OSI model
Content removed for copyright violation. If this is in error, the contributor should go to Talk and explain.
The OSI Model is a layered model for communications and computer network protocol design, developed as part of the Open Systems Interconnection initative. The model divides the functions of a protocol into a series of layers. Each layer has the property that is 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.
In addition to standards for individual protocols in transmission, there are now also interface standards for different layers to talk to the ones above or below (usually operating-system-specific). For example: Winsock and Berkeley sockets between layers 4 and 5, or NDIS and ODI between layers 2 and 3.
The seven layers of the OSI model, in order from the highest to the lowest layers, are:
- 7: The Application Layer
- 6: The Presentation Layer
- 5: The Session Layer
- 4: The Transport Layer
- 3: The Network Layer
- 2: The Data-Link Layer
- 1: The Physical Layer
In real-world protocols, there is some argument as to where the distinctions between layers are drawn; there is no one correct answer. But roughly:
+- - - - - - -+ 7| Application | e.g. HTTP, SMTP, SNMP, FTP, TELNET, FTAM, | | APPC, X.400, X.500, NCP, AppleTalk, AFP, DAP + - - - - - - + 6| Presentation| e.g. TDI, XDR, SNMP, FTP, Telnet, SMTP, NCP, AFP + - - - - - - + 5| Session | e.g. NWLink, NBT, Named Pipes, NetBIOS, | | ASP, ADSP, ZIP, PAP, DLC + - - - - - - + 4| Transport | e.g. TCP, UDP, SPX, | | NetBEUI, ATP, NBP, AEP, RTMP +- - - - - - -+ 3| Network | e.g. IP, IPX, NWLink, NetBEUI, | | DDP + - - - - - - + 2| Data Link | e.g. Ethernet, Token Ring, PPP, ODI, NDIS, | | LocalTalk, TokenTalk, EtherTalk +- - - - - - -+ 1| Physical | e.g. RS-232, ISDN, 10Base-T, electricity, radio + - - - - - - +