This is mentioned in MAC specification 5.7 .
If the Short DRX Cycle is used and [(SFN * 10) + subframe number] modulo (shortDRX-Cycle) = (drxStartOffset)
modulo (shortDRX-Cycle); or
- if the Long DRX Cycle is used and [(SFN * 10) + subframe number] modulo (longDRX-Cycle) = drxStartOffset:
- start onDurationTimer.
For E.g Current subframe number is 9. SFN is 988 , Longdrx cycle is 80 and drxstartoffet in RRC IE as 5
988 * 10 + 9 % 80 should be equal to 5.