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 Open Systems Interconnection (OSI) model is 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.
By defining standard interfaces between each of the seven layers, an individ-
ual layer only has to know about the interface to the layer above, to the layer
below, and to the same layer on the other end of the network. This interface
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.
