digital subscriber line

digital subscriber line

Digital subscriber line
In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology

Digital subscriber line
The bit rate of consumer DSL services typically ranges from 256 kbit/s to 40 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. In ADSL, the data throughput in the upstream direction, (the direction to the service provider) is lower, hence the designation of asymmetric service. In symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal.

Digital subscriber line - History
The theoretical foundations of DSL, like many other forms of communication technology, can be traced back to Claude Shannon's seminal 1948 paper: A Mathematical Theory of Communication. An early patent was filed in 1979 for the use of existing telephone wires for both telephones and data terminals that were connected to a remote computer via a digital data carrier system.

Engineers developed higher-speed DSL facilities such as High bit rate Digital Subscriber Line (HDSL) and Symmetric Digital Subscriber Line (SDSL) to provision traditional Digital Signal 1 (DS1) services over standard copper pair facilities.

A DSL circuit provides digital service

Early DSL service required a dedicated dry loop, but when the U.S

On the subscriber's end of the circuit, inline low-pass DSL filters (splitters) are installed on each telephone to filter the high-frequency "hiss" that would otherwise be heard, but pass voice (5 kHz and below) frequencies

Older ADSL standards delivered 8 Mbit/s to the customer over about 2 km (1.2 mi) of unshielded twisted-pair copper wire. Newer variants improved these rates. Distances greater than 2 km (1.2 mi) significantly reduce the bandwidth usable on the wires, thus reducing the data rate. But ADSL loop extenders increase these distances by repeating the signal allowing the LEC to deliver DSL speeds to any distance.

By 2012 some carriers reported steadily declining numbers of DSL users.

Digital subscriber line - Basic technology
Telephones are connected to the telephone exchange via a local loop, which is a physical pair of wires

For a long time it was thought that it was not possible to operate a conventional phone-line beyond low-speed limits (typically under 9600 bit/s)

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

One of Lechleider's contributions to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL

Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load coil

The commercial success of DSL and similar technologies largely reflects the advances made in electronics over the decades that have increased performance and reduced costs even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the popularity of DSL technology:

The two main pieces of equipment are a Digital subscriber line access multiplexer (DSLAM) at one end and a DSL modem at the other end.

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

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

Digital subscriber line - Naked DSL
A naked DSL (a.k.a. standalone or dry loop DSL) is a way of providing DSL services without a PSTN (analogue telephony) service. It is useful when the customer does not need the traditional telephony voice service because voice service is received either on top of the DSL services (usually Voice over IP) or through another network (mobile telephony).

It is also commonly called a "UNE" for Unbundled Network Element in the USA and known as a ULL service (Unconditioned Local Loop) in Australia. It has 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 115 million in 2010, while the number of cellular subscribers has grown to 277 million (as of 2010). This lack of demand for landline voice service has resulted in the expansion of naked DSL availability.

Naked DSL products are also marketed in some other countries e.g. Australia, New Zealand and Canada.

Digital subscriber line - Typical setup
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

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:

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/s serial interface). Also, some variations of DSL (such as SDSL) use synchronous serial connections.

The DSL transceiver then attempts to synchronize with the DSLAM

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.

The accompanying figure is a schematic of a simple DSL connection (in blue). The right side shows a DSLAM residing in the telephone company's central office. The left side shows the customer premises equipment with an optional router. This router manages a local area network (LAN) off of which are connected some number of PCs. With many service providers, the customer may opt for a modem which contains a wireless router. This option (within the dashed bubble) often simplifies the connection.

Digital subscriber line - Exchange equipment
At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off to 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.

Digital subscriber line - Customer equipment
The customer end of the connection consists of a terminal adaptor or "DSL modem". This converts data between the digital signals used by computers and the 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 ethernet 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 equipment is referred to as a gateway.

Most DSL technologies 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.

Digital subscriber line - Protocols and configurations
Many DSL technologies implement an Asynchronous Transfer Mode (ATM) layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

Digital subscriber line - Protocols and configurations
DSL implementations may create bridged or routed networks

Digital subscriber line - 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

TC-PAM: Trellis Coded Pulse Amplitude Modulation, used for HDSL2 and SHDSL

DMT: Discrete multitone modulation, the most common kind, also known as OFDM (Orthogonal frequency-division multiplexing)

Digital subscriber line - 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 like GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that allow the conversion of copper based ISDN, ADSL and DSL over fiber optics.

DSL technologies (sometimes summarized as xDSL) include:

ISDN digital subscriber line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN.

High-bit-rate digital subscriber line (HDSL / HDSL2), was the first DSL technology that used 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.

Single-pair 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 (Centillium has been acquired by TranSwitch Corp.) 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-bit-rate digital subscriber line 2 (VDSL2), an improved version of VDSL

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.

Internet Protocol subscriber line (IPSL), developed by Rim Semiconductor in 2007, allowed for 40 Mbit/s using 26 AWG copper telephone wire at a 5,500 ft (1,700 m) radius, 26 Mbit/s at a 6,000 ft (1,800 m) radius. The company operated until 2008.

Digital subscriber line - Further reading
Dave Burstein (2002). DSL. John Wiley and Sons. ISBN pp 53–86

Digital subscriber line - Further reading
Digital Subscriber Line. International Engineering Consortium ISBN

Internet access - Digital subscriber line (DSL, ADSL, SDSL, and VDSL)
Digital Subscriber Line (DSL) service provides a connection to the Internet through the telephone network. Unlike dial-up, DSL can operate using a single phone line without preventing normal use of the telephone line for voice phone calls. DSL uses the high frequencies, while the low (audible) frequencies of the line are left free for regular telephone communication. These frequency bands are subsequently separated by filters installed at the customer's premises.

With a symmetric digital subscriber line (SDSL), the downstream and upstream data rates are equal.

Very-high-bit-rate digital subscriber line (VDSL or VHDSL, ITU G.993.1) is a digital subscriber line (DSL) standard approved in 2001 that provides data rates up to 52 Mbit/s downstream and 16 Mbit/s upstream over copper wires and up to 85 Mbit/s down- and upstream on coaxial cable. VDSL is capable of supporting applications such as high-definition television, as well as telephone services (voice over IP) and general Internet access, over a single physical connection.

