<P> Examples of signals communicating supervision or call progress include E and M signaling, SF signaling, and robbed - bit signaling . In physical (not carrier) E and M trunk circuits, trunks were four wire . Fifty trunks would require a hundred pair cable between switches, for example . Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). Tip and ring were the voice - carrying pair, and named after the tip and ring on the three conductor cords on the manual operator's console . </P> <P> In two - way trunks with E and M signaling, a handshake took place to prevent both switches from colliding by dialing calls on the same trunk at the same time . By changing the state of these leads from ground to - 48 volts, the switches stepped through a handshake protocol . Using DC voltage changes, the local switch would send a signal to get ready for a call and the remote switch would reply with an acknowledgment to go ahead with dial pulsing . This was done with relay logic and discrete electronics . </P> <P> These voltage changes on the trunk circuit would cause pops or clicks that were audible to the subscriber as the electrical handshaking stepped through its protocol . Another handshake, to start timing for billing purposes, caused a second set of clunks when the called party answered . </P> <P> A second common form of signaling for supervision was called single - frequency or SF signaling . The most common form of this used a steady 2,600 Hz tone to identify a trunk as idle . Trunk circuitry hearing a 2,600 Hz tone for a certain duration would go idle . (The duration requirement reduced falsing .) Some systems used tone frequencies over 3,000 Hz, particularly on SSB frequency division multiplex microwave radio relays . </P>

Where does information exchange begin in order to produce good customer service