A request for clearance implies readiness for start-up according to the German AIP - either be ready for startup within five minutes or give an estimate for start-up when requesting the ATC clearance.
All UIR/FIR controllers have mode S downlink capability, i.e. ATC knows the selected altitude as well as the actual flown track, ground speed, heading, IAS/Mach and vertical speed if the airplane supports these. Eurocontrol EHS capability list
Descent clearances will usually be given well before the computed top of descent