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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.
  • 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 lower latency but a higher error rate in noisy conditions.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
  • 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.
  • 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.
  • 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".
  • 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.
  • 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).