A method comprises providing six helicopters. The abstracted network map may be used to compute least cost allowable paths through the network. The deterioration due to the repeated rewriting at a start point and an end point of the recording is suppressed by recording a synchronizing signal before the write data andor dummy data after the write data in the way such that thermal stress is less likely to occur. Accordingly, the N-Node VLT system eliminates the disadvantage of Spanning Tree Protocol by allowing link aggregation group terminations on multiple separate distribution or core switches and also supporting a loop-free topology. The second photodetector receives the first-order diffraction light and obtains a focusing error signal.