Multiplexing of Asynchronous Digital Signals Using Pulse Stuffing with Added-Bit Signaling

Abstract

Time-division multiplexing of digital signals with slightly different bit rates requires some method of bit rate equalization, which is termed synchronization. In pulse stuffing synchronization, extra pulses are inserted in each incoming signal as often as required to equalize the bit rates. These extra pulses carry no information and are removed when the signal is demultiplexed. They are identified by sync signals which are multiplexed and transmitted with the information signals using a bit (the added bit) allocated regularly to the synchronizing equipment. The sync signals are redundantly coded so that information loss due to transmission errors in those signals is negligible. A multiplexing system is described which simultaneously accepts binary data at 1.5, 54, and 108 Mb/s. These bit rates accommodate present day voice and television signals, using pulse code modulation. This system allows sufficient deviation of bit rates from nominal to permit the use of room temperature crystals at the terminals. Added-bit signaling also provides for convenient dropping and adding of a portion of the multiplexed information at many points along a high-capacity route. Laboratory tests have demonstrated the feasibility of this system.