VDSL2 (ITU-T G.993.2) is a second-generation version and an enhancement of VDSL. Approved in February 2006, it is able to provide data rates exceeding 100 Mbit/s simultaneously in both the upstream and downstream directions. However, the maximum data rate is achieved at a range of about 300 meters and performance degrades as distance and loop attenuation increases.

Broadband Internet access - Digital subscriber line (DSL, ADSL, SDSL, and VDSL)
to the customer), hence the designation of asymmetric.[ ADSL Theory], Australian broadband news and information, Whirlpool, accessed 3 May 2012 With a symmetric digital subscriber line (SDSL), the downstream and upstream data rates are equal.[ SDSL], Internetworking Technology Handbook, Cisco DocWiki, 17 December 2009, accessed 3 May 2012

Bell Sympatico - Digital Subscriber Line (DSL)
Bell's digital subscriber line (DSL) services are based on ADSL, ADSL2+ and VDSL2 technology

On May 8, 2012, Bell has simplified its FTTN DSL lineup to offer only three plans. The service offerings are as follows:

In regions where FTTN is unavailable, a plan called Bell Internet 5 is available. It is similar to the Fibe 5/1 plan, but uses traditional DSL instead of FTTN DSL. Upload speeds are limited to 800 kbit/s.

Rate-Adaptive Digital Subscriber Line
'Rate-adaptive digital subscriber line' ('RADSL') is a variation of asymmetric digital subscriber line (ADSL) technology. In RADSL the DSL modem adjusts the upstream (networking)|upstream Bandwidth (signal processing)|bandwidth to create a wider frequency band for the downstream traffic. Using this technique the line is more tolerant of errors caused by Noise (physics)|noise and signal loss.

Rate-Adaptive Digital Subscriber Line
As the frequency is adjusted, the upstream bandwidth may be markedly decreased if there is a large amount of line noise or signal degradation - this may reduce the upstream bit rate to as little as 64 kbit/s - the same speed as a single ISDN B channel.

Very high speed digital subscriber line 2
'Very-high-bit-rate digital subscriber line 2' ('VDSL2') is an access technology that exploits the existing infrastructure of copper wires that were originally deployed for plain old telephone service|traditional telephone service as a way of delivering very high speed internet access

Very high speed digital subscriber line 2 - Description
VDSL2 is the newest and most advanced standard of digital subscriber line (DSL) broadband wireline communications. Designed to support the wide deployment of triple play (telecommunications)|triple play services such as voice, video, data, high-definition television (HDTV) and interactive gaming, VDSL2 is intended to enable operators and carriers to gradually, flexibly, and cost-efficiently upgrade existing xDSL infrastructure.

The protocol is standardized in the International Telecommunication Union telecommunications sector (ITU-T) as Recommendation G It has been announced as finalized on 27 May 2005, and first published on 17 February Several corrections and amendments have been published in 2007 through 2011.

VDSL2 is an enhancement to very-high-bit-rate digital subscriber line (VDSL), Recommendation G It permits the transmission of asymmetric and symmetric aggregate data rates up to 200Mbit/s downstream (networking)|downstream and upstream (networking)|upstream on twisted pairs using a bandwidth up to 30MHz.

VDSL2 deteriorates quickly from a theoretical maximum of 250Mbit/s at source to 100Mbit/s at and 50Mbit/s at , but degrades at a much slower rate from there, and outperforms VDSL. Starting from its performance is equal to ADSL2+.

Asymmetric digital subscriber line|ADSL-like long reach performance is one of the key advantages of VDSL2. LR-VDSL2 enabled systems are capable of supporting speeds of around 1–4Mbit/s (downstream) over distances of 4–5km (2.5–3miles), gradually increasing the bit rate up to symmetric 100Mbit/s as loop-length shortens. This means that VDSL2-based systems, unlike VDSL1 systems, are not limited to short local loops or MTU/MDUs only, but can also be used for medium range applications.

Channel bonding|Bonding (ITU-T G.998.x) may be used to combine multiple wire pairs to increase available capacity, or extend the copper network's reach.

Very high speed digital subscriber line 2 - Profiles
The standard defines a wide range of profiles that can be used in different VDSL deployment architectures; in the central office, in the cabinet or in the building for example.

Very high speed digital subscriber line 2 - Vectoring
Vectoring is a transmission method that employs the coordination of line signals for reduction of Crosstalk (electronics)|crosstalk levels and improvement of performance

Very high speed digital subscriber line 2 - Vectoring
Although technically feasible at the moment vectoring is incompatible with local-loop unbundling but future standard amendments could bring a solution.

Asymmetric digital subscriber line
It does this by utilizing frequencies that are not used by a voice telephone call.ANSI T Network and Customer Installation Interfaces – Asymmetric Digital Subscriber Line (ADSL) Metallic Interface

Asymmetric digital subscriber line
At the telephone exchange the line generally terminates at a digital subscriber line access multiplexer (DSLAM) where another frequency splitter separates the voice band Signal (electronics)|signal for the conventional Telecommunications network|phone network. Data carried by the ADSL are typically routed over the telephone company's data network and eventually reach a conventional Internet Protocol network.

Asymmetric digital subscriber line - Overview
ADSL differs from the less common symmetric digital subscriber line (SDSL). Bandwidth (computing)|Bandwidth (and bit rate) is greater toward the customer premises (known as downstream (networking)|downstream) than the reverse (known as upstream (networking)|upstream).

This is why it is called asymmetric. Providers usually market ADSL as a service for consumers to provide Internet access in a relatively passive mode: able to use the higher speed direction for the download from the Internet but not needing to run servers that would require high speed in the other direction.

