The OSI model reference article from the English Wikipedia on 24-Jul-2004
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OSI model

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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.

This OSI model is roughly adhered to in the computing and networking industry. Its main feature is in the junction between layers which dictates the specifications on how one layer interacts with another. This means that a layer written by one manufacturer can operate with a layer from another. (Assuming that the specification is interpreted correctly.) These specifications are typically known as Request for Comments or "RFC"s in the TCP/IP community. They are ISO standards in the OSI community.

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 most common 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 of 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 Table of Examples
5 Humor
6 See also

Description of Layers

The mnemonics "People Design Networks To Send Packets Accurately", "Please Do Not Throw Sausage Pizza Away", and "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.

The DoD model, developed in the 1970s for DARPA, is a 4-layer model that maps clearly to current common internet protocols. It is based on a more "pragmatic" approach to networking than OSI.

Interfaces

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).

Table of Examples

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Layer Misc. Examples TCP/IP suite AppleTalk suite OSI suite IPX suite SNA UMTS
7 - Application HL7 HTTP, SMTP, SNMP, FTP, Telnet, NTP, RIP, OSPF, BGP AFP, PAP FTAM, X.400, X.500, DAP   APPC  
6 - Presentation TDI, ASCII, EBCDIC, MIDI, MPEG XDR AFP, PAP        
5 - Session Named Pipes, NetBIOS, SIP, SAP, SDP   ASP, ADSP, ZIP   NWLink DLC?  
4 - Transport NetBEUI TCP, UDP, RTP
 
ATP, NBP, AEP, RTMP   SPX, RIP    
3 - Network NetBEUI, Q.931 IP, ICMP, IPsec, ARP DDP   IPX   RRC (Radio Resource Control)
2 - Data Link Ethernet, Token Ring, FDDI, PPP, HDLC, Q.921, Frame Relay, ATM, Fibre Channel   LocalTalk, TokenTalk, EtherTalk Token Bus 802.3 framing, Ethernet II framing SDLC MAC (Media Access Control)
1 - Physical RS-232, V.35, V.34, Q.911, T1, E1 10BASE-T, StarLAN   Localtalk on shielded, Localtalk on unshielded ("phone talk")     Twinax PHY (Physical Layer)

Humor

The 7 layer model has often been extended in a humorous manner, to refer to non-technical issues or problems. A common joke is the 9 layer model, with layers 8 and 9 being the "financial" and "political" layers.

Network technicians will sometimes refer euphemistically to "layer-eight problems," meaning problems with an end user and not with the network.


Adapted from Federal Standard 1037C and previous Wikipedia content.

See also