Dying Gasp is a message (or signal) sent by the customer premises equipment (CPE) DSL device to the digital subscriber line access multiplexer (DSLAM) when a power outage occurs. A DSL interface with dying gasp must derive power for a brief period from another source so that the message can be sent without external power. The dying gasp message will end the session and a new session will be able to be made as soon as power returns and the modem retrains.
Dying Gasp is referenced in section 7.1.2.5.3 of ITU-T Recommendation G.991.2 (12/2003) as the Power Status bit.