[ns] can't access $now

[Ishan Mandrekar <ishan@io.ece.drexel.edu>]

ns-users:

I get a compile error when I try to execute this command:
$ns at [expr $now+$interval] "check"

the error is:
ns: check: can't read "now": no such variable

I've seen $now being used in other scripts to get the current simulation
time. What could be the problem here?

The procedure definition of "check" goes as follows:
proc check { } {
 
  global ns n3 n4 qlimit
....
...
..
.
$ns at [expr $now+$interval] "check"
}

Thanks,
Ishan.