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OSI model

The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. It is also called the OSI seven layer model.

The model divides the functions of a protocol into a series of layers. Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. A system that implements protocol behaviour consisting of a series of these layers is known as a 'protocol stack' or 'stack'. Protocol stacks can be implemented either in hardware or software, or a mixture of both. Typically, only the lower layers are implemented in hardware, with the higher layers being implemented in software.

Usually, the implementation of a protocol is layered in a similar way to the protocol design, with the possible exception of a 'fast path' where the commonest transaction allowed by the system may be implemented as a single component encompassing aspects of several layers.

This logical separation of layers makes reasoning about the behaviour of protocol stacks much easier, allowing the design of elaborate but highly reliable protocol stacks. Each layer performs services for the next higher layer, and makes requests of the next lower layer. An implementation several OSI layers is often referred to as a stack (as in TCP/IP stack).

Table of contents
1 Description of Layers
2 Model in Real World
3 Interfaces
4 Examples

Description of Layers

The mnemonics "Please Do Not Throw Sausage Pizza Away" or "All People Seem To Need Data Processing" may help you remember the layers.

Model in Real World

Real-world protocol suites often do not strictly match the seven-layer model. There can be some argument as to where the distinctions between layers are drawn; there is no one correct answer. However, most protocol suites share the concept of three general sections: media, covering layers 1 and 2; transport, covering layers 3 and 4, and application, covering layers 5 through 7.


In addition to standards for individual protocols in transmission, there are also interface standards for different layers to talk to the ones above or below (usually operating-system-specific). For example, Microsoft Windows' Winsock and Unix's Berkeley sockets and System V Streams are interfaces between applications (layers 5 and above) and the transport (layer 4). NDIS and ODI are interfaces between the media (layer 2) and the network protocol (layer 3).


Layer Misc. Examples TCP/IP suite AppleTalk suite OSI suite IPX suite SNA
7 - Application HTTP, SMTP, SNMP, FTP, Telnet, RIP, OSPF, BGP AFP, PAP FTAM, X.400, X.500, DAP RIP APPC
6 - Presentation TDI XDR, SNMP, FTP, Telnet, SMTP AFP, PAP
5 - Session Named Pipes, NetBIOS ASP, ADSP, ZIP NWLink DLC?
3 - Network NetBEUI IP, ICMP DDP IPX
2 - Data Link Ethernet, Token Ring, FDDI, PPP, HDLC, Frame Relay, ATM, Fibre Channel LocalTalk, TokenTalk, EtherTalk Token Bus 802.3 framing, Ethernet II framing SDLC
1 - Physical RS-232, V.35, V.34, ISDN, 10BASE-T, StarLAN Localtalk on shielded, Localtalk on unshielded ("phone talk") Twinax

Adapted from Federal Standard 1037C and previous Wikipedia content.