[ns] Dynamically change data rate

[Zubiya ..]

Hi,
In Tcl if we want to change the CBR value and data rate dynamically then
how it can be achieved?
i have tried following code but it takes only initialisation value that is
1000

set cbr_rate 1000
proc record {} {

    global  cb_rate cbr

    set ns [Simulator instance]

    set $cb_rate [expr $cb_rate + 50000]
    set time 0.5
    set now [$ns now]
    $cbr set rate_ $cb_rate
    puts "$cb_rate"
    $ns at [expr $now+$time] "record"

}

Thanks in advance
-- 
Regards