The Open Standards Interconnect (OSI) series of models are an attempt by the International Standards Organisation (ISO) to standardise the way that computer systems communicate with each other. Although there are several OSI models, the most widely used one is the OSI reference model. This seven layer model is intended to ensure interoperability between different protocols and methods of communication. The layers of the OSI reference model are: physical, data link, network, transport, session, presentation, and application [Tanenbaum, 1988].
The physical layer (or layer one) is concerned with the transmission of raw binary data over a communications channel. Protocols at this layer define, for example how each side of a communication sends and receives ones and zeros.
The data link layer (or layer two) takes the raw transmission function and converts in into an error free transmission channel. It merely ensures that the stream remains complete, without any regard to structure or content. Most switching devices operate at this layer, as does the Address Resolution Protocol (ARP).
The network layer (layer three) concerns itself with the routing of packets from source to destination and the basic operations of a subnet. Most routers are layer three devices, and the Internet Protocol (IP) resides at layer three.
The transport layer (layer four) has the task of accepting data from the session layer, breaking it into smaller parts if necessary and passing it to the network layer. This layer also reassembles the parts at the destination and ensures that all parts are correctly received. Protocol based switching operates at this level, as does the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
The session layer (layer five) allows users on differing machines to establish sessions between the machines. In addition to ordinary data transfer, the session layer provides enhanced services, such as authentication. This layer is not present in most Internet protocols.
The presentation layer (layer six) ensures that data is encoded in the correct format for the application or transport that is being used. If necessary, this layer performs the necessary translations between different encodings. The IP security protocol, IPSEC, may be an example of the application of layer six.
The application layer (layer seven) is where all end-user applications sit. This layer supports the hundreds of different user protocols used to perform various tasks, such as e-mail, file transfer, et cetera.
This work also makes reference to another, less commonly used, OSI model — the OSI network management model. This model describes the tasks associated with managing modern computer networks, and provides a way to define relationships between various tasks. The network management model is described in Section 2.1.