Determination of the optimum packet length and buffer sizes for the industrial building automation and control networks

This paper addresses two issues of the building automation and control networks (BACnet) intended for industrial buildings and implemented on the Ethernet: the determination of (a) optimum packet length, and (b) optimum buffer size. The delay-throughput characteristics of each of the four priority classes of messages, i.e., life safety, critical equipment, urgent, and normal, have been measured by simulation experiments for various packet lengths and buffer sizes. In this way, optimum packet length and buffer sizes have been deduced. The optimum packet length has been obtained by evaluating the effect of increasing packet size for a particular traffic pattern and offered load. The optimum buffer size at a node corresponds to the best overall performance for messages of all priorities in terms of overflow rates at the buffers of the nodes with respect to increasing offered loads. The optimum packet length has been found to be between 720 and 904 bits. If we assign 720 bits as the optimum packet length, the optimum buffer size at each node is found to be between 400 and 450 packets, i.e., approximately 288 to 321 kbits