[ns] How to use queue-monitor?

[Jingyan Zhu]

Dear all,

>From ns manual, it seems ns supports traces and monitors. However, it 
doesn't show how a monitors can be used clearly, apart from commond: $ns_ 
monitor-queue n1 n2 qtrace optional:sampleinterval .

When runing a 2-node wireless simulation, I added this commond  after 
starting an ftp traffic. And then error turns up saying

"can't read "link_(0:1)": no such variable while executing "$link_([$n1 id]: 
[$n2 id]) init-monitor $self $qtrace $sampleInterval"
      (procedure "_03" line 3)
      (Simulator monitor-queue line 3)
      invoked from within
"$ns monitor-queue $node_(0) $node_(1) $qtrace"
      (file "two-nodes.tcl" line 108)

Can anyone give me a hint how to solve this problem? I'd like to use 
monitors to count packets transmitted and received, and got stuck here :( 
Thanks a lot.

Yours,
Jingyan

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