Definition of OSI Terms

Address : The identifier of a . Also known as a service access point address.

Association : The relationship between two (N)-entities formed when an (N-1)-connection is established between them.

Blocking : The combining of multiple service data units with added protocol control information into one .

Concatenation : The combining of multiple protocol data units into one service data unit for the layer below.

Connection : A relationship established by the (N)-Layer between two (N+ 1 )-entities for the transfer of data.

Connection endpoint : The end of a connection within a service access point. Used to support multiple connections in one service access point.

Connectionless /connection-oriented : Two different modes of communication between systems. Connection-oriented means that a logical connection is first established between the communicating systems to enable the data to be transferred. Connectionless means that a single message, containing routeing information, is passed without first establishing a connection.

Entity : An active element within the (N)-Layer which communi• cates with a peer element by making use of the services provided by the (N-1)-Layer.

Expedited data transfer The transfer of data between peer

211 212 Definition of OSI Terms entities with priority over data being transferred normally.

Flow control : A function to control the flow of data between peer entities or between layers such that the receiving entity can limit the rate at which data is transmitted by the sending entity.

Interconnection : A relationship between open systems allowing them to communicate and interwork.

Interface : The boundary between two adjacent layers.

Interface control information (ICI) : Information passed across an interface to coordinate interactions between two adjacent layers.

Layer : A subdivision of the OSI architecture. The (N)-Layer provides services to the (N+1)-Layer and makes use of services provided by the (N-1)-Layer.

Multiplexing : An (N)-Layer function where a single (N-1)• connection is used to support multiple (N)-connections. Demulti• plezing restores the multiple (N)-connections.

Primitive : A means of identifying an elementary function provided as part of a layer service. Primitives are used either by the (N)-Layer to select a particular service from the (N-1)-Layer, or by the (N-1 )-Layer to indicate that a service has been selected.

Protocol : A set of rules governing the format and sequence of information exchanged between communicating peer entities.

Protocol control information (PCI) : Information exchanged between communicating peer entities to coordinate their operation.

Protocol data unit (PDU) : A unit of data spedfled in a protocol and consisting of protocol control information and optionally a user data field.

Quality of service (QOS) : A description of t.he set of required Definition of OSI Terms 213 operating characteristics provided to an (N)-entity by the (N-1 )• service. Usually applies to a particular connection and is negotiated and agreed upon at the time of establishing the connection.

Relay : An intermediate open system with only the lower three layers implemented, used to interconnect end open systems.

Service : A facility which a layer provides to the layer above. The total set of all facilities provided by the (N)-Layer is known as the (N)-service.

Service access point (SAP) The point of interaction between entities in adjacent layers.

Service data unit (SDU) : A unit of data which is transferred from one end of a connection to the other with no loss of identity or meanmg.

Splitting An (N)-Layer function where multiple (N-1)• connections are used to support a single (N)-connection. Recom• bining restores the single (N)-connection.

Standards Promotion and Application Group (SPAG) : A group representing European companies with an interest in OSI, set up to consider implementation issues of OSI.

Subnetwork : The OSI representation of a real communications network excluding the end open systems.

System : A set of hardware, software, processes, human operators and all other functions required to form an autonomous unit capable of performing information processing.

User data (UD) : Data passed between (N)-entities on behalf of (N+l)-entities. For example, an (N)-SDU will form user data for the (N)-PDU. Glossary

Architecture : The overall structure of a system and the set of rules governing the interactions between parts of that system.

Circuit-switching : A means of interconnecting multiple end-user systems by the switching of physical connections.

Data terminal equipment (DTE) : A real end system whose network entity has no routeing or relaying functions.

Data circuit-terminating equipment (DCE) : An intermediate system whose network entity has only a routeing and relaying function.

Frame : A delimited stream of data bits transferred at the data link level.

HDLC (High-level ) : A set of procedures defining the exchange of data at the data link level.

LAN () : A network linking computer systems and workstations within a limited geographical area, usually using private physical connections.

Packet : The meaning of packet varies depending on the context in which it is used. In packet-switching terms, it is a unit of data transferred through a network. In X.25 terms, it is a unit of data with a predefined format which is transferred at the packet level of X.25.

