Broadband Internet Access

Digital Subscriber Line

The origins of Digital Subscriber Lines date back to 1988, when an engineer at Bell Labs devised a way to carry a digital signal over the unused frequency spectrum available on the twisted pair cables running between the telephone company's central office and the customer premises. Implementation of DSL could permit an ordinary telephone line to provide digital communication without interfering with voice services. However, the management of incumbent local exchange carriers (ILEC) were not enthusiastic about it, since DSL was not as profitable as installing a second phone line for consumers who preferred simultaneous dial-up internet and voice connections, also the broadband data connection would cannibalize existing ISDN customers. This changed in the late 1990s when cable television companies began marketing broadband Internet access. Realizing that most consumers would prefer broadband Internet to dial-up Internet, ILECs rushed out the DSL technology they had delayed implementing for over a decade as an attempt to win market share from the broadband Internet access offered by cable television operators.

As of 2005, DSL is 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 faster than DSL in most commercial situations. Older ADSL standards can deliver 8 Mbit/s over about 2 km (1.24 miles) of unshielded twisted pair copper wire. The latest standard ADSL2+ can deliver more than 20 Mbit/s over similar distances. Many customers, however, are located farther than 2 km (1.24 miles) from the central office, which reduces the amount of bandwidth available (thereby reducing the data rate) on the wires. 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 from 100 to 200 households).

Operation

The local loop of the Public Switched Telephone Network was initially designed to carry POTS voice communication and signaling, as the concept of data communications as we know it today did not exist. For reasons of economy, the phone 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 Shannon capacity of the POTS channel.

At the local exchange (UK terminology) or central office (US terminology) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore according to the Nyquist theorem 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 local loop connecting the central office to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. 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 integrated circuits and disk drives have been getting faster and cheaper, the cost of digging trenches in the ground for new wires remains expensive. All flavors of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Not long ago, the cost of such signal processing would have been prohibitive but because of VLSI technology, the cost of installing DSL on 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 and analog modems, can share the wires with DSL. Only one DSL modem can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection is to use a router that establishes a connection between the DSL modem and local Ethernet 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.

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 voltage signal of a suitable frequency range which is then applied to the phone line.

In addition the subscriber may need to install a DSL filter (known variously as a "filter" or "micro-filter") between the DSL modem and telephones if using the POTS service on the same line. This prevents the modem signals from interfering with voice reception, and vice versa. 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 degradation of the DSL signal (especially if more than 5 analogue devices are connected to the line) because 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.