1 Bibliography The works listed here are those which the authors have found most valuable in the study of data communications and its application. A particular problem associated with the literature in this field is to keep acquainted with the rapidly increasing range of hardware available for data communications. In this respect, the reader is advised to consult the manufacturer's own publications, the trade press (it is worth noting that the journals Datamation, Computer Decisions and Data Processing frequently present comparisons of available equipment) and the various industry equipment guides (for example, Auerbach, Datapro, etc.). Abrahamson, M., and Kuo, F. F. (eds), Computer-Communications Networks (Prentice-Hall, Englewood Cliffs, N.J., 1973). Barber, D. L. A. (ed), 'Communications Networks', Proceedings of the European Computing Conference on Communications Networks (Online Conferences, Uxbridge, 1975). Barber, D. L. A., and Davies, D. W., Communications Networks for Computers (Wiley, Chichester, 1973). Becker, H., Functional Analysis of Information Networks: A Structured Approach to the Data Communications Environment (Wiley, Chichester, 1974). Blackman, M., 'Standards for the Design of Real Time Systems', Comput. Bull., Series 2, 7 (1976) p. 30. Blackman, M., The Design of Real Time Applications (Wiley, Chichester, 1975). Fayen, E. G., and Lancaster, F. W., Information Retrieval On-Line (Melville, Los Angeles, 1973). Fox, J. (ed), Communications, Networks and Teletraffic (Wiley, Chichester, 1972). Gentle, E. C. Jr (ed), Data Communications in Business - An Introduction (American Telephone and Telegraph Co., New York, 1965). Gibbons, T. K., Integrity and Recovery in Computer Systems (National Computing Centre, Manchester, 1976). Hebditch, D. L., Data Communications - An Introductory Guide (Elek Scientific, London, 1975). Johnson, T., 'Electronic Post for Switching Data', New Scient., 70 (1976) p. 351.
2 188 PLANNING FOR DATA COMMUNICATIONS Latini, F., Tempo Reale e Calcolatore Elettronico (Etas Kompass, Milan, 1969). Laver, M., Computers, Communications and Society (Oxford University Press, 1975). Lefkovitz, D., File Structures for On-Line Systems (Hayden, New Jersey, 1967). Lefkovitz, D., Data Management for On-Line Systems (Hayden, New Jersey, 1974). Martin, J., Programming Real Time Computer Systems (Prentice-Hall, Englewood Cliffs, N.J., 1965). Martin, J., Design of Real Time Computer Systems (Prentice-Hall, Englewood Cliffs, N.J., 1967). Martin, J., Telecommunications and the Computer (Prentice-Hall, Englewood Cliffs, N.J., 1969). Martin, J., Teleprocessing Network Organisation (Prentice-Hall, Englewood Cliffs, N.J., 1970). Martin, J., Introduction to Teleprocessing (Prentice-Hall, Englewood Cliffs, N.J., 1972). Martin, J., Systems Analysis for Data Transmission (Prentice-Hall, Englewood Cliffs, N.J., 1972). Martin, J., Design of Man-Computer Dialogues (Prentice-Hall, Englewood Cliffs, N.J., 1973). Martin, T. H., A Feature Analysis of Interactive Retrieval Systems (Stanford University Press, 1974). Nielsen, N. R., The Implications of Star Computing Networks, Information Processing 74 (North Holland, Amsterdam, 1974). Oguchi, B., 'Computers, Telecommunications and Information Networks', Comput. Networks, 1 (1976) p. 2. Pritchard, J. A. T., An Introduction to On-Line Systems (National Computing Centre, Manchester, 1973). Pritchard, J. A. T., Selection and Use of Terminals in On-Line Systems (National Computing Centre, Manchester, 1974). Rubin, M. L., Handbook of Data Processing Management, vol. 5, Advanced Technical Concepts (Petrocelli, New York, 1975). Steinacker, 1., Dokumentationssystems Dialogfunktion und Systementwurf (De Gruyter, Berlin, 1975). Tarrant, J. J. (ed), Data Communications and Business Strategies (Petrocelli, New York, 1975). Unger, C. (ed), Command Languages: Proceedings of the IFIP Working Conference on Command Languages, Sweden 1974 (North Holland, Amsterdam, 1975). Wessel, A. E., Computer Aided Information Retrieval (Melville, Los Angeles, 1975). Wilkes, M. V., Timesharing Computer Systems (Macdonald, London, 1972). Yourdin, E., Design of On-Line Computer Systems (Prentice-Hall, Englewood Cliffs, N.J., 1972). First European Congress on Documentation Systems and Networks (Commission of the European Communities, London, 1973). Second European Congress on Information Systems and Networks (Commission of the European Communities, London, 1975).
3 BIBLIOGRAPHY 189 'Distributed Intelligence in Data Communications', EDP Analyser, 11 (1973). 'Structures for Future Systems', EDP Analyser, 12 (1974). 'Progress towards International Data Networks', EDP Analyser, 13 (1975). 'Soon: Public Packet Switched Networks',EDP Analyser, 13 (1975). Handbook of Data Communications (National Computing Centre, Manchester, 1975). Computers and Telecommunications (Organisation for Economic Cooperation and Development Information Studies, H.M.S.O., 1973). Computer Networks (Science Research Council, London, 1974).
