D-Link DFL-2500 - Manual (Page 348)

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 purpose
Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data-Link
Layer 1 Physical
The different layers perform the following functions:
Application Layer Defines the user interface that supports applications directly.
Protocols: HTTP, FTP, DNS, SMTP, Telnet, SNMP, etc.
Presentation Layer Translates the various applications to uniform network formats that the
rest of the layers canunderstand.
Session Layer Establishes, maintains and terminates sessions across the network.
Protocols: NetBIOS, RPC, etc.
Transport Layer Controls data flow and provides error-handling. Protocols: TCP, UDP,
etc.
Network Layer Performs addressing and routing. Protocols: IP, OSPF, ICMP, IGMP,
etc.
Data-Link Layer Creates frames of data for transmission over the physical layer and
includes error checking/correction. Protocols: Ethernet, PPP, etc.
Physical Layer Defines the physical hardware connection.
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