Peer-to-Peer Communication

For data to travel from the source to the destination, each layer of the OSI reference model at the source must communicate with its peer layer at the destination. This form of communication is referred to as peer-to-peer communication. During this process, the protocols at each layer exchange information, called protocol data units (PDU), between peer layers.
Data packets on a network originate at a source and then travel to a destination. Each layer depends on the service function of the OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field. It then adds whatever headers the layer needs to perform its function. As the data moves down through Layers 7 through 5 of the OSI reference model, additional headers are added. The grouping of data at the Layer 4 PDU is called a segment.

The network layer provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer moves the data through the internetwork by encapsulating the data and attaching a header to create a datagram (the Layer 3 PDU). The header contains information required to complete the transfer, such as source and destination logical addresses.

The data link layer provides a service to the network layer by encapsulating the network layer datagram in a frame (the Layer 2 PDU). The frame header contains the physical addresses required to complete the data link functions, and the frame trailer contains the FCS. The physical layer provides a service to the data link layer, encoding the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.Network devices such as hubs, switches, and routers work at the lower three layers. Hubs
are at Layer 1, switches are at Layer 2, and routers are at Layer 3.