Interval Training :

90 sec on, 1 min off, 90 sec on, 45 sec off, 90 sec on, 30 sec off, 90 sec on, 15 sec off, 90 sec on, 30 sec off, 90 sec on, 45 sec off, 90 sec on.

Found on CrossFit Endurancehttp://www.crossfitendurance.com/90sec-ladder/