Panel Switch - Interoffice Signaling

Interoffice Signaling

For panel-office-to-panel-office calls, the two offices communicated by revertive pulsing. For calls within a single panel office, the two halves of the office also communicated by revertive pulsing.

For compatibility with manual offices, Panel Call Indicator (PCI) signaling was used. PCI used multilevel DC pulses, for a bit to baud ratio of 2:1. PCI signalling lit lamps on the B operator's desk at the terminating manual office. Another type of signaling, Call Annunciator, used speech recorded on strips of photographic film to announce the called number to the answering operator.

PCI continued in use for tandem purposes, decades after its original purpose had disappeared. In the 1950s Auxiliary Senders were added, to allow storing more than eight digits, and sending by MF for Direct Distance Dialing. MF was also used by Crossbar and ESS exchanges.