Glossary

Advanced Research Projects Agency (ARPA) An agency of the US government that initiates and funds large research projects. The agency is now known as DARPA (Defense ARPA), which indicates the application area of its research. American Standards Committee for Information Interchange In normal usage the term ASCII refers to a particular character code that is widely available on computer peripherals. The letters stand for the committee that first formulated the codes. See IA5. Asynchronous framing A communication in which the characters are transmitted, and received, independently of each other. Automatic Repeat Request (ARQ) A technique used in error control that requires the receiver to request the retransmission of an message received with errors. bandwidth The information-carrying capacity of a channel; it is measured in hertz (Hz). The bandwidth is the difference between the highest and lowest frequencies that can be used on the channel. base band The frequency range in which a signal is actually generated, as opposed to the frequencies that might be used to transmit the signal after modulation. baud The rate at which a signal may change. bit The basic unit of information used in computers and digital communications. burst A sequence of information in a transmission; an error burst is a sequence of information that has been corrupted. Carrier Sense Multiple Access (CSMA) A mechanism for accessing a broadcast communication medium which tries to avoid transmission collision by not transmitting if there is already a transmission in progress. The presence of another transmission is detected by sensing the carrier of the transmission.

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CCITT

Computer Communications

(Comite Consultatif Internationale de Telegraphique et Telephonique). An international organisation of PTTs which decides on standards for use in telecommunications. The CCITT is part of the United Nations. channel A path for communication. A channel connects an information source with an information receiver. The channel can be made up of several physical circuits and pieces of equipment. character This is a common name for a small number of bits that are grouped together for transmission and manipulation. Normally a character will consist of 8 bits; values of 5, 6, and 7 bits have been used. A character contains enough bits to encode all of a particular set of printed symbols. ASCII is such an encoding. check bits The redundant information added to the end of a message to enable the receiver to check the message for possible transmission errors. circuit A physical connection used for communication. common carrier A term used in the USA to describe companies, licensed by the Government, that provide the basic telecommunication facilities. concentrator A device that multiplexes several inputs on to one output (and vice versa) but uses local storage to provide a buffer against surges in traffic. A packet switched network node is a concentrator. CRC The Cyclic Redundancy Check (CRC) is an algorithm for computing check bits over a message. The check bits are often placed at the end of a message in a field called 'CRC'. Data Communications Equipment (DCE) The term used to describe the node in an X.25 network. Data Terminal Equipment (DTE) The term used to describe the host in an X.25 network. datagram A packet of information, transferred in a computer network, which is independent of all other packets. The packet contains the source and destination address. digital A system is digital if it can only use data of discrete values, such as 0 and 1. An analogue system can use continuously varying values. Since computers are digital all computer communications are digitally orientated. EBCDIC (Extended Binary Coded Decimal Interchange Code). The name given to the character code used on IBM computers.

Glossary 163

encryption Encryption is a technique for securing data which is being transmitted. The original data is transformed into some meaningless bit pattern which is then sent over the network. The received bit pattern is then transformed back into the original message. Obviously the two transformations are inverses of each other. It is necessary to keep some aspects of the transformation secret to ensure that only the people allowed to see the original message can perform the transformation on the transmitted bit pattern. forward error correction A technique used in error control, in which a large amount of redundant information is added to the message so that error correction can be done by the receiver. Frequency Division Multiplexing (FDM) A technique for multiplexing a channel by dividing the frequency range. Frequency Shift Keying (FSK) A modulation technique in which digital signals are coded directly into frequencies. Front End Computer (FEC) A small (mini or micro) computer used to handle communications devices for a large general purpose computer. gateway In computer networks a gateway is a computer that is connected to two or more networks and allows traffic to flow between the networks. host A user computer on a network. lAS (International Alphabet number 5). A standard character code of the ISO that closely resembles ASCII. LAN- Local Area Network A network is said to be local if it covers an area of a few square kilometres. This is only a rough guide. A network covering a much larger area is a wide area network. MODEM (MOdulator-DEModulator). The piece of equipment that allows digital communication signals to be transferred over the PSTN by modulation. modulation A technique used in communications to change a base band information signal into a signal more suitable for a particular medium. multiplexing A mechanism for using a single communications channel to carry the signals from a number of other channels. node A computer in a packet switching network that does the store and forward switching. The node is usually separate from the host.

