Simulation of Telecommunication Networks

Rockwell International Science Center; Thousand Oaks, CA

Communication networks belong to a class of physical systems that can be studied effectively with the use of discrete event simulation models. This type of simulation assumes that system state changes occur at only discrete points in time, separated by quiescent periods where nothing is assumed to happen.

Networks of interest usually involve a complexity that extends beyond the restrictive assumptions required of analytic methods. With simulation, the network topology, routing policies, and traffic algorithms can be modified and still obtain valid results. The workings of system elements can be described to any level of detail. These networks involve a number of mutually dependent components working in a concurrent or pseudo-parallel fashion. These system elements (nodes and links) are often identical in function yet operate asynchronously. These features of the real network can be more easily modeled if a problem-oriented language like SIMSCRIPT II.5 is used.

The IEEE 802.4 token passing bus LAN protocol is the emphasis of this model. In this scheme, stations connected to a bus form a logical ring based on the descending order of their addresses (unlike in a ring network in which stations would be physically connected serially). A station is allowed to transmit data only when it receives a "token" (a control frame), which represents a right to access the bus. After transmitting data, a station hands over the token to the next active station in the logical ring. The communication on the bus thus has two phases; i.e., a token-passing phase and a data transfer phase.

Alternatives Considered: None

Benefits of SIMSCRIPT II.5: SIMSCRIPT II.5 is English-like in appearance and contains semantics whose world-view assists the modeler in making the correspondence necessary between the elements of the model and those of the language, that is, the programming is done in a language closer to the concepts involved. SIMSCRIPT II.5 contains semantics (statement types) that closely fit the concepts in communications network simulation.

Customer Quote: "For our applications, no simulation programming should ever be done again in FORTRAN."