There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users

The marketing reasons for an asymmetric connection are that, firstly, most uses of internet traffic will require less data to be uploaded than downloaded

Asymmetric digital subscriber line - Operation
Currently, most ADSL communication is Duplex (telecommunications)|full-duplex

With commonly deployed ADSL over Plain old telephone service|POTS (Annex A), the band from kHz to kHz is used for upstream communication, while 138kHz – 1104kHz is used for downstream communication

The total maximum capacity derived from summing the bits-per-bins is reported by DSL modems and is sometimes termed sync rate

The choices the DSL modem make can also be either conservative, where the modem chooses to allocate fewer bits per bin than it possibly could, a choice which makes for a slower connection, or less conservative in which more bits per bin are chosen in which case there is a greater risk case of error should future signal-to-noise ratios deteriorate to the point where the bits-per-bin allocations chosen are too high to cope with the greater noise present

Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk problems that affect other lines in the same bundle.

There is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the modulation method used.

ADSL initially existed in two versions (similar to VDSL), namely Carrierless Amplitude Phase Modulation|CAP and Orthogonal frequency-division multiplexing|DMT. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installations at the time. However, DMT was chosen for the first ITU-T ADSL standards, G and G (also called G.dmt and G.lite respectively). Therefore all modern installations of ADSL are based on the DMT modulation scheme.

Asymmetric digital subscriber line - Interleaving and fastpath
ISPs (rarely, users) have the option to use Bit-interleaving|interleaving of packets to counter the effects of burst noise on the telephone line

Asymmetric digital subscriber line - Interleaving and fastpath
Fastpath connections have an interleaving depth of 1, that is one packet is sent at a time

Asymmetric digital subscriber line - Installation problems
ADSL deployment on an existing plain old telephone service (POTS) telephone line presents some problems because the DSL is within a frequency band that might interact unfavourably with existing equipment connected to the line

In the early days of DSL, installation required a technician to visit the premises

Commonly, microfilters are only low-pass filters, so beyond them only low frequencies (voice signals) can pass

A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband (that is, POTS telephone-like) devices are connected to the line

DSL signals may be degraded by older telephone lines, surge protectors, poorly-designed microfilters, Repetitive Electrical Impulse Noise, and by long telephone extension cords

Asymmetric digital subscriber line - Transport protocols
* STM-1|Synchronous Transport Module (STM), which allows the transmission of frames of the Synchronous Digital Hierarchy (SDH)

Asymmetric digital subscriber line - Transport protocols
In home installation, the prevalent transport protocol is ATM. On top of ATM, there are multiple possibilities of additional layers of protocols (two of them are abbreviated in a simplified manner as PPPoA or PPPoE), with the all-important Transmission Control Protocol|TCP/Internet Protocol|IP at layers 4 and 3 respectively of the OSI model providing the connection to the Internet.

High bit rate digital subscriber line 2
'High bit rate digital subscriber line 2' ('HDSL2') is a standard developed by the American National Standards Institute (ANSI) Committee T1E1.4 and published in 2000 as 'ANSI T '

Symmetric digital subscriber line
'Symmetric digital subscriber line' ('SDSL') can have two meanings:

* In the wider sense it is a collection of Internet access technologies based on Digital subscriber line|DSL that offer symmetric Bandwidth (computing)|bandwidth Upstream (networking)|upstream and downstream (computer science)|downstream, including IDSL, HDSL, HDSL2, G.SHDSL, and the SDSL variant below. It is considered the opposite of asymmetric digital subscriber line (ADSL) technologies where the upstream bandwidth is lower than the downstream bandwidth.

* In the narrow sense SDSL is a particular proprietary and non-standardized DSL variant that supports data only on a single line and does not support analog calls, see below.

Symmetric digital subscriber line - Proprietary SDSL technology
SDSL is a rate-adaptive digital subscriber line (DSL) variant with T-carrier|T1/E-carrier|E1-like data rates (T1: Data rate units#Megabit per second|Mbit/s, E1: Mbit/s). It runs over one pair of copper wires, with a maximum range of . It cannot co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth.

Symmetric digital subscriber line - Standardization efforts
SDSL is a Proprietary protocol|proprietary technology that was never standardized. As such it usually only interoperates with devices from the same vendor. It is the predecessor of single-pair high-speed digital subscriber line|G.SHDSL which was standardized in February 2001 by International Telecommunication Union|ITU-T with recommendation G

SDSL is often confused with G.SHDSL and High bit rate Digital Subscriber Line|HDSL;

in Europe, G.SHDSL was standardized by ETSI using the name 'SDSL'. This ETSI variant is compatible with the ITU-T G.SHDSL standardized regional variant for Europe.

As there is a standardised successor available, SDSL installations today are considered Legacy system|legacy. Most new installations use G.SHDSL equipment instead of SDSL.

Symmetric digital subscriber line - Target audience
SDSL typically falls between ADSL and T1/E1 in price and was mainly targeted at small and medium businesses who don't need the service guarantees of Frame Relay or the higher performance of a leased line.

Multi-rate symmetric digital subscriber line
'Multi-rate symmetric DSL' ('MSDSL') is a Proprietary protocol|proprietary, non-standardized digital subscriber line technology with a maximum distance of 8,800m (29,000ft). It is capable of multiple transfer rates, as set by the Internet service provider, typically based on the service and/or price. The maximum speed in both directions is about 2 Mbit/s.

Multi-rate symmetric digital subscriber line - Technology
MSDSL is based on 2B1Q transmission with a capacity of 2.32 Mbit/s

High-bit-rate digital subscriber line
Successor technology to HDSL includes HDSL2|HDSL2 and HDSL4, proprietary Symmetric digital subscriber line|SDSL, and G.SHDSL.

High-bit-rate digital subscriber line - Standardization
HDSL was developed for T1 service at Mbit/s by the American National Standards Institute (ANSI) Committee T1E1.4 and published in February 1994 as 'ANSI Technical Report TR-28'

High-bit-rate digital subscriber line - Comparison to legacy T1
Legacy T-carrier|T1 carriers use the line codes alternate mark inversion (AMI) and more recently B8ZS

High-bit-rate digital subscriber line - Comparison to legacy T1
Legacy T1 required repeaters every 35dB of attenuation, equivalent to , depending on conductor gauge and other circumstances. Originally marketed as non-repeated T1, HDSL increased the reach to on a American Wire Gauge|AWG24 local loop. To enable longer HDSL lines, up to four repeaters can be used for a reach of .

