Theory for Some Asynchronous Time-Division Switches

Voice switching systems have most commonly been based on controlling electromechanical switches which select and hold a spatially distinct path for each conversation. The technology used to implement such a space-division network (crossbar or ferreed switches) usually results, in practice, of an individual path having much larger bandwidth than is required for faithful transmission of the signal. The space and cost of these switches makes other solutions desirable for 983 984 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , MARCH 1971 many applications. One line of attack has teen to keep the space-division concept, b u t replace the elcctromcchanical relays with semiconductor switches. These techniques, however, still suffer from the hard-to-grow nature of multistage space division networks. A more promising solution seems to be tb place all the conversations on a wideband bus using time-division techniques. In fact, the 101 Electronic Switching System (ESS) is such a time-division switch. This system uses resonant energy transfer to "move" periodically measured samples of a speech wave from an incoming line to an outgoing one. In the same vein, an asynchronous time-division switch has recently been proposed by J. O. Dimmick, T. G. Lewis, and J. F. O'Neill. 1 This switching arrangement makes use of active energy transfer !>ctween filters rather than resonant energy transfer and allows a variable time slot for transferring each speech sample through the switch. T h e asynchronous nature of this switch allows a more efficient use of processing time than is possible to achieve synchronously.