OSI protocols

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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, X.500, and IS-IS 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.


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 (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.

A good way to memorize all 7 layers of OSI, is to remember the phrase: Please Do Not Throw Sausage Pizza Away. This is PDNTSPA which is the following.

  • Physical
  • Data Link
  • Network
  • Transport
  • Session
  • Presentation
  • Application

Layer 1: physical layer[edit]

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, coaxial cable, optical fiber, wireless, or other transmission media.

Layer 2: data link layer[edit]

The data link layer 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 layer further consists of two sublayers : MAC and LLC.

Layer 3: network layer[edit]

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[edit]

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.

Layer 5: session layer[edit]

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.

Layer 6: presentation layer[edit]

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[edit]

Common-Application Service Elements (CASEs)[edit]

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

Application processes[edit]

Routing protocols[edit]

See also[edit]


  1. ^ FTAM support claimed by IBM, Novell, Sun, Unisys etc, Joint Interoperability Test Command