DOCUMENT:Q103951 30-JUL-2001 [crossnet] TITLE :OSI Model: Vertical Interface Terminology PRODUCT :Windows for Workgroups and Windows NT Networking Issues PROD/VER: OPER/SYS: KEYWORDS: ====================================================================== 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). 1. The IDU consists of a protocol data unit (PDU) and some interface control information (ICI). 2. 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). 3. The layer N PDU becomes the layer N-1 service data unit (SDU)--the data unit that will be "serviced by" layer N-1. 4. 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. Additional query words: ====================================================================== Keywords : ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.