4 Glossary of Terms Authors' Note This section aims not only at defining the meaning of terms used in data communications, but also, wherever possible, at indicating their relative importance and their applicability. Many of the descriptions therefore go beyond normal definitions and attempt to explain data communications practice. Access Method The set of conditions under which user programs make use of the telecommunications facilities provided by the operating system and the hardware. The access method is implemented in the form of a software interface between the user programs and the operating system. This software has such functions as user identification, line control, etc. Acoustic Coupler A device that allows data to be transmitted via a terminal over the public switched telephone network, using a normal voice telephone handset. Alternative Routing A network facility that allows data to have at least two routes to reach their destination. Although the shortest route would normally be allocated at transmission time, alternative routing allows such problems as overloading or breakdown in parts of the network to be circumvented.
5 Amplitude Modulation GLOSSARY OF TERMS 191 A means of transmitting information by varying the amplitude of the carrier wave. When an additional signal (carrying the information to be communicated) is applied to the basic sine wave, the amplitude of the carrier wave changes accordingly. Analogue Transmission The means of transmission where the signal is continuously variable. This is the usual method of transmission today using voice telephone lines. but digital transmission techniques (such as pulse code modulation) are increasingly being introduced, whereby the signal is varied by discrete, and not continuously variable, amounts. Answer-back An action by a terminal to indicate to a sending device that it is ready to accept or acknowledge receipt of the data. ARPANET (ARPA Network) The packet-switching network established by the Advanced Research Projects Agency of the United States Department of Defense. ASCII Abbreviation for American Standard Code for Information Interchange, a transmission code of seven bits for data representation. plus one bit for parity checking. Asynchronous Term used to describe the transmission method, or terminal, whereby each character, or sometimes a small block of characters, is individually synchronised. The interval between each character or block may vary. The synchronisation is achieved by the use of start and stop elements and therefore this method is often also called 'start-stop' transmission. Attenuation Loss of electrical power during transmission between two points, due to the natural electrical resistance of the transmission equipment. Automatic Answering Direct connection to a computer system, which signals standard tones to a user when a satisfactory connection has been made, so that data transmission can commence.
6 192 PLANNING FOR DATA COMMUNICATIONS Automatic (Alling The process of dialling without manual intervention. Terminals may, for example, be dialled directly by the computer system. Also known as 'automatic dialling'. Bandwidth The range of frequencies available for data transmission within a given channel. In general, the greater the bandwidth the higher is the possible rate of transmission. Baseband Transmission Transmission of a signal in unmodulated form, that is, at its original frequencies. Batch Terminals Those types of terminals, the normal mode of which is to transmit large blocks of data accumulated previously either at the terminal (see 'intelligent terminals') or off line. Typically the data are accumulated on magnetic tape, punched paper tape or punched cards. Batch Transmission Data transmission of large volumes of data in a sequence of blocks. This type of transmission contrasts with interactive working and with remote metering. Baud Unit of signalling speed. It is measured in terms of discrete signals per second. When these signals correspond to bits, as is often the case, bauds are then equivalent to bits per second. Baudot Code The transmission code named after Baudot. The code is based on five bits per character. BCD (Binary Coded Decimal) The representation of digits, letters and special characters, using a six-bit code, plus a parity bit. Beads A name for the program modules used for application processing in a data communications system.
7 GLOSSARY OF TERMS Binary Synchronous Communications (BSC) 193 Data communication depending on synchronisation between the transmitting receiving devices, achieved by means of the sending of a defined set of control characters at predefined intervals. Bit Abbreviation for 'binary digit', but widely used and accepted as a word on its own to express a single element of transmission, either a '0' (zero) or a '1' according to binary notation. Bits per Second The commonly used measure for data transfer rate. Often expressed as bps, b.p.s. or BPS. Block A fixed or variable number of characters transmitted as a single unit and usually controlled (for errors) as a unit. Block Character Check An error-control procedure applied to a transmitted block of characters, often using a special character or group of characters added to the block expressly for the purpose. Broadband Communications channel having a bandwidth greater than a voice-grade channel. The data transmission rates are normally much higher, using broadband channels, compared with voice channels. Also known as 'wideband'. Buffer A device used for temporary storage of data, usually primarily to compensate for differences in data flow rates (for example, between a terminal and its transmission line) but also as a security measure to allow re-transmission of data if an error during transmission is detected. Busy Hour The sixty consecutive minutes during a day when the data traffic flow is at a maximum. In interactive systems this commonly occurs at around hours, with a second peak around hours. The pattern for batch transmission, on the other hand, may vary considerably from system to system, depending on the schedule set up for the purpose.
