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The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
. The standardization process began in 1977. While the seven-layer
OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
is often used as a reference for teaching and documentation, the protocols originally conceived for the model did not gain popularity, and only
X.400 X.400 is a suite of ITU-T Recommendations that defines the ITU-T Message Handling System (MHS). At one time, the designers of X.400 were expecting it to be the predominant form of email, but this role has been taken by the SMTP-based Internet e-m ...
,
X.500 X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by the ITU-T, Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). ITU-T was former ...
, and IS-IS have achieved lasting impact. The goal of an open-standard protocol suite instead has been met by the
Internet protocol suite The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the sui ...
, maintained by the
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and ...
(IETF).


Overview

The OSI protocol stack is structured into seven conceptual 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 encapsulation information ( header) to the incoming information before it is passed to the lower layer. Headers generally include address of source and destination, error control information, protocol identification and protocol parameters such as flow control options and sequence numbers.


Layer 1: physical layer

This layer deals with the physical plugs and sockets and electrical specification of signals only. This is the medium over which the digital signals are transmitted. It can be
twisted pair Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...
,
coaxial cable Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a ...
,
optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
,
wireless Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The mos ...
, or other transmission media.


Layer 2: data link layer

The
data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...
packages raw bits from the physical layer into
frames A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (co ...
(logical, structured packets for data). It is specified i
ITU-T Rec. X.212 [ISO/IEC 8886
/nowiki>">SO/IEC 8886">ITU-T Rec. X.212 [ISO/IEC 8886
/nowiki>br>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 layer further consists of two sublayers:
MAC and Logical link control">LLC A limited liability company (LLC for short) is the US-specific form of a private limited company. It is a business structure that can combine the pass-through taxation of a partnership or sole proprietorship with the limited liability of a ...
.


Layer 3: network layer

* Connectionless Network Service (CLNS) �
ITU-T Rec. X.213 [ISO/IEC 8348
/nowiki>]. Signalling Connection Control Part, SCCP is based on X.213. *
Connectionless Network Protocol Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI network layer datagram service that does not require a circuit to be established before data is transmitted, and routes messages to their destinati ...
(CLNP) �
ITU-T Rec. X.233 [ISO/IEC 8473-1
/nowiki>">/nowiki>ISO/IEC 8473-1">ITU-T Rec. X.233 [ISO/IEC 8473-1
/nowiki> * Connection-Oriented Network Service (CONS) �
ITU-T Rec. X.213 [ISO/IEC 8348
/nowiki>]. * Connection-Oriented Network Protocol ( X.25) �
ITU-T Rec. X.233 [ISO/IEC 8878
/nowiki>">/nowiki>ISO/IEC 8878">ITU-T Rec. X.233 [ISO/IEC 8878
/nowiki> This is the use of the X.25 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-IS) - 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
/nowiki>">/nowiki>ISO/IEC 10030">ITU-T Rec. X.116 [ISO/IEC 10030
/nowiki> 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.) So here it takes all routing decisions, it deals with end to end data transmission.


Layer 4: transport layer

The connection-mode and connectionless-mode transport services are specified b
ITU-T Rec. X.214 [ISO/IEC 8072
/nowiki>]; the protocol that provides the connection-mode service is specified b
ITU-T Rec. X.224 /nowiki>ISO/IEC 8073
/nowiki> and the protocol that provides the connectionless-mode service is specified b
ITU-T Rec. X.234 [ISO/IEC 8602
/nowiki>">/nowiki>ISO/IEC 8602">ITU-T Rec. X.234 TP0) * Transport Protocol Class 1 ( TP1) * Transport Protocol Class 2 ( TP2) * Transport Protocol Class 3 (TP3 (protocol)">TP3) * Transport Protocol Class 4 (TP4 (protocol)">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
/nowiki>] * Connection-oriented Session protocol �
ITU-T Rec. X.225 SO/IEC 8327-1
/nowiki>* Connectionless Session protocol �
ITU-T Rec. X.235 [ISO/IEC 9548-1
/nowiki>">SO/IEC 9548-1">ITU-T Rec. X.235 [ISO/IEC 9548-1
/nowiki>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, and half-duplex or Simplex circuit">simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension ...
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
/nowiki>] * Connection-oriented Presentation protocol �
ITU-T Rec. X.226 SO/IEC 8823-1
/nowiki>* Connectionless Presentation protocol �
ITU-T Rec. X.236 [ISO/IEC 9576-1
/nowiki>">SO/IEC 9576-1">ITU-T Rec. X.236 [ISO/IEC 9576-1
/nowiki> 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.


