Priority user protection in multiple priority switching systems

Abstract

There is disclosed a technique for interfacing various subscriber groups having different security levels which subscribers are associated with the same telephone system. According to this invention, every terminal element or subscriber has a programmable frame delay for the out and in registers associated with the switching network. In this manner, each subscriber of the same priority will receive the same delay on the transmit and receive paths. This delay is accommodated by means of an up or down counter which is inserted in the frame sync signal to the appropriate input and output registers. The input and output registers are controlled according to the priority of the subscriber, such that there is a complementary delay in the receive path and the transmit path so that the total delay for the same priority subscriber is equivalent to one complete frame. In this manner, only subscribers of the same priority can communicate with each other. In the event that a subscriber of a different priority is misconnected to another subscriber of a different priority, then these subscribers will not be able to communicate due to the fact that the information received will be unintelligible based on the different frame delays. Thus, the technique presented herein permits false connection to be intelligible only for users of the same precedence level. A false connection to a user of a different precedence level will be completely unintelligible.