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Digital subscriber line
Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access. DSL service can be delivered simultaneously with wired telephone service on the same telephone line since DSL uses higher frequency bands for data. On the customer premises, a DSL filter on each non-DSL outlet blocks any high-frequency interference to enable simultaneous use of the voice and DSL services.
The bit rate of consumer DSL services typically ranges from 256 kbit/s to over 100 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. Bit rates of 1 G bit/s have been reached in trials, [1] but most homes are likely to be limited to 500-800 Mbit/s. 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. Researchers at Bell Labs have reached speeds of 10 Gbit/s, while delivering 1 Gbit/s symmetrical broadband access services using traditional copper telephone lines. These higher speeds are lab results, however. [2][3] A 2012 survey found that "DSL continues to be the dominant technology for broadband access" with 365.1 million subscribers worldwide.


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. 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 location of the DSLAM depends on the tel co, but it cannot be located too far from the user 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.
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. The router manages a local area network (LAN) which connects PCs and other local devices. With many service providers, the customer may opt for a modem which contains both a router and wireless access. This option (within the dashed bubble) often simplifies the connection.


How DSL Works

When you connect to the Internet, you might connect through a regular modem, through a local-area network connection in your office, 
through a cable modem or through a digital subscriber line (DSL) connection. DSL is a very high-speed connection that uses the same wires as a regular telephone line.
Here are some advantages of DSL:
  • ·       You can leave your Internet connection open and still use the phone line for voice calls.
  • ·       The speed is much higher than a regular modem
  • ·       DSL doesn't necessarily require new wiring; it can use the phone line you already have.
  • ·       The company that offers DSL will usually provide the modem as part of the installation.
  • But there are disadvantages:
  • ·       A DSL connection works better when you are closer to the provider's central office. The farther away you get from the central office, the weaker the signal becomes.
  • ·       The connection is faster for receiving data than it is for sending data over the Internet.
  • ·       The service is not available everywhere.

·       In this article, we explain how a DSL connection manages to squeeze more information through a standard phone line -- and lets you make regular telephone calls even when you're online.

Asymmetric digital subscriber line
Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voice band modem can provide. ADSL differs from the less common symmetric digital subscriber line (SDSL). In ADSL, Bandwidth and bit rate 
are said to be asymmetric, meaning greater toward the customer premises (downstream) than the reverse (upstream). Providers usually market ADSL as a service for consumers for Internet access for primarily downloading content from the Internet, but not serving content accessed by others.

Overview
ADSL works by using the frequency spectrum above the band used by voice telephone calls. [1] With a DSL filter, often called splitter, the frequency bands are isolated, permitting a single telephone line to be used for both ADSL service and telephone calls at the same time. ADSL is generally only installed for short distances from the telephone exchange (the last mile), typically less than 4 kilometers (2 mi), [2] but has been known to exceed 8 kilometers (5 mi) if the originally laid wire gauge allows for further [clarification needed] distribution.
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 for the conventional 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.
There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL.

The marketing reasons for an asymmetric connection are that, firstly, most users of internet traffic will require less data to be uploaded than downloaded. For example, in normal web browsing, a user will visit a number of web sites and will need to download the data that comprises the web pages from the site, images, text, sound files etc. but they will only upload a small amount of data, as the only uploaded data is that used for the purpose of verifying the receipt of the downloaded data or any data inputted by the user into forms etc. This provides a justification for internet service providers to offer a more expensive service aimed at commercial users who host websites, and who therefore need a service which allows for as much data to be uploaded as downloaded. File sharing applications are an obvious exception to this situation. Secondly internet service providers, seeking to avoid overloading of their backbone connections, have traditionally tried to limit uses such as file sharing which generate a lot of uploads.

Symmetric digital subscriber line

A symmetric digital subscriber line (SDSL) is a digital subscriber line (DSL) that transmits digital data over the copper wires of the telephone network, where the bandwidth in the downstream direction, from the network to the subscriber, is identical to the bandwidth in the upstream direction, from the subscriber to the network. This symmetric bandwidth can be considered to be the opposite of the asymmetric bandwidth offered by asymmetric digital subscriber line (ADSL) technologies, where the upstream bandwidth is lower than the downstream bandwidth. SDSL is generally marketed at business customers, while ADSL is marketed at private as well as business customers.
What is the difference between ADSL and SDSL?
ADSL is common type of DSL line that offers download speeds many times higher than upload speeds. This is the most common type of DSL used in residential applications.
SDSL is usually more expensive type of DSL that offers upload speeds almost identical as download speeds. This makes it optimal for businesses and server applications (P2P) where upstream speed is important. SDSL may somewhat benefit gaming as well.

VDSL
Very-high-bit-rate digital subscriber line (VDSL)[1] and Very-high-bit-rate digital subscriber line 2 (VDSL2)[2] are digital subscriber line (DSL) technologies providing data transmission faster than asymmetric digital subscriber line (ADSL).
VDSL offers speeds of up to 52 Mbit/s downstream and 16 Mbit/s upstream,[3] over a single flat untwisted or twisted pair of copper wires using the frequency band from 25 kHz to 12 MHz.[4] These rates mean that 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 connection. VDSL is deployed over existing wiring used for analog telephone service and lower-speed DSL connections. This standard was approved by the International Telecommunication Union (ITU) in November 2001.
Second-generation systems (VDSL2; ITU-T G.993.2 approved in February 2006) [5] use frequencies of up to 30 MHz to provide data rates exceeding 100 Mbit/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 local loop attenuation increases.

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