Layer 7: application layer


Common-Application Service Elements (CASEs)

* Association Control Service Element (ACSE) �
ITU-T Rec. X.217 [ISO/IEC 8649
/nowiki>]
ITU-T Rec. X.227 [ISO/IEC 8650-1
/nowiki>">SO/IEC 8650-1">ITU-T Rec. X.227 [ISO/IEC 8650-1
/nowiki>br>ITU-T Rec. X.237 [ISO/IEC 10035-1
/nowiki>]. * Reliable Transfer Service Element (RTSE) �
ITU-T Rec. X.218 SO/IEC 9066-1
/nowiki>br>ITU-T Rec. X.228 [ISO/IEC 9066-2
/nowiki>">SO/IEC 9066-2">ITU-T Rec. X.228 [ISO/IEC 9066-2
/nowiki> * Remote Operations Service Element protocol">Remote Operations Service Element (ROSE) �
ITU-T Rec. X.219 [ISO/IEC 9072-1
/nowiki>]
ITU-T Rec. X.229 [ISO/IEC 9072-2
/nowiki>]. [ TCAP is related to X.219. * Commitment, Concurrency, and Recovery service element (CCRSE) *
Security Exchange Service Element Security is protection from, or resilience against, potential harm (or other unwanted coercive change) caused by others, by restraining the freedom of others to act. Beneficiaries (technically referents) of security may be of persons and social ...
(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 protocol (CMIP) – ISO 9596 /
X.700 The Common Management Information Protocol (CMIP) is the OSI specified network management protocol. Defined iITU-T Recommendation X.711, ISO/IEC International Standard 9596-1 It provides an implementation for the services defined by the Comm ...
* Directory services (DS) –
X.500 X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by the ITU-T, Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). ITU-T was former ...
, later modified for the TCP/IP stack as
LDAP The Lightweight Directory Access Protocol (LDAP ) is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network. Directory serv ...
* File transfer, access, and management (FTAM)FTAM support claimed by IBM, Novell, Sun, Unisys etc
, Joint Interoperability Test Command * Message handling system (MHS) –
X.400 X.400 is a suite of ITU-T Recommendations that defines the ITU-T Message Handling System (MHS). At one time, the designers of X.400 were expecting it to be the predominant form of email, but this role has been taken by the SMTP-based Internet e-m ...
* Virtual terminal protocol (VT) - ISO 9040/9041 * Remote Database Access (RDA) * Distributed Transaction Processing ( OSI TP) * Interlibrary Loan Application Protocol (ILAP) *
Document Transfer And Manipulation A document is a written, drawn, presented, or memorialized representation of thought, often the manifestation of non-fictional, as well as fictional, content. The word originates from the Latin ''Documentum'', which denotes a "teaching" o ...
(DTAM) *
Document Printing Application A document is a writing, written, drawing, drawn, presented, or memorialized representation of thought, often the manifestation of nonfiction, non-fictional, as well as fictional, content. The word originates from the Latin ''Documentum'', w ...
(DPA) * Document Filing and Retrieval (DFR)


Routing protocols

*
Intermediate System to Intermediate System Intermediate System to Intermediate System (IS-IS, also written ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this b ...
(IS-IS) – ISO 10589 (reprinted in RFC 1142) *
End System to Intermediate System End, END, Ending, or variation, may refer to: End *In mathematics: ** End (category theory) ** End (topology) ** End (graph theory) ** End (group theory) (a subcase of the previous) ** End (endomorphism) *In sports and games **End (gridiron footb ...
(ES-IS) – ISO 9542 (reprinted in RFC 995) * Interdomain Routing Protocol (IDRP) – ISO 10747


See also

* Protocol stack * Protocol Wars *
WAP protocol suite Wireless Application Protocol (WAP) is a technical standard for accessing information over a mobile wireless network. A WAP browser is a web browser for mobile devices such as mobile phones that use the protocol. Introduced in 1999, WAP ach ...


References

{{DEFAULTSORT:Osi Protocols Network architecture Reference models