Apparatus and method for secure digital communication

Abstract

The multiplexer in a multichannel, secure digital communication system repetitively embeds a sequence of byte length uniquely coded synchronizing signals in each of n successive frames of one channel of a multichannel, time multiplexed digital bit stream. Redundant protective relay data and associated parity check data are embedded in four of the n frames in a second, preferably adjacent channel, with the remaining of the n frames in the second channel containing additional of the uniquely coded synchronizing signals. The time multiplexed serial bit stream is transmitted to a remote station, preferably by fiber optic communications media. The most recent bits of the received serial bit stream equal in number to the number of bits spanned by the two channels in n frames are retained in n-1 serially connected serial in, serial out shift registers. Sixteen bit portions, a frame apart, of the retained most recent bits of the serial bit stream are applied to n serial in, parallel out shift registers. The parallel outputs of these shift registers corresponding to the frames of the two channels encoded with the synchronizing signals are compared with the sequence of uniquely coded signals stored in addresses in PROMs. When the compared signals match, which will occur once in every n frames, a synchronization signal is generated and the protective relay data and parity check data are read directly from the other parallel outputs of the serial in, parallel out shift registers. Similar rapid resynchronization of voice and data channels in the time multiplexed serial bit stream can be achieved by counting the number of bits in the bit stream from the generation of the synchronization signal in the relay protection channels to the beginning of the time slot for a selected voice or data channel.