A - D E F G H I - L M - P Q - T U - Z Glossary of Selected Terms (E-H)   Section E EBCDIC Extended binary coded decimal interchange code. An 8-bit computer code for representing alphabetic and numeric characters. ebusiness A term used to describe the conduct of business via the Internet. EDI (electronic data interchange) A term used to describe the exchange of standardised and formal business documents and transactions by electronic or data networking means. Electronic purchase orders, invoices and other financial transactions are typically carried out by means of EDI. EFS (error free seconds) A performance measure used to specify the quality of data lines. Higher specification lines require a higher proportion of error free seconds. email A shortening of the term electronic mail---the sending of letters, documents and files by means of a data network or the Internet. EMC (electromagnetic compatibility) EMC is the term applied to a series of specifications which define how telecommunications devices should be immune to electromagnetic disturbance. EMI (electromagnetic interference) Electromagnetic interference is the disturbance caused by a telecommunications device to other neighbouring devices. encryption The coding of data information prior to transmission in order to make the contents of the communication meaningless to third parties intent on overhearing or intercepting the message. ethernet LAN A local area network (LAN) conforming to the IEEE 802.3 standard. The commonest form of LAN used nowadays. There are different variants---10baseT, 10/100baseT, fast ethernet (100baseT) and Gigabit ethernet. extranet A remote site connected to an intranet (internal private company data network) by means of a secure connection across the public Internet. Section F fading The loss of radio signal caused by interference or weather attentuation effects. fault management One of the five main network management functions defined by the ISO management model. The others are accounting, security, performance and configuration management. FDDI (fibre distributed data interface) A type of metropolitan area network (MAN) capable of bitrates up to about 100 Mbit/s. FDDI is typically used to link LAN segments within a large office complex or on a campus site. female connector A plug connector or socket which comprises the `holes' for the `pins' of the connector rather than the `pins' which appear in the corresponding male connector. file transfer The carriage of a complete data file from one location to another---transferred from a location called in jargon the ftp server (file transfer protocol server) to the ftp client. firewall A computer server or similar device installed at the interconnection point between a private IP-network (intranet) and the public Internet and used to `police' the data crossing between the networks. The firewall prevents unwanted outsiders gaining access to the intranet, checks and filters incoming emails for unwanted content (e.g. viruses) and seeks to ensure that malicious attacks on website servers are not successful. flow control The technique used by a data communications protocol to ensure that devices receiving data across a data network are not inundated with a flood of information which they are unable to handle. frequency modulation (FM) A technique used to enable a low bit speed or low bandwidth information signal to be carried on a much higher frequency carrier signal---by adjustment of the latter's frequency by a small amount corresponding to the amplitude of the former. forwarding The process of transport of data packets across a data network. Routers inspect the address of each packet and then forward the packet along the best next hop towards the destination. FRAD (frame relay access device) A device which converts data from a data terminal equipment (DTE) for carriage by a frame relay network. frame The name given to a block or packet of data having received its layer 2 (datalink) protocol header. (If you like, a layer 2 `packet' of data.) frame relay A modern form of packet switching, capable of much higher bitrates than X.25-based packet-switched networks. framing The structuring framework required in digital line systems in order that a number of different users or channels may share the line by time division means. FSK (frequency shift keying) When a carrier signal is frequency modulated with a binary data signal, the effect is merely to alternate the carrier signal between two or more fixed values. This is known as frequency shift keying. Section G gateway Two alternative meanings. Most commonly nowadays, the term gateway is used to describe a device which provides for protocol conversion---particularly of higher layer (e.g. application layer) protocols. A mail gateway might, for example, provide conversion between Internet mail system messages and X.400-format messages. In the early days of the Internet, the term gateway was also used to describe the nodes of a data network. GOS (grade of service) The proportion of calls in a circuit switched network which are lost due to network congestion. Since it is uneconomic to provide networks of enormous uncongested proportion, it is usual to design networks to a given grade of service - typically 1 in 100. GPRS (general packet radio service) An adaptation of the GSM mobile telephone system to incorporate packet data transmission GSM (global system for mobilecommunication) One of the most common types of mobile telephone network technology, and one which allows full roaming of subscribers between networks around the world. Most common in Europe. GUI (graphical user interface) A software program which converts a graphical-style computer screen output to or input from a human computer user into the `hieroglyphic gibberish' that the computer understands. Thus a GUI program might convert a `click' on a `button' into a computer command like `atur'. Section H half duplex A mode of telecommunications transmission in which both directions of communication are possible, but in which only one party may `speak' at any given time. There is no possible to talk at the same time... even if only to `butt in'. Hamming code An error correction/detection code used in datacommunications. Hayes protocol A command language used between a computer and a modem in order that the computer can control the modem. Commands which may be issued include `dial', `dial number ***', `clear', etc. HDLC (high level data link control) A protocol conforming with the open systems interconnection (OSI) model, layer 2, allowing for the error-free conveyance of bits over a single link of a connection. HDSL (high bitrate digital subscriber line) A transmission technique allowing a high speed duplex signal of 2 Mbit/s to be transmitted to customer premises over existing copper lineplant. header The bits within a block or cell which provide for correct delivery of the payload. hexadecimal A 16-state numerical code (values 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F). Hexadecimal numbers (typically prefixed by `0x') are often used as a `shorthand' for binary numbers, into which they are easy to convert. Each digit of a hexadecimal number is simply replaced in turn by its four-digit binary equivalent. host In data communications vocabulary, a host is a computer (of any type---PC, server or mainframe) connected to a data network and running one or more applications which communicate across the data network. Often the host is synonomous with the DTE. The term DTE is more commonly used when describing physical layer and the electrical itnerface. At the software and higher protocol layers, the term host is more commonly used. In the computing world the term host is sometimes used synonomously with `mainframe computer'. html (hypertext markup language) A text `markup' language in which websites and web documents are written. http (hypertext transfer protocol) An application layer protocol used to create hyperlinks between documents and websites held on different physical servers within the Internet. hub A hardware device providing the physical interconnections and bus or ring topology necessary as the basis of a given type of local area network (e.g. ethernet, token ring, etc.) huffman code A code used in datacommunications for data compression. This has the benefit of reducing the overall number of bits which need to be carried across the network, so improving speed of transmission and minimising costs. hyperlink By means of a hyperlink documents and files stored on different physical servers connected a data network such as the Internet can be related to one another. By means of a web browser to interpret the hyperlinks, the separate documents (e.g. different text blocks and pictures) appear on the human user's computer screen to be a single document. By clicking on further hyperlinked text (typically underlined) a fast connection to a related document can be achieved.