clearing

In telephone switching systems, clearing is the controlled sequence of events that terminates an active call and returns all circuit elements to their idle state, ready to handle the next connection. When a caller hangs up or when a call is disconnected for any reason, the switching equipment must perform clearing to release the voice path, reset the signaling channels, and restore the switching matrices to neutral. Without proper clearing, circuits remain falsely occupied and subscribers cannot place new calls.

The clearing process involves several distinct steps. The calling party or called party initiates the sequence by going on-hook (replacing the handset). The switching exchange detects this signal and begins releasing the trunk circuits that formed the voice path. Supervisory signals are sent back through the network to inform the originating exchange that the connection is being torn down. Billing records are finalized. The trunk groups involved are marked as available again. In electromechanical systems, stepper switches and selectors physically return to home position. In electronic exchanges, stored call records are erased from memory.

Clearing in different switching technologies

Electromechanical step-by-step exchanges use cam-driven releasing mechanisms that pull relay armatures to neutral, breaking relay chains and allowing stepper switches to reset. Crossbar exchanges rely on electrical supervisory signals to trigger release coils that restore gate circuits. Digital switching systems perform clearing in software, removing the call record from memory and deallocating timeslots or channels. The underlying principles remain consistent: detecting the disconnect signal, notifying all involved exchanges, breaking the voice path, and restoring availability.

Clearing failures or delays have serious consequences. If a trunk fails to clear properly, it remains seized by phantom traffic, reducing the effective capacity of the exchange. Subscribers hear busy tones even when circuits are available. In legacy networks, stuck calls could persist for minutes, sometimes requiring manual intervention by an operator. Modern systems employ clearing timers that force release after a set duration if normal signals fail, preventing permanent circuit lockups.

The term clearing reflects the physical act of making something empty or unobstructed: the circuit is cleared of its connection, returned to blank state. Related terms include disconnect (the caller action that initiates it), release (the act of freeing the circuits), and seizure (the opposite state, when a circuit is in use). Clearing is essential infrastructure; the speed and reliability of clearing directly determines how many calls an exchange can handle per hour.