PERFORMANCE EVALUATION OF A PICTURE ARCHIVING AND COMMUNICATION-NETWORK USING STOCHASTIC ACTIVITY NETWORKS

The concept of picture archiving and communication systems (PACS) is n ow widely accepted in the medical community. In order to bring the con cept to reality, however, innovative designs and implementations are n eeded. One such design is a fiber-optic star-based PACS. This PACS net work is based on a multiplexed passive star local area network with wa velength-division multiplexing to provide separate logical channels fo r transfer of control and image data. The system consists of an image network (INET), for image transfer at a rate of 140 Mbps, and a contro l network (CNET), operating at 10 Mbps, for mediating the flow of imag e transfers. INET is a circuit switched network devoted solely to imag e transfer, while CNET employs the CSMA/CD protocol for bus arbitratio n. Before such a system can be deployed, an accurate evaluation study must be carried out to estimate its performance characteristics. Such evaluations are complicated both by the complexity of the PACS itself and the varied demands that are placed on such a system. A novel appro ach based on stochastic activity networks, a stochastic extension of P etri nets, is useful in this regard. Stochastic activity networks were used to develop a detailed model of the command and image channels. T he performance of the system was then evaluated under realistic worklo ad conditions. In particular, we were able to estimate a number of imp ortant performance variables including the image response time, comman d channel delay, and queue length at each type of node and the network supervisor. The results 1) show that stochastic activity networks are an appropriate model type for evaluating picture archiving and commun ication systems, 2) delineate the workload conditions under which PACS may effectively operate, and 3) show that even when these conditions are exceeded, the command channel load remains extremely light. Result s of this type are useful both to designers of other PACS networks and those interested in this particular PACS design.