the DSL modem.

In the early days of DSL, installation required a technician to visit the premises. One splitter was installed near the point where the phone line entered the premises, from which a dedicated data line was installed. Today, many DSL vendors offer a self-install option, in which they ship equipment and instructions to the customer. In this case, since no changes are made to the cable plant on the customer premises, all the phone wires are carrying both POTS and DSL signal frequencies; therefore the customer generally needs to plug a splitter into each telephone outlet. However, this can sometimes cause degration of the DSL signal (especially if more than 5 analogue devices are connected to the line) becuase the DSL signal is present on all telephone wiring in the building. A way to circumvent this is to install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer and may not work on some (poorly designed) household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use. As of 2005, establishing new cable modem or satellite broadband service generally does require a visit by a technician to the premises, even when there is existing cable television service to this customer; this constitutes one of the major competitive advantages of DSL over cable broadband service.

At the exchange a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. It also separates out the voice component.

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Protocols and configurations

Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over Ethernet or ATM (PPPoE or PPPoA, also known as PPPoATM), while authenticating with a userid and password and using PPP mechanisms to provide network details.

DSL also has contention ratios which need to be taken into consideration when choosing a plan.

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DSL technologies

The reach-restraints (line length from Central Office to Subscriber) reduce as data rates increase, with technologies like VDSL providing short-range links (typically "fibre to the curb" network scenarios).

Example DSL technologies (sometimes called xDSL) include: