Development of public switch telephone networks, human voice characteristics and certain channel characteristics including twisted pair channels, DTMF signaling, central office switching, trunk circuits, Erlang capacity and blocking probabilities, multiplexing and private branch exchange. Digital telephony, call switching, echo suppression, modems, T1 and E1 trunks, integrated services digital networks (ISDN), digital subscriber lines (xDSL) and VoIP switched networks. Switching techniques. Traffic theory. Signaling and SS7 call control protocol, switching networks and Digital switching techniques