Digital subscriber line

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"DSL" redirects here. For other uses, see DSL (disambiguation).
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Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit

asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access
.

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.

DSL service can be delivered simultaneously with

frequency bands for data transmission. On the customer premises, a DSL filter
is installed on each telephone to prevent undesirable interaction between DSL and telephone service.

The

Gbit/s using traditional copper telephone lines, though such speeds have not been made available for the end customers yet.[2][3][4]

History

It was originally thought that it was not possible to operate a conventional phone line beyond low-speed limits (typically under 9600 bit/s). In the 1950s, ordinary twisted-pair telephone cable often carried four megahertz (MHz)[

Newcastle-upon-Tyne and the Pontop Pike transmitting station. 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. A 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.[5]

The motivation for digital subscriber line technology was the

asymmetric digital subscriber line (ADSL) by placing wide-band digital signals at frequencies above the existing baseband analog voice signal carried on conventional twisted pair cabling between telephone exchanges and customers.[6] A patent was filed by AT&T Bell Labs on the basic DSL concept in 1988.[7]

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 (time delay) incurred by the retransmission of error-containing packets is acceptable.

Consumer-oriented ADSL was designed to operate on existing lines already conditioned for

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.

Older ADSL standards delivered 8 

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 local exchange carrier (LEC) to deliver DSL speeds to any distance.[9]

DSL SoC

Until the late 1990s, the cost of

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. 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.

These advantages made ADSL a better proposition for customers requiring

DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to the popularity of ADSL technology. Some of the first field trials for DSL were carried out in 1996.[10]

Early DSL service required a dedicated dry loop, but when the U.S. Federal Communications Commission (FCC) required incumbent local exchange carriers (ILECs) to lease their lines to competing DSL service providers, shared-line DSL became available. Also known as DSL over unbundled network element, this unbundling of services allows a single subscriber to receive two separate services from two separate providers on one cable pair. The DSL service provider's equipment is co-located in the same telephone exchange as that of the ILEC supplying the customer's pre-existing voice service. The subscriber's circuit is rewired to interface with hardware supplied by the ILEC which combines a DSL frequency and POTS signals on a single copper pair.

Since 1999, some ISPs started to offer microfilters, which were installed indoors and performed the same function as DSL splitters which were installed outdoors, which was to separate ADSL frequencies from POTS frequencies used for phone calls.[11][12] These filters originated out of a desire to make self-installation of DSL service possible, and eliminate early outdoor DSL splitters which were installed at or near the demarcation point between the customer and the ISP.[13]

By 2012, some carriers in the United States reported that DSL remote terminals with fiber backhaul were replacing older ADSL systems.[14]

Operation

Telephones are connected to the telephone exchange via a local loop, which is a physical pair of wires. The local loop was originally intended mostly for the transmission of speech, encompassing an audio frequency range of 300 to 3400 hertz (commercial bandwidth). However, as long-distance trunks were gradually converted from analog to digital operation, the idea of being able to pass data through the local loop (by using frequencies above the voiceband) took hold, ultimately leading to DSL.

The

analog modem
would on a POTS connection. More usable channels equate 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

MPLS
network.

The underlying technology of transport across DSL facilities uses

bits
into certain high-frequency impulses for transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem passes on, in digital form, to its interfaced equipment, such as a computer, router, switch, etc.

Unlike traditional dial-up modems, which modulate bits into signals in the 300–3400 Hz audio baseband, DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This frequency band separation enables DSL service and plain old telephone service (POTS) to coexist on the same cables, known as voice grade cables.[15] On the subscriber's end of the circuit, inline DSL filters are installed on each telephone to pass voice frequencies but filter the high-frequency signals that would otherwise be heard as hiss. Also, nonlinear elements in the phone could otherwise generate audible intermodulation and may impair the operation of the data modem in the absence of these low-pass filters. DSL and RADSL modulations do not use the voice-frequency band so high-pass filters are incorporated in the circuitry of DSL modems filter out voice frequencies.

A DSL modem

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

FTTN
).

Most residential and small-office DSL implementations reserve low frequencies for POTS, so that (with suitable filters and/or splitters) the existing voice service continues to operate independently of the DSL service. Thus POTS-based communications, including

powerline, or Wi-Fi
network on the customer's premises.

The theoretical foundations of DSL, like much of

digital modulation methods
.

Naked DSL

Main article: Naked DSL

Naked DSL is a way of providing only DSL services over a local loop. 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 VoIP) or through another network (E.g., mobile telephony). It is also commonly called an unbundled network element (UNE) in the United States; in Australia it is known as a unconditioned local loop (ULL);[16] in Belgium it is known as "raw copper" and in the UK it is known as Single Order GEA (SoGEA).[17]

It started making a comeback in the United States in 2004 when

SBC,[18] and Verizon's merger with MCI,[19]
those telephone companies have an obligation to offer naked DSL to consumers.