High-bit-rate digital subscriber line - Comparison to other DSL variants
Unlike ADSL, HDSL operates in the baseband and does not allow plain old telephone service|POTS or ISDN to coexist on the wire same pairs. Unlike ADSL, the proprietary SDSL, and G.SHDSL, HDSL is not rate adaptive: the line rate is always 1.544Mbit/s or 2.048Mbit/s. Lower rates at multiples of 64kbit/s are offered to customers by using only a portion of the DS0 channels in the DS1 signal, referred to as channelized T1/E1.

High-bit-rate digital subscriber line - Comparison to other DSL variants
HDSL gave way to new symmetric DSL technologies, HDSL2|HDSL2 and HDSL4, the proprietary SDSL, and G.SHDSL. HDSL2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192kbit/s to 2.3Mbit/s, using a single pair of copper.

Single-pair high-speed digital subscriber line
'Symmetrical high-speed digital subscriber line' ('SHDSL') is a form of digital subscriber line (DSL), a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide

SHDSL features symmetrical data rates in both the upstream (networking)|upstream and downstream (networking)|downstream directions, from 192 kbit/s to 2,312 kbit/s of payload (computing)|payload in 8 kbit/s increments for one pair and 384 kbit/s to 4,624 kbit/s in 16 kbit/s increments for two pairs of wires

An optional extended SHDSL mode allows symmetric data rates up to 5,696 kbit/s on one pair, using the 32-TC-PAM modulation scheme specified in Annexes F and G. Using M-pair bonding, up to four pairs of wires may be bonded to yield data rates up to M×5,696 kbit/s. In this way, a single SHDSL interface using four bonded wire pairs can carry up to 22,784 kbit/s.

The SHDSL payload may be either 'clear channel' (unstructured), T-carrier|T1 or E-carrier|E1 (full rate or fractional), multiple ISDN Basic Rate Interface (BRI), Asynchronous Transfer Mode (ATM) cells or Ethernet packets. A 'dual bearer' mode allows a mixture of two separate streams (e.g. T1 and ATM) to share the SHDSL bandwidth.

Single-pair high-speed digital subscriber line - SHDSL standards
The industry standard for SHDSL is defined by ITU-T recommendation G This was first published in February SHDSL equipment is also known by the standard's draft name of G.SHDSL. Major updates to G were released in December Equipment conforming to the 2003 version of G is often referred to by the standard's draft name of G.SHDSL.bis or just SHDSL.bis. The updated G features:

* Optional support for up to four copper pair connections (M-pair)

* Optional extensions to allow user data rates up to 5696 kbit/s per pair, described in Annexes F and G

* Optional support for dynamic rate repartitioning, allowing flexible change of the SHDSL data rate without service interruption, described in Annex E.10.3

* New payload definitions including Ethernet packet transfer mode (PTM), described in Annex E.11

SHDSL supersedes the older HDSL symmetric DSL technology defined in ITU-T G SHDSL is largely replaced by VDSL2 because of greater bandwidth, less interference and greater utilization.

In Communications in Europe|Europe, a variant of SHDSL was standardized by the European Telecommunications Standards Institute (ETSI) using the name 'SDSL'. This ETSI variant is compatible with the ITU-T SHDSL standardized regional variant for Europe and must not be confused with the usage of the term SDSL in North America to refer to symmetric digital subscriber line.

Very-high-bit-rate digital subscriber line
'Very-high-bit-rate digital subscriber line' ('VDSL' or 'VHDSL') is a digital subscriber line (DSL) technology providing data transmission faster than ADSL over a single flat untwisted or twisted pair of copper wires (up to 52Mbit/s Downstream (networking)|downstream and 16Mbit/s Upstream (networking)|upstream), and on coaxial cable (up to 85Mbit/s down- and upstream) using the frequency band from 25kHz to 12MHz

Very-high-bit-rate digital subscriber line
Second-generation systems (very high speed digital subscriber line 2|VDSL2; ITU-T G approved in February 2006) use frequencies of up to 30MHz to provide data rates exceeding 100Mbit/s simultaneously in both the upstream and downstream directions. The maximum available bit rate is achieved at a range of about 300 meters; performance degrades as the loop attenuation increases.

Very-high-bit-rate digital subscriber line - Development
The concept of VDSL was first published in 1991 through a joint Bellcore-Stanford research study. The study searched for potential successors to the then-prevalent HDSL and relatively new ADSL, which were both 1.5 Mbit/s. Specifically, it explored the feasibility of symmetric and asymmetric data rates exceeding 10 Mbit/s on short phone lines.

Very-high-bit-rate digital subscriber line - VDSL standards
VDSL uses up to 7 different frequency bands, which enables customization of data rate between upstream and downstream depending on the service offering and spectrum regulations. First generation VDSL standard specified both quadrature amplitude modulation (QAM) and discrete multi-tone modulation (DMT). In 2006, ITU-T standardized VDSL in recommendation G which specified only DMT modulation for VDSL2.

ISDN digital subscriber line
'ISDN Digital Subscriber Line' ('IDSL') uses Integrated Services Digital Network|ISDN-based digital subscriber line technology to provide a data communication channel across existing copper telephone lines at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection at 128kbit/s

IDSL is not available in all countries.

ISDN digital subscriber line (IDSL) is a cross between ISDN and xDSL. It is like ISDN in that it uses a single-wire pair to transmit full-duplex data at 128 kbit/s and at distances of up to RRD range. Like ISDN, IDSL uses a 2B1Q line code to enable transparent operation through the ISDN U interface. Finally, the user continues to use existing Customer-premises equipment|CPE (ISDN BRI terminal adapters, bridges, and routers) to make the CO connections.

The big difference is from the carrier's point-of-view. Unlike ISDN, IDSL does not connect through the voice switch. A new piece of data communications equipment terminates the IDSL connection and shunts it off to a router or data switch. This is a key feature because the overloading of central office voice switches by data users is a growing problem for telcos.

The limitation of IDSL is that the customer no longer has access to ISDN signaling or voice services. But for Internet service providers, who do not provide a public voice service, IDSL is an interesting way of using Plain old telephone service|POTS dial service to offer higher-speed Internet access, targeting the embedded base of more than five million ISDN users as an initial market.