Packet-switching A technique for routeing data through a network in discrete blocks (or packets) thus maximising the use of physical connections. The physical connections within the network

214 Glossary 215 are shared, with data for different sources and destinations being interleaved.

Routeing : The means of transferring data from a source to a destination identified by its address. It is not necessary for a source system to have knowledge of the intermediate systems used, but it is necessary for intermediate systems to know the correct 'direction' for each destination system.

Transparent data transfer : The transfer of data between systems such that data can be sent in any format, and the received data will be identical to that sent.

Virtual circuit : A logical path through a network between a source system and a destination system. The two communicating systems appear to have exclusive use of a connection, but a dedicated physical link is not provided. Virtual circuits may be switched (SVC) or permanent (PVC).

X.21 : CCITT recommendation for a general purpose interface between DTE and DCE for synchronous operation on public data networks. (X.21 his provides the same service for networks using the V Series modems.)

X.25 : CCITT recommendation for the interface between DTE and DCE for terminals operating in the packet mode.

X.400 : CCITT recommendations for message handling systems.

WAN (wide area network) : A network linking computer systems over a geographically widespread area using both public and private communication connections. Index abstract syntax, see syntax tion (JTM), 180-1, 192- Abstract Syntax Notation One 4, 196 (ASN.1), see syntax transaction processing (TP), AC, see application service ele• 180, 194, 196 ments Virtual Terminal services (VT), addressing 180, 186-9, 202, 204 see also routeing ARPANET, 9, 10 across OSI networks, 93, 97- architecture 9, 195 layered, 9-11, 13-25, 29 between layers of OSI model, ASN.l, see syntax 37-8 association, 35 domain, 98-9, 195 Association Control (AC), see in , 93, 97-9 application service ele• application, 30-1 ments association, 35, 182-4 asynchronous transmission, see context, 182, 186 data transmission entity, 34-6, 160, 181-3, atomic action, 184-6 185-6 Basic Encoding Rules (BER), process, 34, 115, 179-81 163-5 protocol data unit (APDU), 'black box' approach 181 to system design, 14 service element {ASE), 181-2 blocking, 41 title, 183, 195 British Standards Institution (BSI), , 36, 179-98 6, 26-7 function of, 179-80 implementation of, 196-7 carrier sense, see CSMA/CD requirements for, 180-1 CCITT (International Telegraph specification of, 181-96 and Telephone Consul• application service elements (ASEs) tative Committee), 6, 9, Association Control (AC), 26-7, 98, 206 181-4 see also V Series, X.21, X.25, Commitment, Concurrency X.400 and Recovery (CCR), CCR, see application service ele• 181, 184-6 ments File Transfer, Access and circuit Management (FTAM), packet-switched, 9, 91-2, 109 180-1, 189-92, 196, 204 virtual, 20, 93 Job Transfer and Manipula- circuit-switching, 2-3, 7, 20, 91-2

217 218 Index classes of transport service, 120- 166 3, 128-9, 132, 199, 201 CSMA/CD (carrier sense multi• commands and responses ple access and collision in Data , 70, 80-1 detection), 48, 58-61, Commitment, Concurrency and 72, 87, 200 Recovery, see applica• see also tion service elements communications data compression, 161, 165 communications networks, 6- data encryption, 161, 165 9, 90-2 data link history of data communica• configuration, 70-3 tions, 2-12 connection, 63-4 compliance with OSI, 27-8, 45, entity, 47-8, 50, 52, 54, 60, 202-3 62, 69-70, 75 concatenation, 42 flow control, 65 connection protocol data unit (LPDU), see also individual layer en• 69-76 tries , 64-5 between open systems, 34-7 service multiplexed connections, 42- acknowledged connection• 3, 49, 66, 122 less, 66, 87 connection endpoint, 37 connectionless, 66-7, 76, connectionless mode, 34, 64, 200- 87 1 connection-oriented, 66-8, data link service, 66-7, 76, 87 87 service access point (LSAP), (CLNS), 93, 67 107-8 service data unit (LSDU), physical service, 52-4 63-4, 74-6 session service, 158 , 36, 62-89, 108 transport service, 134 and HDLC, 77-88 connection-oriented mode, 34, 64, and LLC, 86-8, 201, 205-6 76, 200-1 and Network Layer, 64-5, 67, data link service, 66-8, 87 88 network service (CONS), 93, and Physical Layer, 50, 54-5, 108, 112 61-2, 64, 74, 87-8 physical service, 52-3 function of, 62-4 session service, 137 implementation of, 77-88 transport service, 134 requirements for, 64-6 context specification of, 66-76 application context, 182, 186 data link services defined context set, 166 Unacknowledged Connect.ion• presentation context, 161, less Data Transfer, 67-8 Index 219

