[ns] error in implementing in tcl script

[VijayaLaxmi Madasu]

Hi,

This is vijaya, presently doing my project work in simulation of wireless
LAN using ns2.
Presently i got a problem in implementing one of the solution.I'm attaching
the tcl script for which im getting the below error:

can't read "f": no such variable
        while executing
"$ns trace-all $f"
 line 21

 Thanks & Regards
 Vijayalaxmi Madasu