Asymmetric Digital Subscriber Line 2 Plus
'ITU G.992.5' (also referred to as 'ADSL2+' and 'G.DMT.bis+') is an International Telecommunication Union standard for asymmetric digital subscriber line (ADSL) broadband Internet access. The standard has a maximum theoretical download speed of '24 Data rate units|Mbit/s'. Utilizing ITU_G.992.5_Annex_M upload speeds of 3.3 Data rate units|Mbit/s can be achieved.

Asymmetric Digital Subscriber Line 2 Plus - Technical information
ADSL2+ extends the capability of basic Asymmetric digital subscriber line|ADSL by doubling the number of downstream (computer science)|downstream Channel (communications)|channels. The data rates can be as high as 24Data rate units|Mbit/s downstream and up to 1.4Mbit/s upstream depending on the distance from the digital subscriber line access multiplexer|DSLAM to the customer's premises.

ADSL2+ is capable of doubling the frequency band of typical ADSL connections from 1.1MHz to 2.2MHz. This doubles the downstream data rates of the previous G.992.3|ADSL2 standard (which was up to 12Mbit/s), and like the previous standards will degrade from its peak bitrate after a certain distance.

Also ADSL2+ allows port bonding. This is where multiple ports are physically provisioned to the end user and the total bandwidth is equal to the sum of all provisioned ports. So if 2 lines capable of 24Mbit/s were bonded the end result would be a connection capable of 48 Mbit/s download and twice the original upload speed. Not all DSLAM vendors have implemented this functionality.

Asymmetric Digital Subscriber Line 2 Plus - Finland
ADSL2+ and triple play solutions are offered by the major DSL providers (including Elisa Oyj and TeliaSonera) in certain areas. Usually maximum download speed when using ADSL2+ is 24Mbit/s and upload speed 1Mbit/s. Also Nebula, SuomiCom, DNA, KPO, Finnet and many other ISPs provide ADSL2+ connections.

Asymmetric Digital Subscriber Line 2 Plus - Greece
As of January 2007, most DSL Internet service provider|providers started offering ADSL2+ from their proprietary network.

* [ CYTA Hellas] – Double Play Services (2play) – Up to 24Mbit/s downstream, 1Mbit/s upstream.

* Forthnet [ – Offers Double Play services ([ Forthnet ADSL Economy] and [ Forthnet 2play]) – Up to 24Mbit/s downstream, 1Mbit/s upstream.

* Hellas On-Line|Hellas Online ( – Offers double play services ([ hol double-play]) – Up to 24Mbit/s downstream, 1Mbit/s upstream.

* [ Net One] – Offers double play services ([ Net One Για το Σπίτι]) – Up to 24Mbit/s downstream, 1Mbit/s upstream.

* On Telecoms [ ] – Offers triple play services ([ Όλα σε Ένα – All in One]) – Up to 24Mbit/s downstream.

* OTE [ – Offers Double play ([ conn-x TALK]) – Up to 24Mbit/s downstream, 1Mbit/s upstream.

* [ Tellas] (a subsidiary of Wind Hellas) – Offers double play services ([ Tellas Double Play Best Price, Tellas Double Play No Limit] and [ Tellas Double Play Unlimited]) – Up to 24Mbit/s downstream, 1Mbit/s upstream.

* Vivodi [ – Offers triple play services (Vivodi#Triple Play|cableTV [ – Up to 24Mbit/s downstream, 1Mbit/s upstream.

* Vodafone Greece|Vodafone – Offers double play services ([ Vodafone Double Play Plus]) – Up to 24Mbit/s downstream, 1Mbit/s upstream.

Asymmetric Digital Subscriber Line 2 Plus - Hungary
In Hungary, since mid-2006, multiple telecom companies have started offering ADSL2+ services. As of July 2007, T-Com, the biggest Hungarian ISP, is silently upgrading its customers' ADSL connections to ADSL2+, although with no speed changes.

Asymmetric Digital Subscriber Line 2 Plus - Guatemala
Telgua in Guatemala has deployed services across Guatemala City and nearby zones, its deploying ADSL2+ in other cities and now operates the largest ADSL2+ network in the country. Deliver speeds capacity up to 5Mbit/s downstream and 1024kbit/s upstream and increasing.

Asymmetric Digital Subscriber Line 2 Plus - Bangladesh
BTCL – Bangladesh Telecommunications Company Limited Provides Up to 2 Mbit/sdownstream with its ADSL2+ Broadband Internet connection. Although there are new internet service providers such as qubee and banglalion who are also offering up to 2 M/bits/p/s. Other than that several other network providers such as Grameenphone, Aktel, Airtel, Citycell (zoom) and Banglalink.

Asymmetric Digital Subscriber Line 2 Plus - India
[ BSNL] offers ADSL2+ Broadband under the DataOne brand name, with advertised speeds up to 24Mbit/s (download)

Asymmetric Digital Subscriber Line 2 Plus - Turkey
*ADSL2+ ([ Türk Telekom]): Download speed up to 16 Mbit/s and upload 1 Mbit/s for unlimited connection.

*ADSL2+ ([ Smile adsl]): Download speed up to 16Mbit/s and upload 1Mbit/s for unlimited connection.

*ADSL2+ ([ Biri adsl]): Download speed up to 10Mbit/s and upload 1Mbit/s for unlimited connection.

*ADSL2+ ([ Doping adsl]): Download speed up to 16Mbit/s and upload 1Mbit/s for unlimited connection.

*ADSL2+ ([ Turkcell Superonline]): Download speed up to 8Mbit/s and upload 1Mbit/s for unlimited connections.

*ADSL2+ ([ TurkNet]): Download speed up to 8Mbit/s and upload 1Mbit/s for unlimited connection.

Powerline Digital Subscriber Line
'Power-line communication' ('PLC') carries data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. It is also known as 'power-line carrier', 'power-line digital subscriber line' (PDSL), 'mains communication', 'power-line telecommunications', or 'power-line networking' (PLN).