Connection-Oriented Data Trans• in Network Layer, 95, 100, fer, 69 112, 123 data transmission, 47-61 in , 123-5, asynchronous, 47, 51 131 parallel, 50-1 formal description technique (FDT), serial, 50-1 44-5 synchronous, 47, 51, 57-8 framing DCE (data circuit-terminating in Data Link Layer, 63, 65, equipment), 57, 108 74-85 deblocking, 41 FTAM, see File Transfer, Access demultiplexing and Management of OSI connections, 42-3 functional profile Digital (DNA), through 081, 196-7, 204, 209 10 functional standard, see func• directory services, 180, 195-6 tional profile domain functional units addressing domain, 98-9, 195 of presentation services, 167- DTE (data terminal equipment), 8 57, 108-11 of session services, 146-9 DXE, see DCE and DTE gateway, 10 EDC, see error detection and re• covery harmonization EIA RS-232-C, 6, 56 within Network Layer, 96 see also Physical Layer HDLC (High-level data link con• encryption, see data encryption trol), 77-88 entity see also Data Link Layer see also individual layer en• tries ICI, see interface control informa• within 08I model, 32-9 tion enveloping ID U, see interface data unit of service dat.a units, 40 IEEE 802, 9, 77, 86-7, 205 error detection, 19-20 see also Data Link Layer error detection and recovery (EDC), lnstitut.e of Electrical and Elec• 65-6 tronics Engineers (IEEE), error notification, 50, 63, 65 9 , 9, 59-60 integrated services digital net• work, see ISDN File Transfer, Access and Man• int.erface, 14 agement (FTAM), 158, general properties of, 1.5-16 180-1, 189-92, 196, 204 within 08I model, 32-4 finite state machine, 44, 46 interface control information (I CI), flow control 38-40, 75 in Data Link Layer, 65 interface dat.a unit. (IDU), 39-40 220 Index

International Organization for Stan• Layer), 86-8 dardization (ISO), 12, management 26-7, 98 layer management, 194-5 International Telegraph and Tele• OSI management, 180, 194- phone Consultative Com• 6, 201-2 mittee, see CCITT security management, 180, interworking, 10, 30-1 195-6 ISDN (integrated services digital system management, 194-5 network), 11, 57-8, 72, man-machine interface, 23 77 Manufacturing Automation Pro• see also Physical Layer tocol (MAP), 134, 158, ISO, see International Organiza• 196, 204-7, 209 tion for Standardization message handling systems (MHS), see X.400 Job Transfer and Manipulation modes of operation (JTM), 180-1, 192-4, in Data Link Layer, 70, 72-4, 196 82-5 LAN, see local area network of data transmission circuits, Language of Temporal Ordering 3-4 Specification, see LO• of OSI connections, 42-3, 49, TOS 66, 122 LAP, 86 time-division multiplexing (TDM) LAP-B, 77, 86 7 LAP-D, 77 layer network see also individual layer en• see also communications and tries subnetwork n1anagement, 194-5 addressing, 93, 95, 97-9 within OSI model, 32-40, connection, 117 42-45 types of, 119-23 layered architecture, 9-11, 13-25, entity, 64-5 29 flow control, 95, 100, 112, local area network (LAN), 7-11, 123 48-9, 53-4, 59-61, 72, protocol, 96-7, 112-13 77, 86-7, 200-1 protocol data unit (NPDU), LLC ( Layer), 91, 107-8 86-8, 201, 205-6 quality of service, 94-5, 103- see also Data Link Layer 4 LOTOS (Language of Temporal service, 116 Ordering Specification), connectionless ( CLNS), 93, 45, 200 107-8 connection-oriented (CONS), MAC ( 93, 108, 112 Index 221