8 194 Byte PLANNING FOR DATA COMMUNICATIONS A sequence of binary digits representing a given set of numbers and characters according to a defined code. A commonly used byte code is based on eight bits per byte, plus a parity check bit. Carrier Wave The electromagnetic wave which is modulated by a signal containing the information to be transmitted, thereby producing a combined modulated wave. The carrier wave is normally a simple, single, high-frequency wave having a sinusoidal form, whereas the modulating signal containing the information is complex in form. Channel In strict telecommunications terminology, a channel is the physical facility for allowing one-way transmission. This may be a line or a radio link. To allow twoway transmission, therefore, two channels are required (making a 'circuit' - see below). However, in common usage, channel is loosely used to describe any link allowing data transmission, such as a telephone line, a circuit, etc. It should be noted also that the telecommunications use of the word 'channel' differs from the meaning ascribed to it in data processing, where it describes the device attached to the central processing unit handling the movement of data in and out of the machine. Check Bit A bit transmitted with an item of data in order to provide control information (for example, whether the number of bits is even or odd) on the other bits containing the data. If an error occurs, the check bit may not match the new bit pattern and an error is therefore signalled. Circuit The electrical conductors providing a complete path for two-way communication. Circuit Switching The method of establishing a route for communication whereby a complete link between the calling and receiving stations is set up and maintained exclusively for the exchange of information between those two stations, until one of them breaks off transmission or receiving. The circuit is dedicated to the particular link established, independently of the actual use of that link while the connection is maintained. Common Carrier An organisation that has received authority from a high-level regulatory body to
9 GLOSSARY OF TERMS provide public communications services. The term is widely used in the United States, much less so in Europe and elsewhere. Examples of common carriers in the United States are The Bell System and Western Union. 195 Concentrator A communications device that allows data transmitted over a number of lower speed lines to converge and be immediately re-transmitted over a single higher speed line. This is similar in function to a multiplexer (see below) but a concentrator has the additional capability (achieved by being programmable in its own small computer memory) of handling a variety of line speeds and protocols. Furthermore, it may be able to undertake certain communications functions of the network, such as polling terminals, etc. As in the case of a multiplexer, strictly speaking the reverse process of separating a combined signal on a high speed line into separate ones on lower speed lines is called 'de-concentrating', but in practice the term 'concentrator' is used for both processes. Conditioning The improvement of ordinary telephone lines for data transmission, by means of special equipment to suppress echoes, achieve equalisation, etc. Connect Time The elapsed period from the instant a terminal user makes connection with a remote computer to breaking off the connection on completion of his task. During this period the user may well not have been transmitting or receiving data all the time, but the connection will have been kept open in readiness and will have been monitored constantly by the remote computer. Contention A procedure of line control whereby two or more terminals may request simultaneously to transmit data over the same line(s). The computer maintains a queue of requests and allocates the line(s) as availability allows. Control Character A special character (in a given context) that is used to initiate or stop a particular control function, such as 'carriage return'. The control characters associated with a message (q.v.) are often referred to as an 'envelope'. Conversational Mode A method of working at a terminal whereby the user and the computer system proceed step by step, each waiting for response from the other after each has taken action. Also known as 'interactive working', 'interactive mode' or 'interactive computing'.
10 196 CRT PLANNING FOR DATA COMMUNICATIONS Abbreviation for cathode ray tube and used to describe a display screen terminal. Commonly used in the United States, less so in Europe where the term 'VDU' (Visual Display Unit) is more often used. Cyclic Redundancy Check A specific method of detecting transmission errors whereby a check character is used to control the other characters carrying the information, on the basis of an arithmetic calculation utilising the bits in the information characters. Data Circuit The electrical means for providing two-way transmission between two points for the purpose of data transmission. Data Compression (Data Compaction) The process of encoding information in order to reduce its over-all volume. Various techniques can be used, but substantial savings can normally be achieved in terms of storage requirements and transmission times. Data Set In data communications, this is mostly synonymous with a modem, that is, a device which allows a digital terminal device to transmit and receive over an analogue communication channel. However, the term is often used also to describe a limited file of data, especially when referring to data processing. Delay Distortion Distortion caused by the fact that some frequencies travel more slowly than others over the same transmission path. Some components of a signal therefore arrive later than others. Demodulation The process of recovering from a modulated wave the original signal with which the wave was modulated on transmission. This process of demodulation takes place at the interface between the terminal or computer and the communications line. Dial-up A means of obtaining a communications link to a remote station by-dialling a unique series of numbers (or using push buttons), which then set up a circuit connection by means of the public switched network for telephone or telex.
