nterconnection model) The OSI model is an International Standards Organization (ISO) template for a layered network. It describes a "protocol stack," which is a set of rules and formats that interact with each other. For other protocol stacks, see TCP/IP
. See OSI
Layers 7 through 4 are geared more to the application than the lower layers, which are designed to move packets from one place to another no matter what they contain.
Application Layer 7
This top layer defines the language and syntax programs use to communicate with other programs. Layer 7 is 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 Web pages and email messages, making queries, executing remote jobs and obtaining directory information about network resources.
Presentation Layer 6
When data are transmitted between different types of computer systems, Layer 6 manages the conversion. For example, Layer 6 converts between ASCII and EBCDIC data codes as well as between floating point and binary formats. This layer is also used for encryption and decryption. See ASCII
Session Layer 5
Layer 5 provides coordination. The session layer determines one-way or two-way communications. It may mark 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.
Layers 3 through 1 move packets from the sending station to the receiving station.
Network Layer 3
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
Layer 2 is 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
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. See TCP/IP