service access point (NSAP), technical limitations of, 199- 97-9, 102 201 service data unit (NSDU), OSI basic refence model, 12, 14, 90-1, 100-1, 103-5 24-5, 27-31, 45 Network Layer, 36, 90-114 OSI management, 180, 194-6, and Data Link Layer, 64-5, 201-2 67, 88 directory services, 180, 195- and Physical Layer, 49-50, 6 91 layer management, 194-5 and Transport Layer, 120-3 security management, 180, and X.25, 27, 95, 108-13 195-6 function of, 90-3 system management, 194-5 implementation of, 108-13 OSI transaction processing (OSI requirements for, 94-7 TP), 180, 194, 196 specification of, 97-108 network management, see OSI packet, 91-3 management data packet, 19-20 network services, 94-5, 101-8 packet level of X.25, 109 Network Connection Estab• packet-switched circuit, 9, lishment, 102-4 91-2, 109 Network Connection Release, X.25 packet, 109-13 106-7 packet-switched data network (PSDN), Data Transfer, 104 9-11 Expedited Data Transfer, 104- parallel transmission, see data 5 transmission Receipt Confirmation, 104 PCI, see protocol control infor• Reset, 104-5 mation (N)-notation, 32 PDU, see protocol data unit physical 'onion skin' architecture, 10, 13 connection, 47-55, 66 open systems, 30-1 connection endpoint, 50 connection between, 34-7 entity, 47-8, 52-6 intermediate, 36, 96-7 media, 14, 17-18, 47-61 open systems interconnection (OS I), protocol data unit (physical 30-1 PDU), 51-5, 57 applications of, 199-210 quality of service, 50-1, 56, history of, 11-12 60 implementations of, 28-9, service, 50-5 203-9 connectionless, 52-4 implementation specifications connection-oriented, 52-3 for, 14, 27 service data unit (physical omissions from, 201-2 SDU), 47, 50-3, 55, 57 political considerations, 202- Physical Layer, 36, 47-61 3 and CSMA/CD, 48, 58-61 222 Index

