Detectability of packet communications using variants of the X.25 protocol

The low probability of intercept (LPI) performance of a variant of a robust, proven packet transmission protocol, AX.25, is examined. Both AX.25 and its variant are described. In the latter the message is packetized, overhead information is added to it, and it is sent a go-back-N protocol. The system performance is quantified in terms of the distribution function of the increase in detectability (or Q factor). The probability of correct transmission is found to exhibit a sharp threshold as a function of the bit error rate. For all practical purposes the system will either operate very well or not at all. The intercept vulnerability, as measured by the variation in the Q factor, is not very sensitive to a wide range of variations in the amount of overhead per packet. Thus, a system design may ignore the variation in vulnerability brought by an increase in the number of packets used to transmit a given message, concentrate on the optimization of transmission time, and be assured of not being far from optimal