The increasing practical importance of data communications and computer networking has generated a noticeable amount of research on formal methods to support the design, specification, validation, simulation, and implementation of data communication protocol systems;This dissertation presents a language, namely State Architecture Notation (SAN), for specifying models of protocol systems and describes an important companion simulation tool, namely, the State Architecture Simulator (SAS);The syntax and the semantics of SAN are presented. Protocol systems are modelled by specifying an interconnection of the nine basic components defined in SAN: finite-state machine (FSM), pulsed combinational function (CFP), static combinational function (CFS), pulsed delay (DELP), static delay (DELS), queue (QUE), derivative (DER), clock (CLK), and environment (ENV);The design and the implementation of SAS are described. SAS contains two PASCAL programs and a VAX command language procedure that compiles, executes, and reports on simulations of user supplied SAN specifications of protocol systems. SAS is an interactive event driven system allowing users to examine the system status, to assign inputs, and to save system status at a terminal;The operational steps involved in creating and running a SAS executable simulated system from the SAN model are described. Several simulation models, including the start-stop protocol and a sub-system of the ADCCP protocol, were run. The simulation experience demonstrates that the SAN and the SAS are useful tools in protocol design;The evaluation of SAN and SAS is presented along with some suggestions of ways that SAN and SAS can be improved.