When and how is CPDLC established? On a typical commercial trans-Atlantic flight, say from Frankfurt (EDDF) to New York Kennedy (KJFK), on a typical airliner (B777, A380, B787, etc.), when are Controller–Pilot Data Link Communications CPDLC established? 


Is it generally before take-off, or just during the flight? 
Is it only used during oceanic crossings, or for continental flights, too?
Will ATC instruct the pilot when to use CPDLC and when to revert back to voice communications?
Using CPDLC makes life easier for pilots so we typically use it whenever it is available and logon shortly after takeoff.

When departing Frankfurt there is no rush as CPDLC is not available until crossing into Maastricht Radar. Continental USA does not have it so you lose CPDLC after Moncton Center. 

ATC will often remind you, or ask you to logon if you have forgot as it makes life easier for them too. Where VHF is available we usually use a combination of voice and CPDLC. 

Over the ocean, out of VHF range, we have to establish HF radio contact with Shanwick or Gander radio and do a Selcal check to insure we have a backup communication method.