What is DSL (Digital Subscriber Line)?

Digital subscriber line (DSL)

DSL or xDSL is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, but as of 2009 the term digital subscriber line has been widely adopted as a more marketing-friendly term for ADSL, the most popular version of consumer-ready DSL. DSL can be used at the same time and on the same telephone line with regular telephone, as it uses high frequency bands, while regular telephone uses low frequency.

The download speed of consumer DSL services typically ranges from 256 kilobits per second (kbit/s) to 24,000 kbit/s, depending on DSL technology, line conditions and service-level implementation. Typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for the rarer Symmetric Digital Subscriber Line (SDSL).

Voice and data

Comparing DSL & Dial-Up

DSL (VDSL) typically works by dividing the frequencies used in a single phone-line into two primary "bands". The ISP data uses the high-frequency band (25 kHz and above) whereas the voice utilizes the lower-frequency band (4 kHz and below). (See the ADSL article for information on the subdivision of the high-frequency band.) The user typically installs a DSL filter on each phone outlet. This filters out the high frequencies from the phone line, so that the phone only sends or receives the lower frequencies and the user hears only the human voice. The DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other provided filters are used for all voice devices.

History and science

Implementation of digital subscriber line technology originally took place as part of the ISDN specification, later reused as IDSL. Engineers have developed higher-speed DSL connections like HDSL and SDSL to extend the range of DS1 services on copper lines. Consumer-oriented ADSL is designed to operate also on a BRI ISDN line, which itself is another (non-IP) form of digital signal transmission, as well as on an analog phone line.

DSL, like many other forms of communication, stems directly from Claude Shannon's seminal 1948 scientific paper: A Mathematical Theory of Communication. Employees at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone-company central offices and customers on conventional twisted pair cabling.

A DSL Modem

U.S. telephone companies promote DSL to compete with cable Internet. the first DSL service ran over a dedicated "dry loop", but when the FCC required the incumbent local exchange carriers (ILECs) to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over Unbundled Network Element, this allows a single pair to carry data (via a digital subscriber line access multiplexer [DSLAM]) and analog voice (via a circuit switched telephone switch) at the same time. Inline low-pass filter/splitters keep the high frequency DSL signals out of the user's telephone-receivers. Although DSL avoids the voice-frequency band, the nonlinear elements in the phone would otherwise generate audible intermodulation products and impair the operation of the data modem.

Older ADSL standards can deliver 8 Mbit/s to the customer over about 2 km (1·25 miles) of unshielded twisted-pair copper wire. As of 2009, the latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM. Distances greater than 2 km (1·25 miles) significantly reduce the bandwidth usable on the wires, thus reducing the data rate. Use of an ADSL loop extender can increase these distances substantially.

Operation

Regular DSL

Telephone engineers initially developed the local loop of the public switched telephone network (PSTN) to carry POTS voice communication and signaling: no requirement for data communication as we know it today existed. 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 voiceband or commercial bandwidth.

The local telephone exchange (United Kingdom) or central office (United States) generally digitizes speech signals 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 laws of physics, specifically the Shannon limit, cap the speed of data transmission. For a long time engineers believed it impossible to push a conventional phone-line beyond low-speed limits (typically under 9600 bit/s). In the 1950s ordinary twisted-pair telephone-cable often carried 4 MHz television signals between studios, suggesting that the Shannon Limit would allow transmitting many megabits per second. However, these cables had other impairments besides Gaussian noise, preventing such rates from becoming practical in the field. The 1980s saw the development of techniques for broadband communications that allowed the limit to be greatly extended.

The local loop connecting the telephone exchange to most subscribers has the capability 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 higher frequencies used by DSL travel only short distances). The pool of usable channels is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. 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 they are usable.

One of Lechleider's contributions to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL. This allowed Internet Service Providers to offer efficient service to consumers, who benefitted greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where the delivered data must be error free but latency incurred by the retransmission of errored packets is acceptable.

Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, while voice service cannot be maintained past a certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this, phone companies are endeavoring to remove load coils on copper loops that can operate without them, and conditioning lines to avoid them through the use of fiber to the neighborhood or node FTTN.

The commercial success of DSL and similar technologies largely reflects the advances made in electronics, that, over the past few decades, have been getting faster and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the popularization of DSL technology:

• Until the late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of VLSI technology, the cost of the equipment associated with a DSL deployment (a DSLAM at one end and a DSL modem at the other end) lowered significantly.
• A DSL connection can be deployed over existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations.
• In the case of ADSL, competition in Internet access caused subscription fees to drop significantly over the years, thus making ADSL more economical than dial up access. Telephone companies were pressured into moving to ADSL largely due to competition from cable companies, which use DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to popularize ADSL technology.

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 uses a router that establishes a connection between the DSL modem and a local Ethernet, Powerline, or Wi-Fi network on the customer's premises.

