Asymmetric digital subscriber line
Asymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ADSL differs from the less common symmetric digital subscriber line. In ADSL, bandwidth and bit rate are said to be asymmetric, meaning greater toward the customer premises than the reverse. Providers market ADSL as a service for consumers for Internet access for downloading content from the Internet, but not serving content accessed by others. ADSL works by using the frequency spectrum above the band used by voice telephone calls. With a DSL filter 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 only installed for short distances from the telephone exchange less than 4 kilometres, but has been known to exceed 8 kilometres if the laid wire gauge allows for further distribution.
At the telephone exchange, the line terminates at a digital subscriber line access multiplexer where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL are routed over the telephone company's data network and 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 to be more crosstalk from other circuits at the DSLAM end 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, 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, 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, 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. Most ADSL communication is full-duplex. Full-duplex ADSL communication is achieved on a wire pair by either frequency-division duplex, echo-cancelling duplex, or time-division duplex.
FDD uses two separate frequency bands, referred to as the downstream bands. The upstream band is used for communication from the end user to the telephone central office; the downstream band is used for communicating from the central office to the end user. With deployed ADSL over POTS, the band from 26.075 kHz to 137.825 kHz is used for upstream communication, while 138–1104 kHz is used for downstream communication. Under the usual DMT scheme, each of these is further divided into smaller frequency channels of 4.3125 kHz. These frequency channels are sometimes termed bins. During initial training to optimize transmission quality and speed, the ADSL modem tests each of the bins to determine the signal-to-noise ratio at each bin's frequency. Distance from the telephone exchange, cable characteristics, interference from AM radio stations, local interference and electrical noise at the modem's location can adversely affect the signal-to-noise ratio at particular frequencies. Bins for frequencies exhibiting a reduced signal-to-noise ratio will be used at a lower throughput rate or not at all.
The DSL modem will make a plan on how to exploit each of the bins, sometimes termed "bits per bin" allocation. Those bins that have a good signal-to-noise ratio will be chosen to transmit signals chosen from a greater number of possible encoded values in each main clock cycle; the number of possibilities must not be so large that the receiver might incorrectly decode which one was intended in the presence of noise. Noisy bins may only be required to carry as few as two bits, a choice from only one of four possible patterns, or only one bit per bin in the case of ADSL2+, noisy bins are not used at all. If the pattern of noise versus frequencies heard in the bins changes, the DSL modem can alter the bits-per-bin allocations, in a process called "bitswap", where bins that have become more noisy are only required to carry fewer bits and other channels will be chosen to be given a higher burden; the data transfer capacity the DSL modem therefore reports is determined by the total of the bits-per-bin allocations of all the bins combined.
Higher signal-to-noise ratios and more bins being in use gives a higher total link capacity, while lower s