state. Polarity switching results in reversals of the direction of loop current. When disconnect supervision is implemented without answer supervision, polarity reversal may still be used, but this is rarely implemented in public telephone networks. Typically, it is implemented by removal of battery voltage from the line for a short period of time, resulting in loss of loop current. Interruption of loop current is variously called open loop disconnect, or an open switching interval. Additional types of loop signaling are signals to the central office that the subscriber intends to initiate a second simultaneous call, a three-way conference call, or answer a second incoming call. This signal is called flashing or hook flash, and is performed by interrupting the loop for a fraction of a second, typically at least 300 ms. The flash signal is longer than a rotary dial pulse, sometimes called a short flash, and is shorter than the time required for the on-hook condition. Disconnect supervision is also known as calling party control (CPC), and forward disconnect. Kewlstart Kewlstart is a coined term created in the Asterisk PBX open-source software community for an extension of loop start signaling for FXS and FXO telephony interfaces which adds disconnect supervision. This type of on-hook/off-hook supervision was in use in step-by-step systems as well as electronic switching systems in North America for many decades. Disconnect supervision signals the called terminal device, typically an automated customer premises equipment, that the remote calling party has hung up. Disconnect supervision is implemented as an open switching interval (OSI), a period of several hundred milliseconds during which the loop current is interrupted. Some switching systems remove loop battery voltage for about 250 ms within 6 seconds after the far-end party disconnects. See also BORSCHT References Glossary of telecommunications terms Telephony signals