OSI protocols
Encyclopedia
The Open Systems Interconnection (OSI) protocols are a family of information exchange standards developed jointly by the ISO
and the ITU-T
starting in 1977.
While the seven-layer OSI model
is still often referenced, of the protocols themselves only X.400
and X.500
have had much lasting impact. The goal of a series of open, non-proprietary network protocols is now met by the competing TCP/IP stack.
The Manufacturing Automation Protocol
(MAP) user group, focused on real-time control of manufacturing robots of various types, implements layer 1 (physical), a two-sublayer layer 2 (data link) with LLC Type 3 on top of the medium access layer, and then the layer 7 Manufacturing Message System on top. Layers 3 to 6 are not present. This stack is intended just for the robots themselves; the robot controller would load files with a full seven-layer stack with FTAM
file transfer on top. Parts of the Signaling System 7 stack are OSI derivatives.
This is the medium over which the digital signals are transmitted. It can be twisted pair
, coaxial cable
, optical fiber
, wireless
, or other transmission media.
packages raw bits from the physical layer into frames
(logical, structured packets for data). It is specified in ITU-T Rec. X.212[ISO/IEC 8886 ], ITU-T Rec. X.222 and others. This layer is responsible for transferring frames from one host to another. It might perform error checking.
This level is in charge of transferring data between systems in a network, using network-layer addresses of machines to keep track of destinations and sources. This layer uses routers and switches to manage its traffic (control flow control, error check, routing etc.).
[ ISO/IEC 8072[ ISO/IEC 8073[ ISO/IEC 8602
The transport layer transfers data between source and destination processes. Generally, two connection modes are recognized, connection-oriented or connectionless. Connection-oriented service establishes a dedicated virtual circuit and offers various grades of guaranteed delivery, ensuring that data received is identical to data transmitted. Connectionless mode provides only best-effort service without the built-in ability to correct errors, which includes complete loss of data without notifying the data source of the failure. No logical connection, and no persistent state of the transaction exists between the endpoints, lending the connectionless mode low overhead and potentially better real-time performance for timing-critical applications such as voice and video transmissions.
The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex
operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.
This layer defines and encrypts/decrypts data types from the application layer. Protocols such as MIDI, MPEG, and GIF are presentation layer formats shared by different applications.
This keeps track of how each application talks to another application. Destination and source addresses are linked to specific applications.
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
and the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
starting in 1977.
While the seven-layer OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
is still often referenced, of the protocols themselves only X.400
X.400
X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems — more commonly known as "email"....
and X.500
X.500
X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by ITU-T, formerly known as CCITT, and first approved in 1988. The directory services were developed in order to support the requirements of X.400 electronic mail exchange and...
have had much lasting impact. The goal of a series of open, non-proprietary network protocols is now met by the competing TCP/IP stack.
Overview
OSI protocols stacks are split into seven layers. The layers form a hierarchy of functionality starting with the physical hardware components to the user interfaces at the software application level. Each layer receives information from the layer above, processes it and passes it down to the next layer. Each layer adds its own encapsulation information (header) to the incoming information before it is passed to the lower layer. Headers generally include address of destination and source, check sums (for error control), type of protocol used in the current layer, and other options such as flow control options and sequence numbers (used to ensure data is sent in order).The Manufacturing Automation Protocol
Manufacturing Automation Protocol
Manufacturing Automation Protocol was a computer network standard released in 1982 for interconnection of devices from multiple manufacturers. It was developed by General Motors to combat the proliferation of incompatible communications standards used by suppliers of automation products such as...
(MAP) user group, focused on real-time control of manufacturing robots of various types, implements layer 1 (physical), a two-sublayer layer 2 (data link) with LLC Type 3 on top of the medium access layer, and then the layer 7 Manufacturing Message System on top. Layers 3 to 6 are not present. This stack is intended just for the robots themselves; the robot controller would load files with a full seven-layer stack with FTAM
FTAM
FTAM, ISO standard 8571, is the OSI Application layer protocol for File Transfer Access and Management.The goal of FTAM is to combine into a single protocol both file transfer, similar in concept to the Internet FTP, as well as remote access to open files, similar to NFS...
file transfer on top. Parts of the Signaling System 7 stack are OSI derivatives.
Layer 1: physical layer
This layer deals with the physical plugs and sockets and electrical specification of signals.This is the medium over which the digital signals are transmitted. It can be twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
, coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
, optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
, or other transmission media.
Layer 2: data link layer
The data link layerData link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
packages raw bits from the physical layer into frames
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
(logical, structured packets for data). It is specified in ITU-T Rec. X.212
Layer 3: network layer
- Connectionless Network Service (CLNS) – ITU-T Rec. X.213
[ ISO/IEC 8348]. SCCP is based on X.213. - Connectionless Network Protocol (CLNP) – ITU-T Rec. X.233
[ ISO/IEC 8473-1]. - Connection-Oriented Network Service (CONS) – ITU-T Rec. X.213
[ ISO/IEC 8348]. - Connection-Oriented Network Protocol (X.25X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
) – ITU-T Rec. X.233[ ISO/IEC 8878]. This is the use of the X.25 X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
protocol to provide the CONS. - Network Fast Byte Protocol – ISO/IEC 14700
- End System to Intermediate System Routing Exchange Protocol (ES-IS) - ISO/IEC 9452 (reprinted in RFC 995).
