OSI Model: Vertical Interface Terminology (103951)
This article was previously published under Q103951
SUMMARY
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.
| Modification Type: |
Major |
Last Reviewed: |
7/30/2001 |
| Keywords: |
KB103951 |
|