It occurs when there is no voice transmitted in the direction of the measurement and DTX is activated in this direction as well.
For instance : DL RXLEV FULL = -95 and DL RXLEV SUB = -55, means that very little voice is transmitted in the downlink direction and DTX DL is enabled.
FULL : measurement is done on all TCH, even if nothing is carried on it.
SUB : measurement is done on SACCH frames (it always contains data)