164 Computer Communications

noise The extra signals that are always received on a circuit that can cause errors in the received information. open system A system is open if all of the communication procedures and protocols are published for public use. A proprietary communications system is not usually open, even if all of the protocols are published, as the manufacturer will be influenced by commercial pressures when deciding on changes and their availability. The international standards are an open system. packet switching Messages from the user computers (hosts) are broken into small packets. These packets are then switched from node to node in a network to reach the destination host. Packet Switching Service (PSS) The public packet switching network in the UK run by British Telecom. PSS uses X.25 as its access protocol. parity An error-detecting mechanism; often used on single characters. The bits of a character are summed and an extra bit added either to make the total number of Is even (even parity) or odd (odd parity). piggy-back A mechanism used in flow control where acknowledgements are added to data messages using the reverse channel. private circuit A PSTN connection between two sites that bypasses the dialling equipment in the exchange. Thus the circuit can only be used for communication between the two sites but should have a lower error rate than a switched circuit. protocol An agreed format for communication between two or more parties. PSTN The Public Switched Telephone Network. PTT (Postal, Telegraph and Telephone authority). The administration that runs the posts and all public telecommunication systems in a country. In Europe these are all government monopolies. In the USA the PTTs are private companies (common carriers). redundancy Extra information that is added to a message; it is an overhead on the channel to carry the extra information. The extra information is used to implement a protocol. reference model The International Standards Organization uses a reference model to describe the architecture of the protocols it is producing. Generally a reference model is an embodiment of the philosophy and rules for building a system.

Glossary 165

RS232C The name given to the physical terminal interface standard V24 in the USA. store and forward The technique, used in nodes, whereby messages (or packets) are stored in the node memory before being forwarded on a communication channel. synchronous framing A communication in which the characters follow one another immediately; thus the receiver and sender need to have synchronised clocks. Time Division Multiplexing (TDM) A technique for sharing the capacity of a channel by dividing it into slots of equal time. V24 A standard interface for connecting asynchronous character equipment. It is equivalent to the EIA RS232C standard. virtual call An end-to-end connection between two hosts across a packet switched network in which information can be exchanged in a sequenced flow controlled manner. Addressing information is only used when the call is set up.

Bibliography

There are many books available on Computer Communications which have been written for various levels of students and backgrounds. If you can visit a large books hop with a good technical section then you will certainly find more books than I have listed below. If you cannot reach a good bookshop then this list may help you with further study. Chou, W. (ed.) Computer Communications, vols 1 & 2, Prentice-Hall. These two books contain chapters written by various experts. The individual contributions are very good but the overall structure is very weak. Volume 1 concentrates on principles with chapters on network structures, network protocols, various communication technologies (satellite, radio), security and performance analysis. Volume 2 is concerned with applications; it covers a broad spectrum of network usage, including file transfer, terminal traffic, computer mail, and distributed systems. Davies, D.W., Barber, D.L.A., Price, W.L., Solomides, C.M. Computer Networks and their Protocols, Wiley, 1980. This is a comprehensive book which concentrates on the techniques used in computer communications, flow control, packet switching technologies etc., rather than the structure of communication systems. There is a good emphasis on CCITT standards. Denning, D.E.R. Cryptography and Data Security, Addison-Wesley, 1982. This is a very detailed book about the underlying theory and techniques used in cryptography and security in computer systems; written by one of the leading experts in the field. Doll, Dixon R. Data Communications: Facilities, Networks, and Systems Design, Wiley, 1978. This book was written by an American for the American DP manager and student, but as the terminal market is dominated by the US standards and systems the book is good for the reader who is interested in the material covered in chapters 3 to 6 of this book. There is a discussion of the problems in designing terminal networks in the DP environment.

166

Bibliography 167

Gee, K.C.E. Introduction to Local Area Computer Networks, Macmillan, 1983. This is a very readable description of the various types of LAN. It has a good description of the mechanisms used in the three major types of LAN: ethernet, token ring, and slotted ring (Cambridge Ring). Halsall, Fred Data Communications and Computer Networks, Addison-Wesley, 1985. The content ofHalsall's book covers a similar area to that covered by this text. However the treatment of the material and the emphasis on various technical aspects makes the Halsall book a very good complement for further reading. Jennings, Fred Practical Data Communications, Blackwell, 1986. This is an excellent text covering the physical and link aspects of digital communications in depth. The book describes all of the relevent V and X series CCITT recommendations, and the US counterparts. There is a chapter on the ISDN; and very detailed descriptions of the latest MODEM techniques. Kuo, F.F. (ed.) Protocols and Techniques for Data Communication Networks, Prentice-Hall, 1981. A number of leading American researchers have contributed chapters to this book. The subject matter of the various chapters is somewhat disjointed, but the authors have tried to relate to each other's chapters where possible. Most of the authors turn to the ARPA network for examples. Chapters of particular interest as further reading are on: background, transport protocols, terminal and file transfer protocols, routing and flow control, broadcast satellite networks. Schwartz, M. Computer Communication Network Design and Analysis, Prentice-Hall, 1977. A detailed study of the theory of communication network design with mathematical analysis. The optimising techniques for routing and flow control in networks are covered with worked examples. The text is parti­cularly good on the design of terminal networks, as opposed to switched computer networks. Tannenbaum, A. Computer Networks, Prentice-Hall, 1981. This book covers the protocols used in packet switching networks. The author has arranged the text to follow the ISO protocol hierarchy, as far as possible. A very good mix of commercial, PTT and DARPA protocols are covered in some detail.

