Interfaces and services
Function of each layer is to provide services to layer above it.
Entities: Active elements in each layer are called entities - could be software processes or hardware (chips).
Peer entities: Entities in layer n on two machines are called peer entities
Service user, service provider: Layer n+1 uses a service provided by layer n. Therefore layer n is called service provider and n+1 the service user.
SAP (Service Access Point): Layer n SAP is a point where layer n+1 accesses a service. (e.g., SAP is a socket into which modular phones are plugged). Each SAP has an address (i.e., the telephone number)
IDU, SDU, PDU: Interface Data Unit, Service Data Unit and Protocol Data Unit. See figure on next page.
Network software/Interfaces and services.