In the OSI Layer (or Stack) layer 1, the physical layer, defines the physical and electrical characteristics of the transmission. This includes the transmission medium, connectors, and electrical characteristics of the signal.
Layer 2, the data link layer, defines link access management protocols, including transmission, framing, and error control functions. The data link layer addresses only a point-to-point connection between two adjacent nodes in a network.
Layer 3, the network layer, provides network routing, management, and maintenance. This includes end-to-end maintenance of a call, error checking, network monitoring, and packet assembly/disassembly. Layer 3 also performs many other functions central to operation and maintenance of the network.
Layer 4, the transport layer, is the first layer at which the application using the network becomes important. Many computers run multiple applications simultaneously or run multiple sessions of a single application. The transport layer translates between the single data stream of the network layer and multiple data streams of the session layer.
Layer 5, the session layer, controls the exchange of data between two programs running on different computers. When two programs communicate, they set up a "session.” Because computers can run multiple programs or multiple instances of the same program, it is important to set up sessions to track who is communicating with whom.
Layer 6, the presentation layer, encodes and reformats data. It acts as a translator between the programs exchanging information across the network.
Layer 7, the application layer, is the interface between the user’s application software and the network. For example, if database management software running on the user’s computer requires a file located on a remote computer, Layer 7 software will initiate the request to retrieve the remote file.