Collections of Papers These are of special relevance as further reading.

Proceedings of IEEE - Special Issue on Packet Switching. Vol. 66, no. 11, November 1978.

168 Computer Communications

This special issue contains a number of papers specially written by American researchers as a technical introduction to the various aspects of packet switching. Many of the examples are taken from the ARPA network, but this reflects the background of the authors. A number of papers also consider PTT network problems. Some particularly interesting papers include Evolution of Packet Switching, written by Roberts; this is a very readable history of packet switching. Papers on packet radio and satellite networks describe existing networks and their protocols. The paper by Clark et al., on Local Area Networks gives a good background to the technology and protocols of these networks. Proceedings of IEEE - Special Issue on Open Systems Interconnection. Vol. 71, no. 12, December 1983. This issue is a collection of papers written by experts from the standards field. There is a paper for each layer of the ISO reference model, with several overview papers and other topics. Each of the major applications is described in a separate paper. This is an important source of information describing the emerging International Standards for Open Systems Interconnection.

Index

access 108 access control 111 acknowledgement 77 acoustic coupler 32 activities 145 adaptive routing 117 address 123 addressing 62, 88, 111, 139 aloha techniques 95 amplitude 4 analogue 7 application layer 141 application level 109 ARPA network 90, 92, 117 ARPANET 85, 154 ARQ 66, 71, 123, 125 ASCII 17, 41, 154 asynchronous frame 17 asynchronous framing 16 asynchronous MODEM 33 asynchronous terminals 43 authentication 156 automatic bit rate detection automatic repeat request 71

bandwidth 7-8, 47, 108, 113 base band 27, 98 baud 13 BCC 59 Binary Synchronous

Communications 58 bit 23 bit stuffing 122 bits 6 bits per second 14 block 72 Block Character Check 59 bps 14 break 15 British Telecom 7, 24

21

169

broadcast network 102 BSC 58, 121 buffer space 113 burst 65 bus network 98 bus network technology 98

cable network 98 cable network technology 97 Cambridge ring 100 capacity 8-11 carrier sense multiple access 95 cascaded multiplexers 51 CCITT 29, 119 channel 8 channel bandwidth 115 channel capacity 9, 64, 67, 85 channel number 128 character 40 character codes 40 character devices 16 character framing 16 characters 16 check bit 68,72 ciphertext 156 circuit switching 86 CMDR 126 collision 95 collision avoidance mechanism 95 commands 123 common carriers 24 communication networks 104 computer mail 147 computer messages 153 computer network 84 concentration 46 concentrator 46, 114 congestion 116, 134 connectionless 106 connection-orientated 106

170

contexts 146 continuous RQ 75 control 109 control characters 41, 58 controller 54 cosine wave 4 cost 12 CRC 125 CSMA 95 CSMA-CD 98 cyclic redundancy checking 59

D bit 134 D plug 42 Data Encryption Standard 158 data representation 141 data structure 146 datagram 89, 107 DCE 120 deadlock 115 decibel 10 delay 134 demultiplexing 43 dialogue 144 dialogue control 144 dial-up 26 differential phase modulation 30 digital 7 digital network 36 digital signature 157 distributed computing 61 distributed intelligence 62 domain specific 139 DTE 120 dynamic windows 82

EBCDIC 41 echo checking 66, 67 ECMA 142 electromagnetic noise 13 electronic mail 97 encryption 156 end-point 105 entity 147 error burst 65 error control 119 error correction 65 error detection 65 errors 65, 119 ETB 58 Ethernet 98 ETX 58 exchanges 93, 119 exclusive-or 68

Index

FCC 24 FCS 125 FDM 47 FEC 67 Federal Communications

Commission 24 feedback control 117 FEP 51 fibre optics 13, 64, 85 file 57 file access 150 file transfer 146 flag 122 flow control 77, 82, 108, 119 forward error correction 67 fragmentation 135 frames 121 frequency 8, 28 frequency division multiplexing 26,

47 frequency range 7 frequency shift keying 28 front end processor 51 FSK 28 FTAM 150 full duplex 14, 144 fully connected network 86

general format identifier 129 GFI 129 global network address 139 Go-back-N 76, 125 graphic characters 19, 40 guard band 4 7

half duplex 14, 144 Hamming code 68 Hamming distance 66 handshakes 61 HDLC 121 hertz 7 hierarchy 61 high-speed MODEM 33 host 90 host computer 93 hosts 88, 104 hub polling 55

IA5 51 IBM 41, 58, 121 idle RQ 74 IMP 92 information 6, 7 information content 9

information rate 9 information switching 87 initial domain 139 Integrated Services Digital

