The Osi Framework; D.1. The 7 Layers Of The Osi Model - D-Link NetDefend DFL-210 User Manual

Appendix D. The OSI Framework
The Open Systems Interconnection Model defines a framework for intercomputer communications.
It categorizes different protocols for a great variety of network applications into seven smaller, more
manageable layers. The model describes how data from an application in one computer can be
transferred through a network medium to an application on another computer.
Control of data traffic is passed from one layer to the next, starting at the application layer in one
computer, proceeding to the bottom layer, traversing over the medium to another computer and then
delivering up to the top of the hierarchy. Each layer handles a certain set of protocols, so that the
tasks for achieving an application can be distributed to different layers and be implemented
independently.
Figure D.1. The 7 layers of the OSI model
Layer number
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
The different layers perform the following functions:
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data-Link Layer
Physical Layer
Layer purpose
Application
Presentation
Session
Transport
Network
Data-Link
Physical
Defines the user interface that supports applications directly.
Protocols: HTTP, FTP, DNS, SMTP, Telnet, SNMP, etc.
Translates the various applications to uniform network formats that the
rest of the layers canunderstand.
Establishes, maintains and terminates sessions across the network.
Protocols: NetBIOS, RPC, etc.
Controls data flow and provides error-handling. Protocols: TCP, UDP,
etc.
Performs addressing and routing. Protocols: IP, OSPF, ICMP, IGMP,
etc.
Creates frames of data for transmission over the physical layer and
includes error checking/correction. Protocols: Ethernet, PPP, etc.
Defines the physical hardware connection.
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