GETTING THE MOST OUT OF ONE ASPECT OF YOUR DESIGN NEED NOT MEAN GIVING UP ANOTHER.

Implementations of single protocols, such as the SS7 (Signaling System Seven) telephony protocol often split into many layers. Definitions of other protocols specify individual or groups of layers, which may combine to meet the target application, such as with Sigtran (signaling transport). In most cases, a protocol layer feeds a higher protocol layer, an application, or a user at a terminal. Similarly, a protocol layer likely uses the services of a lower protocol layer or a physical device. Thus, a typical protocol layer comprises an upper layer interface-processing module, a state-machine engine that provides the layer's processing capability, and a lower layer interface-processing module.

A single protocol layer may have to serve multiple users accessing either multiple lower layer services or multiple instances of a single service (Figure 1). The interface portions of the protocol layer extend beyond the limits of the layer itself. The user portion contains both the layer's public API (application-program interface) and the layer's functions that implement the API. The layer's service portion contains both the lower layer public API and the functions that access it (Figure 2).

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