
61200305L1-1 ATLAS 550 User Manual B-1
Appendix B
OSI Model and Frame Relay
Technology Overview
This chapter discusses the OSI Model, Frame Relay Protocol, and Transpar-
ent Bit Oriented Protocol (TBOP).
OSI MODEL
The OpenSystems Interconnection (OSI) modelis an internationally accept-
ed standard for communication between multiple vendors’ communication
equipment. It relies on a seven-layer model to allow communication be-
tween communication equipment. Table B-1 describes these layers.
Bydefiningstandardinterfacesbetweeneachofthesevenlayers,anindivid-
uallayer onlyhasto knowaboutthe interfaceto thelayerabove,to the layer
below, and tothe samelayeron theotherend ofthe network.Thisinterface
definition simplifies the process of networking.
Table B-1. Seven-Layer OSI Model
Layer Title Description
Layer 7 Application Contains functions for end-user services. These include FTP, remote file
access, and network management. This is not the application, but the
interface.
Layer 6 Presentation Provides transparent communication by creating code and syntax
compatibility between systems.
Layer 5 Session Takes care of the communication facility provided by the transport layer
(layer 4). Allows sessions to be established, recovered, and terminated.
Layer 4 Transport Provides some error correction and end-to-end flow control. Also decides
best route for the information being transmitted.
Layer 3 Network Determines the method for transmitting data and also deals with routing
the data between networks. Moves data based on addressing.
Layer 2 Data Link Deals with procedures and protocols for controlling the transmission line.
Provides some error detection and correction.
Layer 1 Physical Deals with the electrical, mechanical, and functional control of sending
data over the transmission lines.
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