How the OSI Model Works

A group called the International Standards Organization (ISO) has put together the Open Systems Interconnect (OSI) Reference Model, which is a model that describes seven layers of protocols for computer communications. These layers don't know or care what is on adjacent layers. Each layer, essentially, only sees the reciprocal layer on the other side. The sending application layer sees and talks to the application layer on the destination side. That conversation takes place irrespective of, for example, what structure exists at the physical layer, such as Ethernet or Token Ring. TCP combines the OSI model's application, presentation, and session layers into one which is also called the application layer.

The application layer refers to application interfaces, not programs like word processing. MHS (Message Handling Service) is such an interface and it operates at this level of the OSI model. Again, this segmentation and interface approach means that a variety of email programs can be used on an intranet so long as they conform to the MHS standard at this application interface level.

The presentation layer typically simply provides a standard interface between the application layer and the network layers. This type of segmentation allows for the great flexibility of the OSI model since applications can vary endlessly, but, as long as the results conform to this standard interface, the applications need not be concerned with any of the other layers.

The session layer allows for the communication between sender and destination. These conversations avoid confusion by speaking in turn. A token is passed to control and to indicate which side is allowed to speak. This layer executes transactions, like saving a file. If something prevents it from completing the save, the session layer, which has a record of the original state, returns to the original state rather than allowing a corrupt or incomplete transaction to occur.

The transport layer segments the data into acceptable packet sizes and is responsible for data integrity of packet segments. There are several levels of service that can be implemented at this layer, including segmenting and reassembly, error recovery, flow control, and others.

The IP wrapper is put around the packet at the network or Internet layer. The header includes the source and destination addresses, the sequence order, and other data necessary for correct routing and rebuilding at the destination.

The data-link layer frames the packets-for example, for use with the PPP (Point to Point). It also includes the logical link portion of the MAC sublayer of the IEEE 802.2, 802.3 and other standards.

Ethernet and Token Ring are the two most common physical layer protocols. They function at the MAC (Media Access Control) level and move the data over the cables based on the physical address on each NIC (Network Interface Card). The physical layer includes the physical components of the IEEE 802.3 and other specifications.