The protocol for each layer is concerned with providing a peer-to-peer service with the corresponding layer at the other end of the path (a hop for the lower three layers, end-to-end for the upper four). Each layer uses the services of the layers below it, by communicating via a Service Access Point (SAP).
Peer to peer communication using the services of a lower layer
During peer-to-peer communication, information at the sender (i.e. a Protocol Data Unit, PDU) flows down through each of the lower layers in the same node. At the lowest (physical layer) the information passes over the communications cable to the corresponding physical layer entity.
When information is received, the information (a Service Data Unit, SDU) is passed up to the next higher layer.
The boundaries between adjacent layers in the same system are called Interfaces. Service Primitives are used to pass the information, and the protocol entity to which the information is delivered is called a Service Access Point (SAP).
Examples of SAPs are the type field in the Medium Access Control (MAC) protocol, the address field in HDLC, the protocol field in the IP network header, and the port identifier in UDP and TCP.