11 Direct Distance Diol/ing (DDD) GLOSSARY OF TERMS 197 A telephone facility which allows the user to dial remote telephones, terminals or computers located outside the user's local area, without the aid of an operator. Display Unit Any terminal device that provides visual representation of the data, normally on a television-like screen. Distortion The difference between the shape of the original, transmitted wave or pulse and the received shape, after the signal has traversed the transmission circuit. Distributed Processing An approach to information handling whereby processing and storage tend to be placed at or near the points where transactions occur, but where communications facilities allow links to other data bases and other processing facilities as the need arises. A well-developed distributed processing system has a grid-like network with several small processing machines located at different points in the grid, with a minimum of hierarchical control in the network. Duplex Term describing a communications channel over which independent transmission can take place in each direction simultaneously. Also called 'full duplex', EBCDIC Abbreviation for Extended Binary Coded Decimal Interchange Code, an eight-bit code. Based on BCD (see above), it provides a wider range of numbers, letters and special characters. Echo A reflection of the signal at some electrical discontinuity in the circuit, or at the end-station, returned to the transmitting station and causing some interference to the primary signal. Echo-check A means of error control whereby the transmitted information on receipt at the destination is re-transmitted back to source for comparison with the original.
12 198 Equalisation PLANNING FOR DATA COMMUNICATIONS The process of reducing distortion by compensation for attenuation and time delays at various frequencies in the transmission band. Error Control The general term used to describe various arrangements to detect the presence of errors introduced into the original data by the transmission process. The simplest approaches aim at only detecting such errors, but more sophisticated systems try to correct the detected errors as well, either by requesting re-transmission from source or by carrying out operations on the erroneous data. Facsimile (FAX) The transmission of graphic information such as diagrams, maps, pictures, etc. The image is scanned and encoded at the transmitting station, transmitted in electrical wave or pulse form over the communications line(s), reconstructed and duplicated on paper at the receiving station. Fall-back Procedure An alternative set of operations to be followed in the event of a system error. Such procedures may involve switching to alternative equipment, by-passing certain non-essential operations, use of manual support, etc., and normally cause a reduced level of service until the fault has been corrected. Fail-safe The manner of coping with faults in the system in such a way that a minimum level of service is maintained, that is, not allowing the whole system to fail catastrophically. Such an approach requires an extensive set of fall-back procedures. (Compare 'graceful degradation'.) Frame A group of characters transmitted as a unit. This term is being increasingly used in the context of 'packets' in packet-switching technology. Frequency-division Multiplexing (FDM) A communications technique that uses special equipment (a multiplexer) to transmit a number of signals over a single path by dividing the available bandwidth into narrower bands, each of which is used for a single, separate channel. Although still widely used, this type of multiplexing is tending to be replaced by 'timedivision' multiplexing (see below).
13 Frequency Modulation GLOSSARY OF TERMS 199 A transmission process whereby the original frequency of a carrier wave is varied in step with the amplitude of the signal containing the information. This means of transmission minimises the interference from static and other noise sources. Front-end Processor (FEP) A computer dedicated to communications functions and acting as an interface between a data processing computer and network facilities. It is normally situated physically close to the data processing computer and carries out such functions as line control, message queueing, code conversion, etc. Full Duplex (Same as 'Duplex'). Graceful Degradation The process of gradual breakdown of a system in such a way that sudden catastrophic failure does not occur, but rather a steady deterioration in service. This has the advantage of allowing users to complete jobs and priority activities to be maintained while corrective action is initiated. Special design features and backup facilities have to be built into the system to allow graceful degradation. Half Duplex A communications channel over which transmission can take place in either direction, but not simultaneously. HDLC Abbreviation for 'high-level data link control' procedure, a proposed international standard for frame structures in connection with network protocols. Header The information contained at the start of a message, providing details on destination, priority, etc. Hertz (Hz) The international standard measurement of frequency, expressed in cycles per second.
14 200 PLANNING FOR DATA COMMUNICATIONS Host Computer (Host Processor) A computer which provides services to remote users via network facilities. A terminal user may wish to interrogate several data bases, each of which is stored on a different 'host computer' located at different geographical points in the network. Intelligent Terminals The term used to describe terminals that have sufficient fast access memory for them to be programmable. Such terminals therefore have software capability that can greatly enhance their flexibility and performance, by taking over local control functions such as error control, editing, data compression, etc. Interactive (See 'Conversational Mode'). Interface The point of interconnection between two systems, sub-systems or equipment. The primary purpose of an interface is therefore to provide a bridge between the two parts to be connected in such a way that any incompatibilities inherent in the two parts are resolved by the interface. The means appropriate for anyone interconnection may be hardware, software or both. Typical interfaces in data communications systems are : terminal/network, host computer/network, front-end processor/host computer, etc. Interference General term used to describe the presence of non-random extraneous signals in a communications channel, thereby causing reduced clarity of the desired signal. Interrupt An event that causes the running of a computer program to be temporarily suspended while a higher priority task is executed. This occurs frequently in communications software systems to allow external events to register as quickly as possible. The original program is then resumed after the necessary processing, caused by the interrupt, is completed. Keyboard Send-Receive A terminal of the teletypewriter kind which has a keyboard for input and printer for receiving output. Leased Line A communications line which is reserved exclusively by the telecommunications
15 GLOSSARY OF TERMS 201 authority for the use of a particular customer. Such a line would be set up by the telecommunications authority to connect two locations nominated by the customer, thereby not requiring the customer's data to be allocated to and switched via different circuits on each transmission. Line Any circuit used to transfer data from one point to another. See 'channel' for further explanation. Line Interrupt Trace (LIT) A monitoring method to help in the detection of faults, whereby all interrupts to and from terminals and other equipment are logged continuously over a specified period of time. Line Speed Rate of transfer of information over a given channel. The normal unit of measurement is either 'bits per second' or 'bauds' (these are not always equivalent - see 'baud'). Link A continuous data communications path between two places. The medium used may be telephone, telex or telegraph line, or a microwave connection. The term is commonly interchanged with 'channel', 'circuit' or 'line', which are, however, more specific than the general term 'link' (see also 'channel', 'circuit' and 'line'). Log-in Expression used to describe the procedure whereby a terminal user makes the connection to a remote system, involving an identification procedure, authorisation check, etc. Also called 'log-on'. Loop Checking A method of error control based on comparison of the original data with the transmitted data, which is re-transmitted back to source for the purpose of checking. Similar to 'echo-check'. LRC Abbreviation for 'Longitudinal Redundancy Check'. This method of error control is applied to a block of characters by creating a special control character reflecting each bit position within successive characters of the block. Hence the last special character represents a set of parity checks for each longitudinal bit formation.
