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Definition: OSI model


(Open Systems Interconnection model) The International Standards Organization's OSI model serves as a standard template for describing a network protocol stack (see OSI and ISO). For comparisons between the OSI model and other protocol stacks, see TCP/IP, SDVoE, ATM, NetWare, SNA and SS7.






Upper Layers
Layers 7 through 4 comprise the upper layers of the OSI protocol stack. They are more geared to the type of application than the lower layers, which are designed to move packets, no matter what they contain, from one place to another.

Application Layer 7
This top layer defines the language and syntax that programs use to communicate with other programs. It represents the purpose of communicating in the first place. For example, a program in a client workstation uses commands to request data from a program in the server. Common functions at this layer are opening, closing, reading and writing files, transferring files and email messages, executing remote jobs and obtaining directory information about network resources.

Presentation Layer 6
When data are transmitted between different types of computer systems, the presentation layer negotiates and manages the way data are represented and encoded. For example, it provides a common denominator between ASCII and EBCDIC machines as well as between different floating point and binary formats. Sun's XDR and OSI's ASN.1 are two protocols used for this purpose. This layer is also used for encryption and decryption.

Session Layer 5
Provides coordination of the communications in an orderly manner. It determines one-way or two-way communications and manages the dialog between both parties; for example, making sure that the previous request has been fulfilled before the next one is sent. It also marks significant parts of the transmitted data with checkpoints to allow for fast recovery in the event of a connection failure. In practice, this layer is often incorporated into the transport layer.

Transport Layer 4
Responsible for overall end-to-end validity and integrity of the transmission. The lower layers may drop packets, but the transport layer performs a sequence check on the data and ensures that if a 12MB file is sent, the full 12MB is received.

"OSI transport services" include layers 1 through 4, collectively responsible for delivering a complete message or file from sending to receiving station without error.

Lower Layers
Layers 3 through 1 move packets from the sending station to the receiving station.

Network Layer 3
Establishes the route between sender and receiver across switching points, which are typically routers. The most ubiquitous example is the IP protocol in TCP/IP (see TCP/IP). IPX, SNA and AppleTalk are examples of earlier routable protocols, which means that they included a network and station address in their addressing system. If all stations are contained within a single network segment, then the routing capability in this layer is not required. This layer is also the switching function of the dial-up telephone system. See layer 3 switch.

Data Link Layer 2
Responsible for node-to-node validity and integrity of the transmission. The transmitted bits are divided into frames, most commonly Ethernet. Layers 1 and 2 are required for every type of communications. For more on this layer, see data link protocol.

Physical Layer 1
Passes bits onto and receives bits from the connecting medium (wire, fiber). This layer has no understanding of the meaning of the bits, but deals with the electrical characteristics of the signals and signaling methods.




The TCP/IP Protocol Stack
Using TCP/IP as a model, the sending application (top) hands data to the transport layer, which breaks it up into the packet format required by the network. It stores the sequence number and other data in the packet headers. The network layer adds source and destination data in its header, and the data link layer adds station data. At the receiving side, the corresponding layer reads the headers and discards them.