Network 36 intelligent TDM 49 interactive terminals 39 Interface Message Processors 92 interference 64 internal character codes 41 international standards 104 internet protocol 134 internetworking 136, 138 interrupt 131 interworking 111 ISDN 36 ISO 41, 105, 119

job 152 JTM 152

LAP 125 LAPB 125 layer concept 105 line speeds 17 link 125 link access protocol 125 link level 108 listen-while-talk 98 local clock 19 local exchange 26 local multiplexer 44 local network 97 logical channel 128 loose source routing 135

M bit 132 mail 92 mark 15 marks 144 mechanical terminals 18 medium 8 message 71, 88 message switching 87 message transfer agent 154 microprocessor 50, 63, 97 minicomputer 51, 62, 91 mini-packet 100 MODEM 26, 31, 42, 120 modulation 27 modulator 28 modulo 2 arithmetic 73 modulo numbers 79 monitor 100

Index

monitoring 111 monopoly 24 multi-access 57 multiplexing 43, 46, 89

names 111, 146 naming Ill negative acknowledgement 81 network level 108 network management Ill network organisation 39 network resources 113 N-layer 105 nodes 88, 113 noise 10, 64 NSAP address 139

171

Open Systems Interconnection 104 operating system 113 optical fibres 4 out-of band 131 out-of-sequence 126

packet 96 packet radio 95 packet slots 96 packet switching 90 Packet Switching Stream 93 packets 72 PAD 148 parallel transmission 11 parity 68 partially connected network 87 perfect channel 10 permanent circuits 87 persistent CSMA 95 phase 6, 30 phase modulation 29 physicallevel 108 piggy-back 127 piggy-backed 82 plaintext 156 polling 54 polling regime 54 polynomial codes 72 positive acknowledgement 78, 94 Postal, Telegraph and Telephone

authority 25 p-persistent 95 presentation layer 109, 145 presentation structure 151 Prestel 63 private circuit 34 private network 97

172

processes 104 proforma 153 promiscuous node 156 protocol 41, 58 protocol hierarchy 61 protocollayering 104 protocols 84 PSS 27,85,93 PSTN 25, 42, 65, 86 PTT 25, 85, 119 public key 158 Public Switched Telephone

Network 25 pulse code modulation 36 pure aloha 94

quadrature amplitude modulation 30 quality of service 108

radio network 84 reassemble 92 reassembly 135 record route 135 redundancy 59, 67 reference model 105 REJ 126 relay 137 relays 104 reliability 65 remote job entry 57 remote login 146 remote multiplexer 44 remote multiplexer network 40 remote terminal 40 repeater 1 00 reservation slots 96 reservation technique 96 reset 81, 126 resource 85 resource management 113, 134 resource sharing 92, 104 responses 123 restart 131 retransmission 74 ring network 99 ring network technology 98 RJE 57 RNR 127 roll call polling 54 round robin 43 routing 62, 86, 117, 139 routing table 117 RR 127

Index

RS232C 15,37,43 RS432 15

S70 142 satellite channel allocation 96 satellite network 97 scanner register 43 SDLC 121 secrecy 157 security Ill selective retransmission 76 semantics 145 sequence numbers serial transmission service access point services 105 session layer 144 session level 109

78, 123 12 106

Shannon's equation 10, 64 simplex 14 sine wave 4 skew 12 slotted aloha 95 SNA 121 source routing 135 source select register 101 space 15 speech signal 25 square waves 6 standards 37 star 51 star network 39 start bit 17 station 101 statistical TDM 49 stop bit 17 stop-go 82 store and forward 88, 114 strict source routing 135 STX 58 sub-networks 136 supervisory channel 33 supervisory frame 124 switched digital network 37 switching 45, 86 switching technique 90 SYN 19, 49, 58 synchronisation 18, 95, 109, 144 synchronous character 54 synchronous framing 16 synchronous MODEM 34 syntax 145

TDM 47 Telenet 85 telephone 7 telephone exchanges 26 telephone system 24 Teletex 142, 143 terminal MODEM 31 terminal multiplexer 43 terminal network 40 throughput 114, 133 time division multiplexing 48 time out 60, 74, 94 title 112 token 109, 144 token management 144 traffic load 115 transaction processing 57 transparency 122 transport level 108 trap door 158 Tymnet 85

UART 20,42 United Nations 37 unnumbered frame 124 user agent 154 users 104 USRT 20

Index

V24 15, 37, 42 virtual calls 93, 120 virtual circuit 89, 106, 128 virtual terminal service 147

waveguides 4 waves 4 wide area network 85, 92 window 80, 115 window size 80 wire 4 work specifications 153 workstation 84, 97

X.21 37, 120 X.21 (bis) 37 X.25 77, 78, 87, 108 X.75 137 Xerox 98

173