16 202 Message PLANNING FOR DATA COMMUNICATIONS A unit of data for transmission, containing a meaningful set of information (usually comprising a series of records, blocks or packets) together with control data, such as routing, error checking, etc., information, to allow the message to be transported and processed as an entity in itself. Message Switching A technique of transmitting messages through a network, whereby each message is temporarily stored at each switching facility until an outgoing circuit becomes available, when the message is forwarded to its destination or to the next switching facility. Also called 'store-and-forward' switching. Microwave Term describing the very short wavelengths (30 cm or less) of electromagnetic waves used for data (or voice) transmission. Such waves are of very high frequencies, of 1000 MHz and greater. Modem Acronym for 'modulator-demodulator', a device providing an interface between data processing equipment (such as a terminal, concentrator or computer) and analogue communications channels, applying the information signal to the carrier wave (modulation) and recovering the original information from the modulated carrier wave (demodulation). Modem Sharing Unit (See 'Multiple Access Coupler') Modulation The process of imposing a signal containing the information to be transmitted on to a carrier wave, the amplitude, frequency or phase of which therefore becomes 'modulated' in accordance with the variations in the information being transmitted. MTBF Abbreviation for 'mean time between failure', a measure of the reliability of a system, corresponding to the average period of time during which the system is able to maintain full operational status without breakdown. MTTR Abbreviation for 'mean time to repair', which indicates the average period necessary to bring a system to full operational status again after a breakdown has occurred.
17 Multi-drop GLOSSARY OF TERMS 203 The connection of two or more terminals to a single communications line. This is a means of sharing the capacity of a line among several terminals, which individually would make only low utilisation of the line or would use a lower capacity line. Also called 'multi-point'. Multi-point (See 'Multi-drop') Multiple Access Coupler A device used to connect a number of terminals to the same modem. The number of terminals and their physical separation are restricted. Also called 'modern sharing unit'. MUltiplexer Communications equipment which enables the signals from a number of individual circuits to be combined and transmitted over a common transmission path. This is normally achieved by either of two techniques: frequency-division multiplexing or time-division multiplexing. Strictly, the reverse process of separating out the combined signals for transmission over separate channels is called 'demultiplexing', but the term 'multiplexer' is commonly used to cover both processes. (See also 'frequency-division multiplexing' and 'time-division multiplexing'.) Also spelled 'multiplexor'. Multithread The name used for data communications software that can process two or more messages concurrently. (See also 'Thread') Network A geographically separated set of points (computers, terminals, concentrators, multiplexers, switching nodes) which are interconnected by communications channels, so that there is at least one continuous path between any two points. Network Topology The geometric pattern and structure of a network, as represented by the layout of its equipment (computers, terminals, concentrators, multiplexers, switching nodes) and its communications links. Network topologies are usually described in terms likening them to other geometric forms, for example, 'star-shaped', 'tree-structure', etc.
18 204 Node PLANNING FOR DATA COMMUNICATIONS A junction point in a network from which two or more communications links radiate. If the point has three or more communications links connected and also acts as a switching centre, then it is commonly referred to as a 'switching node'. Noise Unwanted, random signals in a communications system, tending to reduce the clarity of the originating signal and sometimes causing errors. If the unwanted signals are not random in nature, then they are called 'interference'. Non-intelligent Terminals Terminals which are not programmable and which during operation therefore depend entirely for their performance on direct instructions from outside, for example, the human operator or remote computer. (See 'Intelligent Terminals' for contrast.) Also called 'dumb terminals'. Off-line Term applied to equipment or procedures which are operated at a time separate from the main processing stage, often without any direct control by computer. For example, terminal-to-terminal transmission is said to be 'off-line' as is the operation of auxiliary equipment such as punched cards-to-tape conversion. However, the term is also used for some operations which may actually be under computer control, though typically in an off-peak period, for example, large printing runs at a time subsequent to processing. On-line The direct linking of an operation or equipment to a computer system, which immediately accepts any stimulus from the linked operation or equipment, though the 'acceptance' may in fact reject the intervention on grounds of input errors, overload, etc. Examples of on-line working are interactive computing, direct data capture, process control, remote job entry, etc. Packet A block of bits transmitted and switched as a single unit, within the context of a packet-switching network. The content of the packet would normally be information passing through the network, but in addition always contains control information (for example, destination, etc.). In a packet-switching network, a packet represents the fundamental unit of transportation and multiple, separately-routed packets may be needed to make up a complete meaningful message from an enduser point of view.