Typical setup

Example of a DSLAM from 2006

On the customer side, a DSL modem is hooked up to a phone line. The telephone company connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box. The DSLAM cannot be located too far from the customer because of attenuation between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.

DSL Connection schematic

The above figure is a schematic of a simple DSL connection (in blue). The right side shows a DSLAM residing in the telephone company's telephone exchange. The left side shows the customer premises equipment with an optional router. The router manages a local area network which connects PCs and other local devices. The customer may opt for a modem that contains both a router and wireless access. This option (within the dashed bubble) often simplifies the connection.

Exchange equipment

At the exchange, a

handed off to other networking transports. The DSLAM terminates all connections and recovers the original digital information. In the case of ADSL, the voice component is also separated at this step, either by a filter or splitter integrated in the DSLAM or by specialized filtering equipment installed before it.[20] Load coils in phone lines, used for extending their range in rural areas, must be removed to allow DSL to operate as they only allow frequencies of up to 4000 Hz to pass through phone cables.[21][22][23]

Customer equipment

DSL Modem schematic

The customer end of the connection consists of a DSL modem. This converts data between the digital signals used by computers and the analog voltage signal of a suitable frequency range which is then applied to the phone line.

In some DSL variations (for example,

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 the installation of appropriate DSL filters at the customer's premises to separate 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. It is possible for a DSL gateway to integrate the filter, and allow telephones to connect through the gateway.

Modern DSL

PPPoE
.

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.

DSL implementations may create

subnetwork. The earliest implementations used DHCP to provide the IP address to the subscriber equipment, with authentication via MAC address or an assigned hostname. Later implementations often use Point-to-Point Protocol
(PPP) to authenticate with a user ID and password.

Transmission modulation methods

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

DSL technologies

DSL technologies (sometimes collectively summarized as xDSL) include:

  • Symmetric digital subscriber line (SDSL), umbrella term for xDSL where the bitrate is equal in both directions.
  • Asymmetric digital subscriber line
    (ADSL), umbrella term for xDSL where the bitrate is greater in one direction than the other.
    • ANSI T1.413 Issue 2, up to 8 Mbit/s and 1 Mbit/s
    • G.dmt, ITU-T G.992.1, up to 10 Mbit/s and 1 Mbit/s
    • G.lite, ITU-T G.992.2, more noise and attenuation resistant than G.dmt, up to 1,536 kbit/s and 512 kbit/s
    • Asymmetric digital subscriber line 2
      (ADSL2), ITU-T G.992.3, up to 12 Mbit/s and 3.5 Mbit/s
    • Asymmetric digital subscriber line 2 plus
      (ADSL2+), ITU-T G.992.5, up to 24 Mbit/s and 3.5 Mbit/s
    • Very-high-bit-rate digital subscriber line
      (VDSL), ITU-T G.993.1, up to 52 Mbit/s and 16 Mbit/s
    • G.vector crosstalk cancelling feature (ITU-T G.993.5) can be used to increase range at a given bitrate, e.g. 100 Mbit/s at up to 500 meters.[25]
    • G.fast, ITU-T G.9700 and G.9701,[26] up to approximately 1 Gbit/s aggregate uplink and downlink at 100m.[27] Approved in December 2014, deployments planned for 2016.[28][29]
    • XG-FAST, allows for up to 10 Gbps on copper twisted pair lines, but only for lengths up to 30 meters. Real-world tests have shown 8 Gbps on 30-meter long twisted pair lines.[30][31][32]
  • Bonded DSL Rings
    (DSL Rings), a shared ring topology at 400 Mbit/s
  • Cable/DSL gateway
  • Etherloop Ethernet local loop
  • High-speed voice and data link
  • Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing upstream speed
  • Uni-DSL (Uni digital subscriber line or UDSL), technology developed by Texas Instruments, backwards compatible with all DMT standards
  • Hybrid Access Networks combine existing xDSL deployments with a wireless network such as LTE to increase bandwidth and quality of experience by balancing the traffic over the two access networks.[33]

The line-length limitations from telephone exchange to subscriber impose severe limits on data transmission rates. Technologies such as

fiber to the curb
network architectures).

