The O.S.I. Model And What It Has To Do With Networks



Posted: Sunday, November 01, 2009

by
pcinfospace

I should define the word packet before I start. A packet is a message that is sent over the network from one node (Computer) to another node. The packet includes the address of the node that sent the packet, the address of the node the packet is being sent to and data.

I feel that a person that uses networks or wants to understand how they work should at least read about this concept.

It is a "model" because it is a general theory. Defined: O.S.I. stands for Open System Interconnection Reference Model. It is a description of layered communications and computer network protocol design. A protocol is a language need to communicate between one device or piece of hardware and another. It is normally integrated into the operating system of the computer.

The model divides the Network into seven layers. These layers are: Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. Each layer provides service to the layer above it.

The seven layers are:

1. Physical Controls the layout of cables and devices such as repeaters, hubs and Network Interface Cards.

2. Data Link Provides MAC addresses to uniquely identify network nodes and a means for data to be sent over the physical layer in the form of packets. Bridges and switches are layer 2 devices.

3. Network Handles routing network messages from one computer to another.

4. Transport Concerns the transfer of information from one computer to another. The purpose is to that packets are transported reliably and without error. This is where you find the TCP protocol.

5. Session Establishes sessions between network applications. Session layers include: Simplex,

Half-duplex and Full-duplex.

6. Presentation Converts data so that systems that use different data formats can exchange information.

7. Application Allows applications to request network services. This layer represents the programming interfaces programs such as Microsoft Office or QuickBooks use to request network services. Some Application Layer protocols include: DNS, FTP,SMTP ,SMB, NFS and Telnet.

The above information may or may not be easy to understand. It is however, a general definition of what every bit and packet of information goes through. It could serve as a means of troubleshooting. If you understand which layer is causing the problem you may be able to zero in on why the network isn't working.

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