and Data Link Layer, 50, 54- Connection Release, 167, 172- 5, 61-2, 64, 74, 87-8 4 and ISDN, 57-8 Expedited Data Transfer, 166 and Network Layer, 49-50, functional units of, 167-8 91 Normal Data Transfer, 166, and Token Bus, 48, 60-1 170-1 and , 49, 60-1 Provider Abort, 167, 175-6 and V.24, 51, 56-7 Presentation Connection, 166, and V.28, 56 168-70 and X.21, 56-7 Synchronization, 167 and X.21 bis, 56-7 Token Management, 167 function of, 47-50 Typed Data Transfer, 166 implementation of, 55-61 User Abort, 167, 174-5 requirements for, 50-1 primitive specification of, 51-5 see also service physical services, 52-5 types of, 42-5 Data Transfer, 53-4 procedures Reset, 54 in Data Link Layer, 70, 85-7 point-to-point links, 2, 19 process, 34 polling, 66, 73 protocol presentation within OSI model, 34 connection, 181, 183-4 protocol control information (PCI), context, 161, 166 38-41, 75 entity, 137 protocol data unit (PDU), 39-42 see also individual layer en• protocol data unit (PPDU), 175, 177 tries protocol machine, 44 service, 161-3 PSDN, see packet-switched data service data unit (PSDU), network 165 public switched telephone net• syntax, 161, 163-6 work (PSTN), 5, 11, 91 , 160-78 and , 163 quality of service (QOS}, 50-1, and X.400, 176 64, 94, 103, 118 function of, 160-1 see also individual layer en• implementation of, 176 tries requirements for, 161-3 queue model specification of, 163-77 for Network Layer, 99-101 presentation services, 161-3, 166- for Transport Layer, 123-5 77 Activity Management, 167 reassembling, 41 Alter Context, 166, 170-2 recombining Capability Data Exchange, of OSI connections, 42-3 166 relay, 36, 47, 50, 59-60, 94, 96 Index 223 routeing, 18-20, 9Q-4, 99 Expedited Data Transfer, 145 see also addressing functional units of, 146-9 RS-232-C, see EIA RS-232-C Give Control, 142, 155-7 Give Token, 142 SAP, see service access point Normal Data Transfer, 144- SDLC (Synchronous data link 5, 151-2 control), 77 Orderly Release, 145, 152-3 security management, 180, 195-6 Please Token, 142 separation, 42 Provider-Initiated Abort, 145, segmenting, 41 154-5 sequence number, 20 Provider-Initiated Exception sequencing Reporting, 145 in Data Link Layer, 63, 65, Quarantine, 158 74-7, 8Q-1 Session Connection, 144, 149- serial transmission, see data trans• 51 mission subsets of, 139, 146-9, 199 service Typed Data Transfer, 145 see also individual layer en• User-Initiated Abort, 145, tries 153-4 types of, 43 User-Initiated Exception Re• within OSI model, 34, 37, 42 porting, 145 service access point (SAP), 37-8 service data unit (SDU), 39-42 session see also individual layer en• activities, 141 tries connection, 137, 144 SNA (System Network Architec• entity, 117, 124-5 ture), 10, 77, 202-3 protocol data unit (SPDU), SPAG (Standards Promotion and 143-4 Application Group), 197, S'lrvice, 138-9 209 connectionless, 158 splitting connection-oriented, 137 of OSI connections, 42-3, 66 synchronization, 138-41, 157- subnetwork, 90-2, 96-7, 99, 112- 9 13, 115 tokens, 141-2 subsets Session Layer, 136-59 of session service, 139, 146-9, and Presentation Layer, 163 199 function of, 136-8 subsystem implementation of, 158-9 within OSI model, 31-3 requirements for, 138-9 switching, 90 specification of, 140-58 circuit-switching 2-3, 7, 20, session services, 138-58 91-2 Capability Data Exchange, packet-switching, 9, 91-2 145 synchronous transmission, see data 224 Index

transmission function of, 115-16 syntax, 161 implementation of, 134 abstract syntax, 163-6 requirements for, 116-17 Abstract Syntax Notation specification of, 118-34 One (ASN.l), 163-5, 200 transport services, 118-20, 128- transfer syntax, 165-6 34 system management, 194-5 Connection Establishment, 128- System Network Architecture, see 31 SNA Connection Release, 132-4 explicit, 133-4 Technical Office Protocols (TOP), implicit, 132-3 134, 158 Data Transfer, 131-2 timeout Expedited Data Transfer, 132 in Data Link Layer, 70-1 time sequence diagram, 43, 45 user data (UD), 38-40 Token Bus, 48, 60-1, 87, 205 see also Physical Layer V Series recommendations, 6, 56- Token Ring, 9, 49, 60-1, 72, 87, 7 200 see also Physical Layer see also Physical Layer V.24, 51, 56-7 tokens V.28, 56 in Session Layer, 141-2 virtual call, 93 TOP, see Technical Office Proto• , 20, 93 cols virtual filestore, 189-90 transaction processing, 202 see also File Transfer, Access OSI transaction processing and Management (OSI TP), 180, 194, 196 virtual terminal, 186-7 transport Virtual Terminal services (VT), entity, 90, 95-6, 100, 119 180, 186-9, 202, 204 flow control, 123-5, 131 protocol data unit (TPDU), WAN (wide area network), 7-8 117, 123-34 quality of service, 118-19 X.21, 56-7 service, 125 see also Physical Layer classes of, 120-3, 128-9, X.21 his, 56-7 132, 199, 201 see also Physical Layer connectionless, 134 X.25,9,27,56,77,86,95, 108-13, connection-oriented, 134 201 service access point (TSAP), see also Network Layer 123-4 X.400 service data unit (TSDU), message handling syst.eml'l (MHS), 119-20, 134 27, 134, 176, 196, 206-9 Transport Layer, 115-35 zero-bit insertion, 78-9 and Network Layer, 120-3