A wide range of power-line communication technologies are needed for different applications, ranging from home automation to Internet access which is often called broadband over power lines (BPL)

A number of difficult technical problems are common between wireless and power-line communication, notably those of spread spectrum radio signals operating in a crowded environment. Radio interference, for example, has long been a concern of amateur radio groups.

Powerline Digital Subscriber Line - Basics
Power-line communications systems operate by adding a modulated carrier signal to the wiring system. Different types of power-line communications use different frequency bands. Since the power distribution system was originally intended for transmission of AC power at typical frequencies of 50 or 60 Hertz|Hz, power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power-line communications.

The main issue determining the frequencies of power-line communication is laws to limit interference with radio services

Data signaling rate|Data rates and distance limits vary widely over many power-line communication standards

Powerline Digital Subscriber Line - Long haul, low frequency
Utility companies use special coupling capacitors to connect radio transmitters to the power-frequency AC conductors

On some powerlines in the former Soviet Union, PLC-signals are not fed into the high voltage line, but in the ground conductors, which are mounted on insulators at the pylons.

While utility companies use microwave and now, increasingly, fiber optic cables for their primary system communication needs, the power-line carrier apparatus may still be useful as a backup channel or for very simple low-cost installations that do not warrant installing fiber optic lines.

power-line carrier communication (PLCC) is mainly used for telecommunication, tele-protection and tele-monitoring between electrical substations through power lines at high voltages, such as 110 kV, 220 kV, 400 kV. The major benefit is the union of two applications in a single system, which is particularly useful for monitoring electric equipment and advanced energy demand management|energy management techniques (such as OpenADR and OpenHAN).

The modulation generally used in these system is amplitude modulation. The carrier frequency range is used for audio signals, protection and a pilot frequency. The pilot frequency is a signal in the audio range that is transmitted continuously for failure detection.

The voice signal is compressed and filtered into the 300Hz to 4000Hz range, and this audio frequency is mixed with the carrier frequency. The carrier frequency is again filtered, amplified and transmitted. The transmission power of these HF carrier frequencies will be in the range of 0 to +32 Decibel|dbW. This range is set according to the distance between substations.

PLCC can be used for interconnecting private branch exchanges (PBXs).

To sectionalize the transmission network and protect against failures, a wave trap is connected in series with the power (transmission) line

A coupling capacitor is used to connect the transmitters and receivers to the high voltage line. This provides low impedance path for carrier energy to HV line but blocks the power frequency circuit by being a high impedance path. The coupling capacitor may be part of a capacitor voltage transformer used for voltage measurement.

power-line carriers may change its transmission system from analog to digital to enable Internet Protocol devices. Digital power-line carrier (DPLC) was developed for digital transmission via power lines. DPLC has the required quality of bit error rate characteristics and transmission ability such as transmitting information from monitored electric-supply stations and images.

power-line carrier systems have long been a favorite at many utilities because it allows them to reliably move data over an infrastructure that they control. Many technologies have multiple applications. For example, a communication system bought initially for automatic meter reading can sometimes also be used for load control or for demand response applications.

A PLC carrier repeating station is a facility, at which a power-line communication (PLC) signal on a Electric power transmission|powerline is refreshed. Therefore

the signal is filtered out from the powerline, Demodulation|demodulated and Modulation|modulated on a new Carrier wave|carrier frequency, and then reinjected onto the powerline again. As PLC signals can carry long distances (several 100 kilometres), such facilities only exist on very long power lines using PLC equipment.

PLC is one of the technologies used for automatic meter reading

In a one-way (inbound only) system, readings bubble up from end devices (such as meters), through the communication infrastructure, to a master station which publishes the readings. A one-way system might be lower-cost than a two-way system, but also is difficult to reconfigure should the operating environment change.

In a two-way system (supporting both outbound and inbound), commands can be broadcast out from the master station to end devices (meters) – allowing for reconfiguration of the network, or to obtain readings, or to convey messages, etc

Powerline Digital Subscriber Line - Home control (narrowband)
Power-line communications technology can use the electrical power wiring within a home for home automation: for example, remote control of lighting and appliances without installation of additional control wiring.

Typically home-control power-line communication devices operate by modulating in a carrier wave of between 20 and 200 Hertz|kHz into the household wiring at the transmitter

The universal powerline bus, introduced in 1999, uses pulse-position modulation (PPM). The physical layer method is a very different scheme than the X10. LonTalk, part of the LonWorks home automation product line, was accepted as part of some automation standards.

Powerline Digital Subscriber Line - Low-speed narrow-band
Narrowband power-line communications began soon after electrical power supply became widespread. Around the year 1922 the first carrier frequency systems began to operate over high-tension lines with frequencies of 15 to 500kHz for telemetry purposes, and this continues. Consumer products such as baby alarms have been available at least since 1940.

In the 1930s, ripple carrier signalling was introduced on the medium (10–20 kV) and low voltage (240/415V) distribution systems.

For many years the search continued for a cheap bi-directional technology suitable for applications such as remote meter reading

Since the mid-1980s, there has been a surge of interest in using the potential of digital communications techniques and digital signal processing. The drive is to produce a reliable system which is cheap enough to be widely installed and able to compete cost effectively with wireless solutions. But the narrowband powerline communications channel presents many technical challenges, a mathematical channel model and a survey of work is available.

Applications of mains communications vary enormously, as would be expected of such a widely available medium

Control and telemetry applications include both 'utility side' applications, which involves equipment belonging to the utility company up to the domestic meter, and 'consumer-side' applications which involves equipment in the consumer's premises

A project of EDF, France includes demand management, street lighting control, remote metering and billing, customer specific tariff optimisation, contract management, expense estimation and gas applications safety.

There are also many specialised niche applications which use the mains supply within the home as a convenient data link for telemetry. For example, in the UK and Europe a TV audience monitoring system uses powerline communications as a convenient data path between devices that monitor TV viewing activity in different rooms in a home and a data concentrator which is connected to a telephone modem.

Powerline Digital Subscriber Line - Medium-speed narrow-band
The Distribution Line Carrier (DLC) System technology used a frequency range of 9 to 500kHz with data rate up to 576 kbit/s.

