The OSI Reference Model

The Open Systems Interconnect (OSI) Reference Model has seven layers. Each layer defines a function performed when data is transferred between applications across a network. These layers are usually pictured as a stack of blocks, leading to the common term "protocol stack." Application Layer application programs that use the network Presentation Layer standardizes data presented to the applications Session Layer manages sessions between applications Transport Layer provides error detection and correction Network Layer manages network connections Data Link Layer provides data delivery across the physical connection Physical Layer defines the physical network media Each layer of the stack defines a function that may be performed by any number of protocols. Any given protocol may perform multiple functions. Each protocol communicates with a peer that is an equivalent implementation of the same protocol on a remote system. Each protocol layer is only concerned with communication to a peer at the other end of a link. For example, e-mail is an application level protocol that communicates with a peer e-mail application on a remote system. The e-mail application does not care whether or not the physical layer is a serial modem line or a twisted pair ethernet connection. Information is passed down through the layers until it is transmitted across the network, where it is passed back up the stack to the application at the remote end. Each layer relies on the other layers to perform their functions. The individual layers do not care how the other layers operate. They only need to know how to pass information up or down from one layer to another.