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A collection of technical terms and abbreviations.

Technical terms

  • Attenuation - The amount by which the signal level is reduced over its path from one point to another. In the context of broadband, this means the reduction in signal level over the length of the telephone line.
  • Backhaul - The intermediate link between a subnetwork and the core network of a system. In the context of ADSL, the backhaul is the pipe between the exchange and the ATM cloud. See How ADSL works for more explanation.
  • bRAS profile - A BT term for a download speed limit which is applied to a DSL connection, based on the connection (sync) speed. The bRAS profile includes some overheads, and after these are deducted, what's left is the IP profile which is the practical speed limit. See IP/bRAS profile.
  • Downstream - Refers to the movement of data towards the user from a remote location.
  • Ethernet - A family of computer networking technologies for wired LANs. The standards include several different types of cable and connector, but the dominant style in the broadband context is the RJ45 connector with twisted-pair cable (e.g. Cat5e).
  • Fastpath - The normal configuration of an ADSL connection which has not had interleaving applied. Compared to interleaving, fastpath delivers lower latency but a higher error rate in noisy conditions.
  • Firewall - A software or hardware device which is intended to protect a system against intrusion. A firewall can be a separate piece of equipment positioned between a LAN and the outside world; it can be included in another device such as a router; or it can consist of software running on a computer.
  • Firmware - The 'operating system' of a hardware device, stored in read-only memory or in non volatile read/write memory. In the latter case, the firmware can be upgraded to fix bugs or to change the capabilities of the device. With many types of device, the firmware is actually a computer operating system, using an embedded form of Linux.
  • Homeplug - A networking device which makes use of domestic electrical wiring to carry ethernet signals. Two or more devices plugged into mains sockets provide a link between different parts of the building without the need for extra cabling. 'Homeplug' is a trademark of the Homeplug Power Alliance.
  • Hub - A networking device which links together a number of computers and associated devices. Normally incorporates several ethernet ports. See also Switch.
  • Interleaving - A technique for improving the quality of a connection which is subject to interference. The data stream is separated into small packets and rearranged together with error correction procedures. This reduces the error rate but increases the latency ('ping') of the connection. See Interleaving explained. See also Fastpath.
  • IP profile - A BT term for a download speed limit which is applied to a DSL connection, based on the connection (sync) speed. The IP profile is derived from the bRAS profile with an allowance for overheads. See IP/bRAS profile.
  • Local loop - The connection between the telephone exchange and the customer's premises.
  • MAC address - See MAC(2)
  • Microfilter - Also known as 'ADSL filter', 'DSL filter' or 'Splitter'. A device which plugs into a telephone socket and separates the voice signals from the DSL signals. The voice side has a low-pass filter which blocks frequencies over about 5 kHz, and the DSL side is unfiltered.
  • Modem - Modulator/demodulator. A device which enables digital data to be carried over an analogue link. Outgoing digital data is modulated onto an analogue signal and transmitted along a telephone line (typically); incoming analogue signals from the telephone line are demodulated in order to reconstruct their digital content. For a link to work in this way, there must be compatible modems, or equivalent functionality, at both ends of the link.
  • Modem/router - A device which combines a DSL modem and a router in a single package. Most modem/routers also include a network switch and a firewall, to make them into an all-in-one solution for internet access.
  • Noise margin - The amount by which the SNR exceeds a specified base level of SNR. In a DSL connection, the base level of SNR is the level below which a stable connection is not likely to be possible.
  • Powerline adaptor - See Homeplug.
  • Ringwire - One conductor in the cable which connects the master socket to extension sockets. The ringwire carries the telephone ringer signal, which is generated in the master socket. For the considerable majority of telephones the ringwire serves no purpose because the telephones generate their own ringer signal electronically, and it can often improve ADSL performance by disconnecting the ringwire.
  • Router - A device which passes packets of data between networks. A router steers data to the correct destinations as specified in the packet headers. The term 'router' is often used loosely to refer to a modem/router.
  • SNR margin - See Noise margin.
  • Switch - A networking device which performs the same function as a hub, but with some extra intelligence. It remembers the IP address of each connected device, and which port it's connected to, so it doesn't have to repeatedly poll the network.
  • Target noise margin - Also known as Target SNR Margin. The initial value of noise margin which is applied at the time when a DSL connection is being negotiated. If a connection is subject to interference which makes the connection unstable, the target noise margin can be increased to give more noise margin 'headroom', to make the connection more stable. This will have the effect of reducing the connection speed. See also DLM.
  • Test socket - In an NTE5A master socket the faceplate can be removed to expose an internal socket into which you can plug a modem/router and/or telephone. As the extension sockets are wired into the removeable faceplate, this action disconnects all of the extensions, as an aid to diagnosis of problems. See Trouble shooting.
  • Upstream - Refers to the movement of data away from the user towards a remote location.


