Because of the complexity of networking, the Open System Interconnection or OSI, 7-layer model was developed to compartmentalize the functions of a network into layers. The layers consist of the Physical layer, corresponding to the actual electrical connections or radio transmitters/receivers, the Datalink layer, which corresponds to the ability to send and receive data "frames", as well as uniquely identify workstations, the Network layer, which allows the transport of routable data "packets", making network-to-network communication possible, the Transport layer comes next, defining various types of packet flow. The Session, Application, and Presentation layers, are usually mixed together somewhat and are present in the applications that you run over the network.