19 Packet-switching GLOSSARY OF TERMS 205 A means of transmitting data through a network whereby the data are sent in discrete amounts (packets), each of which is transported independently to its destination. Each packet therefore contains sufficient information within itself to allow it to be handled separately by the network and to be assembled together with other packets at the appropriate destination to form a complete meaningful message. The channels over which a given packet is transmitted may immediately afterwards be utilised for other non-associated traffic and any following associated packets are handled by the network as separate data for transportation. Packet Terminal A terminal which itself can handle packets for sending or receiving via a network. This implies a capability for breaking down raw data into standard packets for transmission and likewise for assembling packets into meaningful messages. Such a terminal will therefore be of the intelligent type (see 'intelligent terminals') and computers themselves, from a networking viewpoint, may be one type of packet terminal. Parallel Bit Transmission A method of transmission whereby the bits representing a given character are Simultaneously transmitted over several parallel paths. Parity The condition of having an even or odd number of bits in a given set of data. Parity check A technique of error control whereby the number of bits in a given set of data is made to be either an odd ('odd parity') or even ('even parity') number. A special extra bit position (the 'parity bit') allows the addition of a bit, if required, to bring the total to the desired odd or even parity. If, after subsequent transmission and processing, the desired parity is no longer present for a given set of bits, it is then known that some error must have occurred. Password A set of characters which uniquely identify a user to a given computer system. A password is used to control access of users to files, processing facilities, etc., and is normally one of the main security checks applied in an on-line data communications system. Phase Modulotion A transmission technique whereby the phase angle of the carrier wave is varied
20 206 PLANNING FOR DATA COMMUNICATIONS proportionally with the instantaneous value of the signal containing the information. Ping-ponging A term used to describe the situation in packet-switching networks where a packet is repeatedly transmitted back and fore between two switching nodes. Point-to-point Connection The direct linking via communications line between two fixed stations, such as terminals or computers, without the use of intermediate switching facilities. Polling A method of controlling the use of communications links by terminals. A central control device 'asks' each terminal in turn if it has data to send. The control device may also inform (called 'addressing') a terminal that there is a message for it. Polling represents an alternative approach to that of contention. Port That part of a computer or data communications equipment acting as a connection point for input/output devices (for example, terminals). Private Line Commonly used to mean the same as 'leased line', namely, a communications line that is reserved exclusively by the telecommunications authorities for the use of a particular customer. It is interesting to note, however, that there still do exist some genuine 'private lines', which are not leased out by the telecommunications authorities but were installed by individual organisations in special circumstances. Notable examples of these are many railways, certain steelworks and military organisations. Private Network The term used to describe any network which is based on a configuration of leased lines and which has been set up exclusively for a single subscriber. Many examples exist of such networks, notably those of banks, airlines, large multinational manufacturing companies, etc., but the trend nowadays is towards greater use of the public switched facilities for data transmission. Protocol A set of standards defining the procedure and information content for the exchange of data between two parts of a network.
21 PSN GLOSSARY OF TERMS Abbreviation for 'public switched network', the publicly available dial-up telephone service of the telecommunications authorities. PTT Abbreviation for the French term for the post, telegraph and telephone administrations, which act as the common carriers for telecommunications in many European and other countries. Pulse Code Modulation (PCM) A technique for converting an analogue signal into a digital one, achieved by a sampling procedure which generates an eight-bit code. PCM equipment is increasingly being installed for voice transmission, so that eventually considerable benefits will be obtained for data communications as very high transmission speeds are possible with this technique. Furthermore, the digital nature of PCM will make it easier to interface data processing equipment, including terminals, to networks. Queue A set of data awaiting processing or transmission. Incoming messages, for example, might be arriving at a rate faster than they can be processed or forwarded to another destination and are therefore temporarily stored until facilities for processing or transmission become available. Real-time A mode of data processing whereby the input data instantaneously arising from an external event are immediately accepted and processed by the system. This element of immediate processing - usually implying updating a file - differentiates real-time from 'on-line'. Furthermore, in real-time working the results of the processing are often immediately fed back to the source in time to affect the function of the environment at the source; examples of this type of real-time system are to be found in process-control applications. 207 Reasonableness Check A technique of error control whereby data are examined to see if they lie within a feasible range of expected values. For example, a sales order for 100 million washing machines would be queried as a result of a reasonableness check. Redundancy Check The term used to describe any type of error control technique that makes use of extra characters or bits specially created to provide control information, but not contributing to the information content of the message itself.
