[ns] Question about prevTime_ in the multistate error model

["jahn-isi" <jahn@ISI.EDU>]

Is there any reason why multiple multistate error modules share one prevTime_
variable?

Currently, the multistate error module transits from one state to another based
on how long it stays at one state by the equation, now - prevTime_ where
prevTime_ remembers the last transition time. And this stay time is computed
when each packet is delivered over its link.

Since the prevTime_ is defined as a static variable meaning that it is shared by 
multiple multistate modules when the simulated network creates multiple error 
modules for multiple links, only one randomly selected multistate module can 
transit from one state to another.

Thanks.
-jahn