Protocol functions are divided into layers. This layered approach allows for better segregation of protocol functions as well as software modularity. Layers are important in communications networks because they allow software upgrades to be deployed without affecting every node in the network. Only those devices which utilize functions within the specific layer need to be upgraded. In a communications network, the first three layers are the most critical. All layers above the first three have no effect on the network itself. They are resident in end node software and are transparent to network devices (such as routers and switches).

The first layer is commonly referred to as the physical layer. This is where the user data and the information appended by the protocol are converted to either optical or electrical form and transmitted over the network. In some cases, the physical layer may append information as well (as is the case with T-1). This appended information is referred to as overhead and is usually minimal.

The second layer is known as the data link layer. The function of this layer is to provide node-to-node communications. The protocols which operate at this layer are not concerned wiht the contents of the user data or the data which resides at higher layers. In fact, protocols at this layer have no visibility to a data unit's final destination. The data link's only concern is the transmission of data between any two devices within a network.

The network access layer is responsible for the transfer of data between the host computer and the network. It is the function of this layer to ensure reliable transfer of data from the source, through the network, to the final destination. Addressing for both the source and the destination can be found at this layer.

There are two other layers which interact with the previously discussed layers. They reside at the layers above the network access layer and provide services between the source and the destination. They are not concerned with the transmission of the data unit through the network; they work at a higher level.

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Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 29-35.