22 208 PLANNING FOR DATA COMMUNICATIONS Remote Batch The transmission of jobs from a terminal at a given location to a remote computer for processing on that computer. The input data and instructions for processing would normally be prepared off-line at the terminal site and then submitted in online mode to the remote computer via a telecommunications link. The terminal may, or may not, then remain on-line to the computer while the data are processed and the results perhaps prepared for transmission back to the terminal. Remote Metering The on-line collection of data by means of a device connected directly to the process being measured. Examples of such applications are monitoring of water levels, collection of telephone usage data, etc. Repeater A device installed as an interface between two circuits, in order to receive signals from one of the circuits and retransmit them on the other circuit, normally with the purpose of amplifying signals that have become too weak and/or to correct distortions that may be affecting the signal. Response Time The period of time that elapses between the sending of a message from a terminal to a remote computer and the receipt of the answer at the terminal sent from the remote computer. In practice, this would be measured for a keyboard terminal by the interval of time between depressing the last key for sending the message and the display of the first character of the response. The response time factor is normally regarded as an important measure of system performance, but it should be noted that it is made up of several quite separate elements, namely, the performance of the terminal, the communications links, switching nodes, queueing at the remote computer, processing at the remote computer (including file searching). Reverse Channel A facility for error control in a given circuit whereby an extra channel is dedicated for the use of control information, which flows in the opposite direction to that of the primary information being transmitted. RJE Abbreviation for 'remote job entry', the activity of submitting batch processing tasks from a terminal to a remote computer. (See also 'remote batch'.) Routing The process of defining and initiating the communications path along which a message or packet is to be transmitted in order to arrive at a given destination.
23 SDLC GLOSSARY OF TERMS 209 Abbreviation for 'synchronous data link control', a detailed set of standards for data exchange using synchronous transmission. Selective Calling The facility of specifying which terminal (or other device) is to receive a given message on a line to which two or more terminals are connected. This selective calling facility would be exercised by the transmitting station. Serial Transmission A means of transmission whereby the bits representing a given character are sent one at a time sequentially over the communications channel (Compare 'parallel bit transmission'.) Signal-to-noise Ratio A measure of the relative strengths of the signal compared with the noise for a given transmission channel. The ratio is expressed in decibels as the signal power divided by the total noise power. Simplex Term describing a communications channel over which transmission takes place in one direction only. Single Thread The name used for data communications software that can process only one message at a time, other messages being queued. (See also 'thread' and 'multithread'.) Spooling A data processing method of buffering output to await transmission or further processing. Instead of being immediately transmitted or processed, which might block output channels, etc., needed for more urgent purposes, the data are stored temporarily on mass storage devices, from which they are subsequently recalled for transmission or processing at a time when the system resources are more freely available. Start-Stop Transmission (See 'Asynchronous')
24 210 PLANNING FOR DATA COMMUNICATIONS Store-and-forward (See 'Message Switching') Synchronous Term used to describe the transmission method, whereby the sending and receiving stations are kept in step with each other by a timing device whether or not data are being transmitted, thereby allowing data to flow at a fixed rate as required and not depending on individual synchronisation of each character or block of characters. Switched Line A communications line, one or both ends of which are connected to switching exchanges or nodes, thereby providing the facility for different traffic routings. The major examples of switched lines are the public dial-up services of the telecommunications authorities, but leased line networks may also have switched lines built into them. Teu Abbreviation for 'transmission control unit', the device acting as an interface between a computer and a modem, performing such functions as parity checking and bit serialisation. Also called 'communications controller' or 'communications control unit'. Teleprocessing A term, originally introduced by IBM, but now in common use, which describes the joint application of telecommunications and data processing, that is, the use of remote computers via communications links. Teletype (TTY) The registered trade mark of the teletypewriter products of the Teletype Corporation, often used however to denote any terminal of the kind having a keyboard, character printer and paper tape sender/punch. Telex A worldwide exchange service, using leased teletypewriters connected via a switched public network capable of operating in the range bits per second. Terminal Any device which via data communications links can be used for the input and/or output of data to a system of which it is part.
25 Thread GLOSSARY OF TERMS 211 The path a message takes through the processing programs or modules in a data communications system. Time-division Multiplexing (TDM) Communications technique using special equipment (a multiplexer) to transmit a number of signals over a single path by transmitting them sequentially at different instants of time. Time-out The predetermined interval of time during which a device waits before taking further action, for example, the period of time during which a central polling device awaits a response from a terminal it has just polled. If there is no response within this interval, the next terminal is polled. Time Sharing A mode of remote data processing whereby two or more terminal users are able simultaneously to make use of a central computer system for different purposes. The resources of the central computer are allocated sequentially in brief intervals of time to each user. Transmission Code A defined set of rules for representing data, normally by bit patterns, for the purposes of transmission and processing. Well-known codes are ASCII, Baudot, BCD and EBCDIC (described elsewhere in this Glossary). Trunk A single or multi-channel communications link between two switching stations. Turnaround Time The time necessary to reverse the direction of transmission on a half duplex circuit. However, in data processing the term is also loosely used to mean the period of time between submission of work to a computer system and the receipt of results. Unattended Operation A facility of a piece of communications equipment to accept and transmit messages without human intervention.