A project called Real-time Energy Management via Powerlines and Internet (REMPLI) was funded from 2003 to 2006 by the European Commission.

In 2009, a group of vendors formed the PoweRline Intelligent Metering Evolution (PRIME) alliance. As delivered, the physical layer is OFDM, sampled at 250kHz, with 512 differential phase shift keying channels from 42–89kHz. Its fastest transmission rate is kilobits/second, while its most robust is 21.4 kbit/s. It uses a convolutional code for error detection and correction. The upper layer is usually IPv4.

In 2011, several companies including distribution network operators (Électricité de France#Distribution network .28RTE and ErDF.29|ERDF, Enexis), meter vendors (Sagemcom, LandisGyr) and chip vendors (Maxim Integrated, Texas Instruments, STMicroelectronics) founded the G3-PLC Alliance to promote G3-PLC technology

Powerline Digital Subscriber Line - Transmitting radio programs
Sometimes PLC was used for transmitting radio programs over powerlines. When operated in the AM radio band, it is known as a carrier current system.

Powerline Digital Subscriber Line - High-frequency (≥ 1 MHz)
High frequency communication may (re)use large portions of the radio spectrum for communication, or may use select (narrow) band(s), depending on the technology.

Powerline Digital Subscriber Line - Home networking (LAN)
Power line communications can also be used in a home to interconnect home computers and peripherals, and home entertainment devices that have an Ethernet port. Powerline adapter sets plug into power outlets and establish an Ethernet connection using the existing electrical wiring in the home. (Power strips with filtering may absorb the power line signal.) This allows devices to share data without the inconvenience of running dedicated network cables.

Powerline Digital Subscriber Line - Home networking (LAN)
The most widely deployed powerline networking standard is from the HomePlug Powerline Alliance

Powerline Digital Subscriber Line - Broadband over power line
Broadband over power line (BPL) is a system to transmit two-way data over existing AC MV (medium voltage) electrical distribution wiring, between transformers, and AC LV (low voltage) wiring between transformer and customer outlets (typically 110 to 240V). This avoids the expense of a dedicated network of wires for data communication, and the expense of maintaining a dedicated network of antennas, radios and routers in wireless network.

BPL uses some of the same radio frequencies used for over-the-air radio systems. Modern BPL employs frequency-hopping spread spectrum to avoid using those frequencies actually in use, though early pre-2010 BPL standards did not. The criticisms of BPL from this perspective are of pre-OPERA, pre-1905 standards.

The BPL OPERA standard is used primarily in Europe by ISPs. In North America it is used in some places (Washington Island, WI, for instance) but is more generally used by electric distribution utilities for smart meters and load management.

Since the ratification of the IEEE 1901 LAN standard and its widespread implementation in mainstream router chipsets, the older BPL standards are not competitive for communication between AC outlets within a building, nor between the building and the transformer where MV meets LV lines.

Powerline Digital Subscriber Line - Automotive uses
Power-line technology enables in-vehicle network communication of data, voice, music and video signals by digital means over direct current (DC) battery power-line

Prototypes are successfully operational in vehicles, using automotive compatible protocols such as CAN-bus, Local Interconnect Network|LIN-bus over power line (DC-LIN) and [DC-bus].[ DC-LIN Over Power line]

LonWorks power line based control has been used for an HVAC system in a production model bus.

The SAE J1772 committee developing standard connectors for plug-in electric vehicles proposes to use power line communication between the vehicle, off-board charging station, and the smart grid, without requiring an additional pin; SAE International|SAE and the IEEE Standards Association are sharing their draft standards related to the smart grid and vehicle electrification.

Powerline Digital Subscriber Line - Standards
Within homes, the HomePlug AV and IEEE 1901 standards specify how, globally, existing AC wires should be employed for data purposes. The IEEE 1901 includes HomePlug AV as a baseline technology, so any IEEE 1901 products are fully interoperable with HomePlug AV, HomePlug GreenPHY or the forthcoming HomePlug AV2 specification (under development now and expected to be approved in Q1 2011).

Powerline Digital Subscriber Line - Standards organizations
Several competing organizations have developed specifications, including the HomePlug Powerline Alliance, Universal Powerline Association (defunct) and HD-PLC Alliance. On December 2008, the ITU-T adopted Recommendation G.hn/G.9960 as a standard for high-speed powerline, coax and phoneline communications. The National Energy Marketers Association was also involved in advocating for standards.

Powerline Digital Subscriber Line - Standards organizations
In July 2009, the IEEE P1901 working group approved its draft standard for broadband over power lines

Internet in the United Kingdom - Digital subscriber line (DSL)
Asymmetric digital subscriber line (ADSL) was introduced to the UK in trial stages in 1998 and a commercial product was launched in In the United Kingdom, most telephone exchange|exchanges, local loops and backhaul (telecommunications)|backhauls are owned and managed by BT Wholesale, who then wholesale connectivity via Internet service providers, who generally provide the connectivity to the Internet, support, billing and value added services (such as web hosting and ).

Internet in the United Kingdom - Digital subscriber line (DSL)
Some exchanges, numbering under 1000, have been upgraded to support Symmetric Digital Subscriber Line|SDSL products

Very-high-bitrate digital subscriber line - VDSL standards
A VDSL connection uses up to seven frequency bands, so one can allocate the data rate between upstream and downstream differently depending on the service offering and spectrum regulations. First generation VDSL standard specified both quadrature amplitude modulation (QAM) and discrete multi-tone modulation (DMT). In 2006, ITU-T standardized VDSL in recommendation G which specified only DMT modulation for VDSL2.

TekSavvy - Digital subscriber line (DSL) Internet
TekSavvy offers DSL service using Bell lines in Ontario and Quebec, Telus lines in Alberta and British Columbia, and Bell Aliant lines in New Brunswick, Newfoundland and Labrador|Newfoundland, Nova Scotia and Prince Edward Island.

