The OSI Model
Understanding The OSI Model
The Open System Information (OSI) model describes the relationship between the services and protocols necessary for the top down transmission data across a network in whatever form. The function of the seven layers in the OSI model is to move application data reliably across the network.
The OSI model consists of a stack of seven layers which can be thought of as two parts. The bottom four "lower" layers are concerned with the communication of raw data. The top three "higher" layers are concerned with the the networking of applications.
| Layer | Description | Associated Protocols or Services |
| Application | The application layer contains applications that invoke network services. E.g. E-mail, file transfer, web browser & server. | SMTP
- Simple Mail Transfer Protocol.
FTP - File Transfer Protocol. HTTP - Hypertext Transfer Protocol. |
| Presentation | The presentation layer handles the representation of data between nodes and is concerned with the format of stored data. It includes data presentation, data security and data compression. | HTML - Hypertext Mark-up Language. |
| Session | The session layer provides services for higher layers that are used to conduct a session. These include establishing, conducting, managing and ending the session. | |
| Transport | The transport layer transports messages from one end of the network to the other. The transport layer frequently resides at the boundary of the delivery mechanism (public IP network) and the enterprise network. Services provided by the transport layer include addressing, connection management, flow control and buffering. | SPX
- Sequenced Packet Exchange.
TCP - Transmission Control Protocol. |
| Network | The network layer handles all problems associated with transmitting a packet from one node to another. The job of the network layer is send and receive packets. | X.25
- Widely used in Europe.
|
| Data Link | The data link layer is concerned with the transmission of frames, rather than bits, across the physical link. | HDLC
- High Level Data Link Control.
|
| Physical | The physical layer is responsible for the transmission of bits across a communication channel e.g. coaxial cable, fibre optic cable, wireless or satellite. |
Each layer in the OSI model
provides services for the layer below. The layers each are responsible
for "communicating" with adjacent
layers.
Thus, the physical layer is concerned with moving bits and maintaining the physical connection.
The data link layer is responsible for transferring packets within frames across a specific physical link and recovering certain transmission errors that it can detect. The data link layer is only aware of its peer at the termination of the link. Data link layer addressing is concerned with transmitting a frame from one point on a network to the next.
The network layer is aware of several peers, each data link being responsible for moving packets between links and sending them to their final destination. The function of the network layer address is to move a single packet from one end of the network to its final destination.
The transport layer is known as the "keystone" as it aware of the entire network. It is the first end-to-end layer. Its counterpart is located at the packets destination. The transport layer relies on the network layer to move packets across intermediate (third party) nodes.
Combined, the bottom five layers of the OSI model provide capabilities for: -
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Handling differences in various form of data representation between computers.
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Ensuring that data is not stolen and that the network remains secure.
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Making sure that the network's resources are efficiently used.
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Managing dialog through the synchronisation of application activity.
Back 24 June, 2000