OSI Model: Vertical Interface Terminology

VERTICAL INTERFACE TERMINOLOGY

The active elements in each network layer are called entities, and are typically implemented by means of a software process. Entities in the corresponding layers on different machines are called peer entities.

The layer N-1 entities implement the services used by layer N. These services are exposed by means of "well-known" service access points (SAPs). A SAP is actually just a mechanism, such as an API, MS-DOS interrupt, or a direct entry point address, by which the layer above can programmatically access a particular function (service) implemented by the layer below.

For data transfer services, OSI defines a standard set of rules that govern the interface between layers:

• The layer N entity passes an interface data unit (IDU) to the layer N-1 entity by means of the appropriate service access point (SAP).

      - The IDU consists of a protocol data unit (PDU) and some
        interface control information (ICI).
  

      - The PDU is the data that the layer N entity wishes to
        pass across the network to its peer entity, it consists
        of the layer N header and the data that layer N received
        from the layer above (N+1).
  

      - The layer N PDU becomes the layer N-1 service data unit
        (SDU)--the data unit that will be "serviced by" layer N-1.
  

      - The ICI is control information, such as the length of
        the SDU, that the layer below needs to do its job.
  

• When layer N-1 receives the layer N IDU, it "strips off" and "considers" the ICI, adds its header information (for its peer entity across the network), adds ICI for the layer below, and passes the resulting IDU to the layer N-2 entity by means of the appropriate SAP.