'Dying Gasp' is one special signal to tell DSLAM that CPE is going to 'die'.
Normally, when CPE losing of power, it will send 'Dying Gasp' to DSLAM such that DSLAM may do some housekeeping or else.
So, if CPE want to implement Dying Gasp, it need a 'big' capacitor to store a little power to be used during losing power. But, DSLAM still have other methods to monitor the CPE existences.
Normally, ADSL PCI cards or ADSL USB modems do not implement Dying Gasp.