- Intermediate System to Intermediate System Intra-domain Routing Protocol (IS-ISIS-ISIntermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
) - ISO/IEC 10589 (reprinted in RFC 1142), later adapted for the TCP/IP model. - End System Routing Information Exchange Protocol for use with ISO/IEC 8878 (SNARE) – ITU-T Rec. X.116
[ ISO/IEC 10030].
This level is in charge of transferring data between systems in a network, using network-layer addresses of machines to keep track of destinations and sources. This layer uses routers and switches to manage its traffic (control flow control, error check, routing etc.).
Layer 4: transport layer
The connection-mode and connectionless-mode transport services are specified by ITU-T Rec. X.214- Transport Protocol Class 0 (TP0)
- Transport Protocol Class 1 (TP1)
- Transport Protocol Class 2 (TP2)
- Transport Protocol Class 3 (TP3)
- Transport Protocol Class 4 (TP4)
- Transport Fast Byte Protocol – ISO 14699
The transport layer transfers data between source and destination processes. Generally, two connection modes are recognized, connection-oriented or connectionless. Connection-oriented service establishes a dedicated virtual circuit and offers various grades of guaranteed delivery, ensuring that data received is identical to data transmitted. Connectionless mode provides only best-effort service without the built-in ability to correct errors, which includes complete loss of data without notifying the data source of the failure. No logical connection, and no persistent state of the transaction exists between the endpoints, lending the connectionless mode low overhead and potentially better real-time performance for timing-critical applications such as voice and video transmissions.
Layer 5: session layer
- Session service – ITU-T Rec. X.215
[ISO/IEC 8326 ] - Connection-oriented Session protocol – ITU-T Rec. X.225
[ISO/IEC 8327-1 ] - Connectionless Session protocol – ITU-T Rec. X.235
[ISO/IEC 9548-1 ]
The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex
Simplex circuit
In telecommunication, the term simplex circuit has the following meanings:#A circuit that provides transmission in one direction only.#Deprecated definition: A circuit using ground return and affording communication in either direction, but in only one direction at a time.The above two definitions...
operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.
Layer 6: presentation layer
- Presentation service – ITU-T Rec. X.216
[ISO/IEC 8822 ] - Connection-oriented Presentation protocol – ITU-T Rec. X.226
[ISO/IEC 8823-1 ] - Connectionless Presentation protocol – ITU-T Rec. X.236
[ISO/IEC 9576-1 ]
This layer defines and encrypts/decrypts data types from the application layer. Protocols such as MIDI, MPEG, and GIF are presentation layer formats shared by different applications.
Common-Application Service Elements (CASEs)
- Association Control Service Element (ACSE) – ITU-T Rec. X.217
[ISO/IEC 8649 ], ITU-T Rec. X.227[ISO/IEC 8650-1 ], ITU-T Rec. X.237[ISO/IEC 10035-1 ]. - Reliable Transfer Service Element (RTSE) – ITU-T Rec. X.218
[ISO/IEC 9066-1 ], ITU-T Rec. X.228[ISO/IEC 9066-2 ]. - Remote Operations Service ElementRemote operations service element protocolThe Remote Operations Service Element is the OSI service interface, specified in , that provides remote operation capabilities, allows interaction between entities in a distributed application, and upon receiving a remote operations service request, allows the receiving entity to attempt the...
(ROSE) – ITU-T Rec. X.219[ISO/IEC 9072-1 ], ITU-T Rec. X.229[ISO/IEC 9072-2 ]. TCAPTransaction Capabilities Application PartTransaction Capabilities Application Part, from ITU-T recommendations Q.771-Q.775 or ANSI T1.114 is a protocol for Signalling System 7 networks. Its primary purpose is to facilitate multiple concurrent dialogs between the same sub-systems on the same machines, using Transaction IDs to differentiate...
is related to X.219. - Commitment, Concurrency, and Recovery service element (CCRSE)
- Security Exchange Service Element (SESE)
This keeps track of how each application talks to another application. Destination and source addresses are linked to specific applications.
Application processes
- Common management information protocolCommon management information protocolThe Common Management Information Protocol is the OSI specified network management protocol.Defined in . It provides an implementation for the services defined by the Common Management Information Service specified in , allowing communication between network management applications and...
(CMIP) – ISO 9596 / X.700 - Directory services (DS) – X.500X.500X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by ITU-T, formerly known as CCITT, and first approved in 1988. The directory services were developed in order to support the requirements of X.400 electronic mail exchange and...
, later modified for the TCP/IP stack as LDAP - File transfer, access, and management (FTAM)
- Message handling system (MHS) – X.400X.400X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems — more commonly known as "email"....
- Virtual terminal protocol (VT) - ISO 9040/9041
- Remote Database AccessRemote Database Access- Purpose :RDA describes the connection of a database client to a database server. It includes features for* communicating database operations and parameters from the client to the server,* in return, transporting result data from the server to the client,...
(RDA) - Distributed Transaction Processing (OSI TP)
- Interlibrary Loan Application Protocol (ILAP)
- Document Transfer And Manipulation (DTAM)
- Document Printing Application (DPA)
- Document Filing and Retrieval (DFR)
Routing protocols
- Intermediate System to Intermediate System (IS-IS) – ISO 10589 (reprinted in RFC 1142)
- End System to Intermediate System (ES-IS) – ISO 9542 (reprinted in RFC 995)
- Interdomain Routing Protocol (IDRP) – ISO 10747
See also
- Internet protocol suiteInternet protocol suiteThe Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
- OSI modelOSI modelThe Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
- Protocol stackProtocol stackThe protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
- TCP/IP model
- WAP protocol suite
- X.25 protocol suite