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 the 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 needed 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 (Media Access Control) 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 have packets transported reliably and without error. This is where you find the TCP (Transmission Control 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 uses different data formats can exchange information. In other words, the presentation layer transforms data so that the application layer can understand it.
7. Application – Allows applications to request network services. This layer represents the programming interfaces programs such as Microsoft Office or QuickBooks 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.