Mobile phone

The original Picturephone system put-upon modern-day crossbar and multi-frequency operation. Lines and trunks were six wire, one and only couple off each way for video and one and only couple off two way for audio. MF handle signaling along the audio partner off was supplemented past a video supervisory Signal (VSS) looping just about along the video quad to secure continuity. More complex protocols were afterwards adoptive for conferencing.