OSI Reference Model

   A networking reference model defined by the ISO (International Organization for Standardization) that divides computer-to-computer communications into seven connected layers. Such layers are known as a protocol stack.

   Each successively higher layer builds on the functions of the layers below, as follows:

   • Application layer 7

   The highest level of the model. It defines the manner in which applications interact with the network, including database management, e-mail, and terminal-emulation programs.

   • Presentation layer 6

   Defines the way in which data is formatted, presented, converted, and encoded.

   • Session layer 5

   Coordinates communications and maintains the session for as long as it is needed, performing security, logging, and administrative functions.

   • Data-link layer 2

   Validates the integrity of the flow of data from one node to another by synchronizing blocks of data and controlling the flow of data.

   • Transport layer 4

   Defines protocols for structuring messages and supervises the validity of the transmission by performing some error checking.

   • Network layer 3

   Defines protocols for data routing to ensure that the information arrives at the correct destination node.

   • Physical layer 1

   Defines the mechanism for communicating with the transmission medium and interface hardware.