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OSI protocols

Information exchange standards

Wikipedia contributorsAdapted for Fathomly · View authors
5 min read
Source updated December 31, 2025

The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T. The standardization process began in 1977.

While the seven-layer OSI model 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.500, and IS-IS have achieved lasting impact. The goal of an open-standard protocol suite instead has been met by the Internet protocol suite, maintained by the Internet Engineering Task Force (IETF).

01Overview

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.

OSI model
Layer Protocol data unit (PDU) Function
Host
layers		 7 Application Data High-level protocols such as for resource sharing or remote file access, e.g. HTTP.
6 Presentation Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption
5 Session Managing communication sessions, i.e., continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes
4 Transport Segment Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing
Media
layers		 3 Network Packet, Datagram Structuring and managing a multi-node network, including addressing, routing and traffic control
2 Data link Frame Transmission of data frames between two nodes connected by a physical layer
1 Physical Bit, Symbol Transmission and reception of raw bit streams over a physical medium

02Layer 1: physical layer

This layer deals with the physical plugs, sockets, electrical/optical specifications and the required line codes.

The physical layer includes the medium over which the digital signals are transmitted. It can be twisted pair, coaxial cable, optical fiber, wireless, or other transmission media.

04Layer 3: network layer

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.

05Layer 4: transport layer

The connection-mode and connectionless-mode transport services are specified by ITU-T Rec. X.214 [ISO/IEC 8072]; the protocol that provides the connection-mode service is specified by ITU-T Rec. X.224 [ISO/IEC 8073], and the protocol that provides the connectionless-mode service is specified by ITU-T Rec. X.234 [ISO/IEC 8602].

  • 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.

06Layer 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, and 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.

07Layer 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.

08Layer 7: application layer

Common-Application Service Elements (CASEs)

This keeps track of how each application talks to another application. Destination and source addresses are linked to specific applications.

Application processes

09Routing protocols

Watch videos about OSI protocolsExplainers and documentaries on YouTube (opens in a new tab)

Sources and credits

This article is adapted from the Wikipedia article “OSI protocols”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

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