Before the summer of 2011, TekSavvy could only access Bell's Performance speed tier, with maximum speeds of 5Mb/s

On March 20, 2013, TekSavvy added a 50 Mbit/s tier, the fastest FTTN tier that Bell started offering a month earlier. TekSavvy offers the same FTTN DSL speeds available from Bell. The only exceptions are that for the slowest plans, 6 Mbit/s regular DSL and 7 Mbit/s FTTN DSL is sold by TekSavvy instead of Bell's 5 Mbit/s plans. All plans offer a choice between 300 GB or unlimited Internet access, while the 6 and 7 Mbit/s plans offer a lower-priced 75 GB option.

Internet in Bulgaria - Digital subscriber lines (DSL)
Asymmetric digital subscriber line (ADSL) technology was introduced in Bulgaria after the privatization of the state monopoly Bulgarian Telecommunications Company (BTC) in 2004

Internet in Croatia - Digital Subscriber Line (DSL)
In Croatia ADSL was introduced in 2000 by the German owned operator T-Com, formerly HT (Hrvatski telekom, meaning Croatian telecom)

* Iskon Internet (owned by T-HT) - part of the network is locally based (in major urban areas), and partly services are run by local loop over T-Com copper lines, TriplePlay plan Iskon.TV, but only on the local part of the network for now.

* Optima Telekom d.d. - part of the network is locally based (in major urban areas), and partly services are run by local loop over T-Com copper lines, TriplePlay plan is called OptiTV. The reach for TriplePlay is extended gradually.

* Metronet telekomunikacije d.d. - has TriplePlay

Digital subscriber line access multiplexer
A 'digital subscriber line access multiplexer' ('DSLAM', often pronounced dee-slam) is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques.

Digital subscriber line access multiplexer - Path taken by data to DSLAM
# Customer premises: DSL modem terminating the ADSL, SHDSL or VDSL circuit and providing LAN interface to a single computer or LAN segment.

# Local loop: the telephone company wires from a customer to the telephone exchange or to a serving area interface, often called the last mile (LM).

#* Main distribution frame (MDF): a wiring rack that connects outside subscriber lines with internal lines. It is used to connect public or private lines coming into the building to internal networks. At the telco, the MDF is generally in proximity to the Utility tunnel|cable vault and not far from the telephone switch.

#* DSL filter|xDSL filters: DSL filters are used in the telephone exchange to split voice from data signals. The voice signal can be routed to a Plain old telephone service|POTS provider or left unused whilst the data signal is routed to the ISP DSLAM via the HDF (see next entry).

#* Handover distribution frame (HDF): a distribution frame that connects the last mile provider with the service provider's DSLAM

#* DSLAM: a device for DSL service. The DSLAM port where the subscriber local loop is connected converts analog electrical signals to data traffic (Upstream (networking)|upstream traffic for data upload) and data traffic to analog electrical signals (Downstream (networking)|downstream for data download).

Digital subscriber line access multiplexer - Role of the DSLAM
The DSLAM equipment collects the data from its many modem ports and aggregates their voice and data traffic into one complex composite signal via multiplexing. Depending on its device architecture and setup, a DSLAM aggregates the DSL lines over its Asynchronous Transfer Mode (ATM), frame relay, and/or Internet Protocol network (i.e., an IP-DSLAM using PTM-TC [Packet Transfer Mode - Transmission Convergence]) protocol(s) stack.

The aggregated traffic is then directed to a telco's Backbone network|backbone switch, via an access network (AN), also called a Network service provider|Network Service Provider (NSP), at up to 10 Gbit/s data rates.

The DSLAM acts like a network switch since its functionality is at Data link layer|Layer 2 of the OSI model

A DSLAM may or may not be located in the telephone exchange, and may also serve multiple data and voice customers within a neighborhood serving area interface, sometimes in conjunction with a digital loop carrier. DSLAMs are also used by hotels, lodges, residential neighborhoods, and other businesses operating their own private telephone exchange.

In addition to being a data switch and multiplexer, a DSLAM is also a large collection of modems. Each modem on the aggregation card communicates with a single subscriber's DSL modem. This modem functionality is integrated into the DSLAM itself instead of being done via an external device like a 20th-century voiceband modem.

Like traditional voice-band modems, a DSLAM's integrated DSL modems are usually able to probe the line and to adjust themselves to electronically or digitally compensate for forward echoes and other bandwidth-limiting factors in order to move data at the maximum possible connection rate.

This compensation capability also takes advantage of the better performance of balanced line DSL connections, providing capabilities for LAN segments longer than physically similar unshielded twisted pair (UTP) Ethernet connections, since the balanced line type is generally required for its hardware to function correctly

Digital subscriber line access multiplexer - Bandwidth versus distance
The following is a rough guide to the relation between wire distance (based on 0.40 mm copper and Asymmetric Digital Subscriber Line 2 Plus|ADSL2+ technology) and maximum data rate

Digital subscriber line access multiplexer - Hardware details
Customers connect to the DSLAM through ADSL modems or DSL Router (computing)|routers, which are connected to the Public switched telephone network|PSTN network via typical unshielded twisted pair telephone lines. Each DSLAM has multiple aggregation cards, and each such card can have multiple Computer port (hardware)|ports to which the customers' lines are connected. Typically a single DSLAM aggregation card has 24 ports, but this number can vary with each manufacturer.

The most common DSLAMs are housed in a telephone company|telco-grade chassis, which are supplied with (nominal) 48 volts direct current|DC. Hence a typical DSLAM setup may contain power converters, DSLAM chassis, aggregation cards, cabling, and upstream links.

On the upstream trunk (ISP) side many early DSLAMs used Asynchronous Transfer Mode|ATM—and this approach was standardized by the DSL Forum—with Gigabit Ethernet support appearing sometime later. Today, the most common upstream links in these DSLAMs use Gigabit Ethernet or multi-gigabit Optical fiber|fiber optic links.

Digital subscriber line access multiplexer - IP-DSLAM
Internet Protocol|IP-DSLAM stands for Internet Protocol Digital Subscriber Line Access Multiplexer. User traffic is mostly IP based.

Digital subscriber line access multiplexer - IP-DSLAM
Traditional 20th century DSLAMs used Asynchronous Transfer Mode (ATM) technology to connect to upstream ATM routers/switches

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