26 212 USASCII PLANNING FOR DATA COMMUNICATIONS Abbreviation for 'United States of America Standard Code for Infonnation Interchange', the transmission code of seven bits for data representation plus one bit for parity checking, being another term for 'ASCII'. User A term loosely used and therefore subject to many interpretations. The most fruitful meaning is when it refers to the person who ultimately has actually to apply transmitted information to his daily work - sometimes called the 'end-user'. The term is also frequently used to mean the tenninal operator, who mayor may not be an 'end-user'. Furthermore, from a common carrier viewpoint, all customers are 'users', whether they are computer professionals carrying out computer-to-computer transmission, terminal subscribers or computer service companies. Value Added Network The term applied to a network operated by an organisation other than a telecommunications authority or common carrier, though using facilities of those organisations, and offering services which go beyond simply providing communications facilities, for example, access to host computers, information retrieval, etc. VDU Abbreviation for 'visual display unit', another term for a terminal having a television-type (cathode ray tube) of screen for displaying data, often with a keyboard for input, file enquiry or computation work. Virtual Call The term used to describe a connection between two parts of a network (typically between a terminal and a computer) whereby the two parts are able to interact as if a specific circuit were dedicated to them throughout the transmission, whereas in reality a logical connection only is established with appropriate back-up facilities (such as intermediate storage), the actual physical circuits being allocated according to route availability, overload conditions, other traffic, etc. Virtual Terminal A concept applied in networking, whereby a set of logical functions for terminal operation is defined so that support by the network and by host computers for user terminals is buffered by an internal network procedure (the 'virtual terminal protocol'). This alleviates the problem of having different tenninal types connected to a network in order to access different types of computer, each of which would otherwise have to be capable of supporting each type of terminal.
27 Voice Grade Channel GLOSSARY OF TERMS 213 Term used to describe a channel which is 4000 Hz wide and capable of carrying speech signals in the range Hz. Such a channel can, however, carry analogue or digital data, with a maximum rate of about 10 kbits per second. VRC Abbreviation for 'vertical redundancy check', a method of error control whereby each character is checked for parity (even or odd), a parity bit being specially added to each character for this purpose. The same letters of abbreviation, VRC, are also used, however, for 'Visible Record Computer' and care should therefore be taken to avoid confusion. WATS Acronym for 'Wide Area Telephone Service', which refers to a special service of telephone companies in the United States whereby a customer can, by use of an access line, make as many calls as he wishes to telephones in a specific zone on a dial basis for a flat monthly charge. Wideband (See 'Broadband')
28 Index Access control 113, 149 Accounting applications 23,32-3,38 Acoustic couplers 71 Airline reservations 30-1 Amplitude 70 Amplitude modulation 70-1 Analogue-digital conversion 33, 71 ANSI (American National Standards Institute). 121 Applications 11,12,17-19,20,23-5, 30-6,38,53-4,64-5,122-3,127, 161-2,168-9,176 Applications software 102-3,147 ARP A (Advanced Research Project Agency) 74,93-4 Assessing risk Asynchronous 69,72 AT&T (American Telephone and Telegraph Company) 93 Auditors 182,184 Automated library 25 Automatic logging 57 Back-up 10,39,47,73,110, 154-5, Badge readers 9,64 Banking applications 11,23,31,64-5 Banking terminals 62,64-5 Bar-code readers 65 Basic access software 104,111 Batch processing 16,32,34 Baud 69 Baudot 68 Bell System 93 Binary 65 Binary characters 73 Binary signals 71 Bits 65 Bits per second (bps) 69 Breakdown 105-6,110,128,146-7,150, 177 Buffering 19,27-8,73,106 Bundespost 90 Burglar alarm systems CADUCEE 90 Card punch 63 Card reader 63 Carrier wave 71 CCIR (International Radio Consultative Committee) 89 CeITT (International Telegraph and Telephone Consultative Committee) 94, 99, 100,109 CCITT Alphabet No.2 66,68 CEPT (European Conference of Postal and Telecommunications Administrations) 86,89 Character checking 68 Checkpoint 152 Checkpoints, human 172 Chemical industry applications 33 CIGALE 90 CITEL (Inter American Conference on Telecommunications) 99 Clustering 17, 84 CN/CP (Canadian National/Canadian Pacific Telecommunications) 89 Codes 65,158-9 Coding Command language ,167 Commercial networks 32,94-7 Communications controller 73 Communications links 65-71,78-82,84, 147,150 Communications processor 73 Communications software 61,74, Compatibility 132,141 Computer workload 25-6 Concentrator 61,72-3,74,82,84,101, 107,111