  • ADSL - Asymmetric Digital Subscriber Line. A protocol for broadband connections over telephone lines, in which greater provision is made for downstream data transfer than upstream.
  • ADSL2, ADSL2+ - Extended versions of ADSL. See Wikipedia article.
  • ATM - Asynchronous Transfer Mode. A switching protocol which is designed to unify telecommunication and computer networks. Data is encoded into small fixed-sized cells, and combined using time division multiplexing.
  • dB - Decibel. A measure of gain or loss in a system. See Decibels in the context of ADSL
  • CRC - Cyclic Redundancy Check. A technique for detecting errors in data transmission. When the receiving equipment detects a CRC error in a packet of data, it requests a retransmission of that packet. For more information see Error correction article.
  • DACS - Digital Access Carrier System. A system for carrying two telephone signals over a single cable pair, now deprecated. See DACS article.
  • DLM - Dynamic Line Management. A process which continually monitors the performance of a DSL connection, and adjusts the interleaving status and target noise margin as appropriate in order to try to maintain the best possible stable connection. Originally used by BT Wholesale, a form of DLM is also used by some LLU operators. See How DSL Max Works.
  • DMT - Discrete Multi Tone. A method of converting digital data into tones or frequencies that can be carried over telephone wire. DMT in ADSL Technology article
  • DSL - Digital Subscriber Line. A generic name for a group of technologies which enable the transmission of digital data over a telephone line.
  • DSLAM - Digital Subscriber Line Access Multiplexer. A network device, usually housed in a telephone exchange, which connects multiple user DSL interfaces into the high speed ATM backbone. See How ADSL works.
  • FEC - Forward Error Correction. A technique for reducing errors in data transmission over noisy channels. Commonly used in conjunction with interleaving. FEC errors are errors which have been autocorrected by the use of FEC, and therefore do not require retransmission of the data. For more information see Error correction article.
  • FTTC - Fibre To The Cabinet. A hybrid arrangement for telephone lines; fibre-optic cable links the exchange to street cabinets, and copper cable links the cabinets to user premises. Used for BT Infinity and similar offerings from other ISPs.
  • FTTP - Fibre To The Premises. Fibre-optic cable links the exchange directly to user premises.
  • HEC - Header Error Control. A technique for detecting and partially correcting errors in data transmission. For more information see Error correction article.
  • ISDN - Integrated Services Digital Network. A set of standards for simultaneous digital transmission of voice, video, data and other services over a telephone line. Used in some countries for internet access, but largely superseded by DSL.
  • LAN - Local Area Network. A network of computers and associated devices which is entirely within a limited area, such as a house, school or office building.
  • LLU - Local Loop Unbundling. An ISP installs its own equipment in a telephone exchange to process ADSL traffic. See MPF and SMPF.
  • MAC(1) - Migration Access Code. A code number which enables a customer to migrate from one ISP to another in a seamless fashion. The losing ISP gives the code to the customer, who then gives it to the new ISP. After that, the new ISP manages the changeover.
  • MAC(2) - Media Access Control. A MAC address is a globally unique reference number for a network device. The number is most commonly allocated by the device manufacturer and stored in a read-only memory in the device or in its firmware.
  • MPF - Metallic Path Facility. A form of LLU in which the ISP takes over responsibility for voice operations as well as ADSL. Also known as "Full unbundling". See also SMPF.
  • MSAN - Multi-Service Access Node. A network device, usually housed in a telephone exchange, which connects users' telephone lines to the core network. Combines several different types of service, such as telephone, ISDN and DSL, into a single port.
  • NAS - Network Attached Storage. An external hard disk storage device which connects to a LAN, providing shared storage facilities for computers and other devices on the LAN.
  • POTS - Plain Old Telephone Service. The basic analogue voice telephone service.
  • SDSL - Symmetric Digital Subscriber Line. A protocol for broadband connections over telephone lines, in which equal provision is made for downstream and upstream data transfer.
  • SMPF - Shared Metallic Path Facility. A form of LLU in which the ISP takes over the handling of ADSL traffic, but BT retains responsibility for voice traffic. See also MPF.
  • SNR - Signal to Noise Ratio. The signal level divided by the noise (interference) level. Usually expressed in dB.
  • VDSL - Very-high-bit-rate Digital Subscriber Line. A protocol for broadband connections over telephone lines, coaxial or fibre-optic cable.
  • VDSL2 - Very-high-speed Digital Subscriber Line 2. An enhanced version of VDSL, currently being employed on BT FTTC lines.
  • WAN - Wide Area Network. A telecommunications network which covers a broad area and links together several localities.
  • Wi-Fi or WiFi - A means of linking together electronic devices by wireless. Can be used within a LAN or to connect to a WAN by means of a hotspot. "Wi-Fi" is a trade mark of the Wi-Fi Alliance.
  • WLAN - Wireless Local Area Network. A LAN in which the devices are attached by wireless connections (normally Wi-Fi).