We have implemented the coupling between OPNET and VSS. The couling is done via asynchronous and synchronous interfaces applying the synchronization protocol mentioned above. With these communication mechanisms a wide range of applications, especially in ATM traffic management sector, can be covered. We have used CASTANET for the functional verification of an ATM accounting unit. The user has to specify how high-level protocol data units (PDU) and abstract data types that are used in OPNET should be mapped to VHDL input signals.

To support the development of interface modules for OPNET and VSS in the future, a proper interface description needs to be developed. This description has to specify:

• what kind of synchronization protocol to use,
• the latency of reaction to synchronous requests, and
• a mapping rule (conversion routine) for abstract data types and protocol data units.

Based on this description, core interface models can be automatically generated. Building blocks will be taken from a library of generic protocol classes and conversion routines.

Because of the timescale problem, VSS is obviously a bottlenck in the co-simulation. Moreover, the number of events that VSS has to evaluate is an order of magnitude higher compared to the system-level simulation in OPNET. Thus, applying cycle-based simulation techniques (Cyclone) seems to be a promising approach.