An Extension to State Machine Modeling: The Concept of Coupled State Machines

A layer in a communication system has at least three main interfaces: (1) A Service Access Point (SAP) for pro-viding services to a user of an upper layer, (2) an in-verse SAP for accessing services provided by a lower layer, and (3) a logical interface for communication purposes to a remote peer entity, which we call Connection Endpoint (CEP). The CEP actually specifies the communication pro-tocol. In its entirety, the full specification of the commu-nication layer results in a huge, complex state machine. It would be beneficial to individually specify the different interfaces of the communication layer and smoothly cou-ple the specifications. Current modeling languages like the Unified Modeling Language (UML) do not offer a satisfying solution. In this paper, an extension is presented, which pro-vides an approach to couple finite state machines. It enables a modeler not only to specify different aspects of a commu-nication layer but also to describe their coordination and synchronization. Furthermore, the concept of coupled state machines can be used for other purposes as well, e.g. for specifying Application Programming Interfaces and model-ing the observer pattern