MIL-STD 188-220 speci es the procedures, protocols, and parameters needed for interoperability of Digital Message Transfer Devices DMTD's and applicable control systems. One of 188-220's Link Layer mechanisms is the Net Access Control Algorithm which in turn de nes 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

more »

... 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 tra c conditions o ered 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 de nition 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.