Performance evaluation of MIL-STD 188-220 interoperability standard for digital message transfer device subsystems

MIL-STD 188-220 specifies the procedures, protocols, and parameters needed for interoperability of digital message transfer devices (DMTDs) and applicable control systems. One of 188-220's link layer mechanisms is the net access control algorithm which in turn defines the net access delay (NAD). MIL-STD 188-220 provides several NAD algorithms to handle "the time a station with a message to send shall wait to send a frame after the TP timer has expired." Supported by an ARL contract, the University of Delaware compared the performance of randomized (R-NAD) and prioritized (P-NAD) NAD algorithms using the OPNET simulation package. Simulation results comparing R-NAD and P-NAD under varying traffic conditions (offered loads, priority distributions, etc.) are presented. Results show that R-NAD outperforms P-NAD in both access delay and collision statistics. More importantly, this simulation exposes 188-220's ambiguous definition of the P-NAD algorithm, an ambiguity that continues even in the later versions of the standard. This ambiguity can result in a P-NAD implementation that actually encourages packet collisions.