Terabit DSL, is a technology that proposes the use of the space between the dielectrics (insulators) on copper twisted pair lines in telephone cables, as waveguides for 300 GHz signals that can offer speeds of up to 1 terabit per second at distances of up to 100 meters, 100 gigabits per second for 300 meters, and 10 gigabits per second for 500 meters.[34][35] The first experiment for this was carried out with copper lines that were parallel to each other, and not twisted, inside a metal pipe meant to simulate the metal armoring in large telephone cables.[36][37]

See also

References

  1. ^ "PC Mag". 10 February 1998.
  2. ^ Owano, Nancy (10 July 2014). "Alcatel-Lucent sets broadband speed record using copper". Phys.org.
  3. ^ Brian, Matt (10 July 2014). "Researchers get record broadband speeds out of old-school copper wire". Engadget.
  4. ^ Tarantola, Andrew (18 December 2013). "The Next Generation of DSL Can Pump 1Gbps Through Copper Phone Lines". Gizmodo.
  5. ^ John E. Trombly; John D. Foulkes; David K. Worthington (May 18, 1982). "Audio and full duplex digital data carrier system". US Patent 4,330,687 (published March 14, 1979).
  6. ^ Shamus, Ronald. "EE535 Homework 3". Worcester Polytechnic Institute. Archived from the original on April 12, 2000. Retrieved September 15, 2011.
  7. ^ US 4924492, Richard D. Gitlin; Sailesh K. Rao & Jean-Jacques Werner et al., "Method and apparatus for wideband transmission of digital signals between, for example, a telephone central office and customer premises", published May 8, 1990 
  8. doi:10.1109/49.93088
    .
  9. ^ "Home". www.strowger.com.
  10. ^ "Network World". 16 September 1996.
  11. ISBN 978-1-60267-000-6
    .
  12. ^ "Network World". November 1999.
  13. ISBN 9781420013078
    .
  14. ^ Om Malik (Apr 24, 2012). "DSL Death March Continues". Gigaom.com. Archived from the original on 2013-06-02. Retrieved 2019-10-21.
  15. ^ "PC Mag". 10 February 1998.
  16. ^ ULL (unconditioned local loop). Whirlpool.net.au. Retrieved on 2013-09-18.
  17. ^ "Next Generation Fibre" (PDF). Archived from the original (PDF) on 2017-10-19.
  18. ^ "Federal Communications Commission Approves SBC/AT&T Merger". www.sbc.com. October 31, 2005.
  19. ^ "Verizon MCI merger". Archived from the original on April 2, 2007.
  20. ISBN 9781420013078
    .
  21. ^ ISDN User Newsletter. Information Gatekeepers.
  22. ISBN 978-1-58705-087-9
    .
  23. ISBN 978-1-58705-119-7
    .
  24. ^ "G.993.2 : Very high speed digital subscriber line transceivers 2 (VDSL2)".
  25. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2018-12-27. Retrieved 2013-12-12.{{cite web}}: CS1 maint: archived copy as title (link)
  26. ^ "New ITU broadband standard fast-tracks route to 1 Gbit/s". ITU-T. 2013-12-11. Retrieved 2014-02-13.
  27. ^ Spruyt, Paul; Vanhastel, Stefaan (2013-07-04). "The Numbers are in: Vectoring 2.0 Makes G.fast Faster". TechZine. Alcatel Lucent. Archived from the original on 2014-08-02. Retrieved 2014-02-13.
  28. ^ "G.fast broadband standard approved and on the market". ITU-T. 2014-12-05. Retrieved 2014-12-07.
  29. ^ Hardy, Stephen (2014-10-22). "G.fast ONT available early next year says Alcatel-Lucent". lightwaveonline.com. Retrieved 2014-10-23.
  30. ^ Anthony, Sebastian (October 18, 2016). "XG.fast DSL does 10Gbps over telephone lines". Ars Technica.
  31. S2CID 33169617
    – via IEEE Xplore.
  32. ^ "NBN attains 8Gbps speeds over copper in XG-FAST trial with Nokia". ZDNET.
  33. ^ Broadband Forum (2016-07-01). "TR-348 Hybrid Access Broadband Network Architecture" (PDF). Archived (PDF) from the original on 2022-10-09. Retrieved 2018-07-01.
  34. ^ Chirgwin, Richard. "DSL inventor's latest science project: terabit speeds over copper". www.theregister.com.
  35. S2CID 53927909
    – via IEEE Xplore.
  36. ^ "Terabits-Per-Second Data Rates Achieved at Short Range". ieeespectrum.
  37. S2CID 216327606
    .
  38. ^ Matsumoto, Craig (2005-09-13). "Valley Wonk: DSL Man". Light Reading. Retrieved 2014-02-19.

Further reading

External links

Wikimedia Commons has media related to Digital Subscriber Line.
  • ADSL Theory—Information about the background & workings of ADSL, and the factors involved in achieving a good sync between your modem and the DSLAM.
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