|  | 1.1 | The OSI Model
| 1.1.3 | Peer-to-peer communications
Each layer uses its own layer protocol to communicate with its peer layer in another system. Each layer's protocol exchanges information, called protocol data units (PDUs), with its peer layers. A layer can use a more specific name for its PDU. For example, in TCP/IP the transport layer of TCP communicates with the peer TCP function by using segments. Each layer uses the services of the layer below it in order to communicate with its peer layer. The lower layer service uses upper layer information as part of the PDUs that it exchanges with its peer. The TCP segments become part of the network layer packets (datagrams) that are exchanged between IP peers. In turn, the IP packets become part of the data link frames that are exchanged between directly-connected devices. Ultimately, these frames become bits, as the data is finally transmitted by the hardware that is used by the physical layer protocol. Each layer depends on the services of the OSI reference model layer that is below it. In order to provide this service, the lower layer uses encapsulation to put the protocol data unit (PDU) from the upper layer into its data field, then it can add whatever headers and trailers the layer wishes to use to perform its function. As an example, the network layer provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer has the task of moving that data through the internetwork. It accomplishes this task by encapsulating the data within a packet. This packet includes a header containing information that is necessary to complete the transfer, such as source and destination logical addresses. The data link layer in turn provides a service to the network layer. It encapsulates the network layer packet in a frame. The frame header contains information that is necessary to complete the data link functions (e.g. physical addresses). And finally, the physical layer provides a service to the data link layer: It encodes the data link frame into a pattern of 1s and 0s for transmission through the medium (usually a wire). -