2.1 Protocol Layers

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 that use 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).

Each layer can be independent, transparent to the layers above and below. The only requirement is that the interface used to communicate between the layers must be compatible with the adjacent layers.

The operating system provides the interface between layers, which helps provide some standardization. In the upper layers, a proprietary interface is typically used to support the specific functions of the protocol. The lower layers can be connection-oriented, whereas the upper layers...