• 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 (Asymmetric Digital Subscriber Line) is a technology for transmitting digital information at a high bandwidth on existing phone lines to homes and businesses.
• Unlike regular dialup phone service, ADSL provides continuously-available, "always on" connection.
• 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 downstream than upstream.
• ADSL is generally offered at downstream data rates from 512 Kbps to about 6 Mbps.
• 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.
• ADSL works by using the frequency spectrum above the band used by voice telephone calls.
• 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, typically less than 4 kilometers.
• At the telephone exchange, the line generally terminates at a digital subscriber line access multiplexer (DSLAM) where another frequency splitter separates the
a frequency band that might interact 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.
• ADSL communication is full-duplex. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD), or time-division duplex (TDD).
• ADSL initially existed in two versions (similar to VDSL), namely CAP and DMT.
• ADSL deployment on an existing plain old telephone service (POTS) telephone line presents some problems because the DSL is within unfavorably with existing equipment connected to the line. It is therefore necessary to install appropriate frequency filters at the customer's premises to avoid interference between the DSL, voice services, and any other connections to the line (for example intruder alarms). This is desirable for the voice service and essential for a reliable ADSL connection.
• ADSL defines three "Transmission protocol-specific transmission convergence (TPS-TC)" layers:
Synchronous Transport Module (STM), which allows the transmission of frames of the Synchronous Digital Hierarchy (SDH)
Asynchronous Transfer Mode (ATM)
Packet Transfer Mode
• ADSL simultaneously accommodates analog (voice) information on the same line.
• A form of ADSL, known as Universal ADSL or G.lite, has been approved as a standard by the ITU-TS.
• ADSL was specifically designed to exploit the one-way nature of most multimedia communication in which large amounts of information flow toward the user and only a small amount of interactive control information is returned.
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