7 OSI Layers The physical layer defines all the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. It responds to service requests from the network layer and issues service requests to the physical layer. The network layer is the third layer out of seven in the OSI model and the third layer out of five in the TCP/IP model. In the TCP/IP reference model it is called the Internet layer. In all of the models, the network layer responds to service requests from the transport layer and issues service requests to the data link layer. the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the network layer The session layer is level five of the seven level OSI model. It responds to service requests from the presentation layer and issues service requests to the transport layer. The presentation layer is the sixth level of the seven layer OSI model. It responds to service requests from the application layer and issues service requests to the session layer. The application layer is the seventh level of the seven-layer OSI model, and the fifth layer of the five-layer TCP/IP model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.