trunk up down (ATM) (TUD)

Trunk Up Down (TUD) is a protocol in ATM networks that monitors a trunk and detects when it goes up or down. ATM switches periodically send test signals from each trunk port to check the quality of the trunk. If the required signals are not detected in a trunk, the TUD declares the trunk "Down". If the trunk comes back up, the TUD detects this and declares the trunk "Up".