Timer T309, in milliseconds, is the interval this port allows for the underlying signaling channel (VPI/VCI) to recover after a failure before Q.93b clears every active SVC that was using it. This matters operationally because it is a blast-radius control: a short T309 tears down calls quickly on a flapping link, while a longer T309 rides through brief outages without dropping established connections but leaves calls in limbo longer. Cascade switches commonly default this to around 90000 milliseconds, so a 2-second signaling glitch would not trigger mass call clearing but a 3-minute outage would.