Interfaces and Services

The function of each layer is to provide services to the layer above it. The active elements in each layer are often called entities. An entity can be a software entity ( such as a process ) or a hardware entity ( such as an intelligent 1/0 chip ). Entities in the same layer on different machines are called peer entities. The entities in layer n implement a service used by layer n+1. In this case layer n is called the service provider and layer n+1 is called the service user. Layer n may use the services of layer n-1 in order to provide its service. It may offer several classes of service, for example, fast, expensive communication and slow, cheap communication.

Services are available at SAPs (Service Access Points), the layer n SAP is the places where layer n+1 can access the services offered. Each SAP has an address that uniquely identifies itself. To make this point clearer, the SAPs in the telephone system are the sockets into which modular telephones can be plugged and the SAP addresses are the telephone numbers of these sockets. To call someone, caller's SAP address should be known. Similarly, in the postal system, the SAP addresses are street addresses and post office box numbers. To send a letter, the addressee's SAP address should be known.

In order for two layers to exchange information, there has to be an agreed upon set of rules about the interface. At a typical interface, the layer n+1 entity passes an IDU (Interface Data Unit) to the layer n entity through the SAP. The IDU consists of an SDU (Service Data Unit) and some control information. The SDU is the information passed across the network to the peer entity and then up to layer n+1. The control information is needed to help the lower layer do its job (e.g. the number of bytes in the SDU) but is not part of the data itself.

In order to transfer the SDU, the layer n entity may have to fragment it into several pieces, each of which is given a header and sent as a separate PDU (Protocol Data Unit) such as a packet. The PDU headers are used by the peer entities to carry out their peer protocol. They identify which PDUs contain data and which contain control information, provide sequence numbers and counts, and so on.