Once upstream and downstream channels are established, a subscriber can connect to a service such as an Internet service provider.

Naked DSL

Dry-loop DSL or "naked DSL," which does not require the subscriber to have traditional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC, and Verizon's merger with MCI, those telephone companies have an obligation to offer naked DSL to consumers.

Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of telephone landlines in the US dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million.. This lack of demand for landline service has resulted in the expansion of naked DSL availability.

Typical setup and connection procedures

Physical connection must come first. On the customer side, the DSL Transceiver, or ATU-R, or more commonly known as a DSL modem, is hooked up to a phone line. The telephone company(telco) connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box. The location of the DSLAM depends on the telco, but it cannot be located too far from the user because of attenuation, the loss of data due to the large amount of electrical resistance encountered as the data moves between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.

When the DSL modem powers up it goes through a sync procedure. The actual process varies from modem to modem but generally involves the following steps:

1. The DSL Transceiver does a self-test.
2. The DSL Transceiver checks the connection between the DSL Transceiver and the computer. For residential variations of DSL, this is usually the Ethernet (RJ-45) port or a USB port; in rare models, a FireWire port is used. Older DSL modems sported a native ATM interface (usually, a 25 Mbit serial interface). Also, some variations of DSL (such as SDSL) use synchronous serial connections.
3. The DSL Transceiver then attempts to synchronize with the DSLAM. Data can only come into the computer when the DSLAM and the modem are synchronized. The synchronization process is relatively quick (in the range of seconds) but is very complex, involving extensive tests that allow both sides of the connection to optimize the performance according to the characteristics of the line in use. External, or stand-alone modem units have an indicator labeled "CD", "DSL", or "LINK", which can be used to tell if the modem is synchronized. During synchronization the light flashes; when synchronized, the light stays lit, usually with a green color.

Modern DSL gateways have more functionality and usually go through an initialization procedure very similar to a PC boot up. The system image is loaded from the flash memory; the system boots, synchronizes the DSL connection and establishes the IP connection between the local network and the service provider, using protocols such as DHCP or PPPoE. The system image can usually be updated to correct bugs, or to add new functionality.

Equipment

The customer end of the connection consists of a Terminal Adaptor or in layman's terms "DSL modem". This converts data from the digital signals used by computers into a voltage signal of a suitable frequency range which is then applied to the phone line.

In some DSL variations (for example, HDSL), the terminal adapter connects directly to the computer via a serial interface, using protocols such as RS-232 or V.35. In other cases (particularly ADSL), it is common for the customer equipment to be integrated with higher level functionality, such as routing, firewalling, or other application-specific hardware and software. In this case, the entire equipment is usually referred to as a DSL router or DSL gateway.

Some kinds of DSL technology require installation of appropriate filters to separate, or "split", the DSL signal from the low frequency voice signal. The separation can take place either at the demarcation point, or with filters installed at the telephone outlets inside the customer premises. Either way has its practical and economical limitations. See ADSL for more information about this.

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. In the case of ADSL, the voice component is also separated at this step, either by a filter integrated in the DSLAM or by a specialized filtering equipment installed before it. The DSLAM terminates all connections and recovers the original digital information.

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), while authenticating with a userid and password and using PPP mechanisms to provide network details.

DSL technologies

The line-length limitations from telephone exchange to subscriber impose more restrictions on higher data-transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures). Technologies likes GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that allow the conversion of copper based IDSN, ADSL and DSL over fiber optics.

Example DSL technologies (sometimes called xDSL) include:

• ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN.
• High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables.
• Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow is equal in both directions.
• Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardized replacement for early proprietary SDSL.
• Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greater in one direction than the other.
• Asymmetric Digital Subscriber Line 2 (ADSL2), an improved version of ADSL
• Asymmetric Digital Subscriber Line 2 Plus (ADSL2+), A version of ADSL2 that doubles the data rates by using twice the spectrum.
• Asymmetric Digital Subscriber Line Plus Plus (ADSL++), technology developed by Centillium Communications for the Japanese market that extends downstream rates to 50 Mbit/s by using spectrum up to 3.75 MHz.
• Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing up stream speed
• Very High Speed Digital Subscriber Line (VDSL)
• Very High Speed Digital Subscriber Line 2 (VDSL2), an improved version of VDSL
• Etherloop Ethernet Local Loop
• Uni-DSL (Uni Digital Subscriber Line or UDSL), technology developed by Texas Instruments, backwards compatible with all DMT standards
• Gigabit Digital Subscriber Line (GDSL), based on binder MIMO technologies.
• Universal High bit rate Digital Subscriber Line (UHDSL) using fiber optics. Developed in 2005 by RLH Industries, Inc. Converts HDSL-1, 2 or 4 copper service into fiber optic HDSL service.

Transmission methods

Transmission methods vary by market, region, carrier, and equipment.

• 2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL
• CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL, used for HDSL
• DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM (Orthogonal frequency-division multiplexing)