[ns] How to modify the bandwidth during simulation (urgent)

[haicheng flammy]

Hi, everyone
 
I want to change the bandwidth of a link during simulation.  How can I do it?
 
for example, I used:
.........
set link [$ns duplex-link n0 n1 2Mb 20ms DropTail]
.........
$ns at 1.0 [$link set bandwidth 1Mb]
 
But it doesn't work.  Why? How can I do it?
 
Thanks a lot

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