Change the value of dataRate_ in Mac/802_11

hi all,

i want to change via tcl the value of rate in Mac/802_11 ...

For exemple when i set :
Mac/802_11 set dataRate_ 1Mb
the dataRate is really 1 Mb but after if i want to change the datarate to 2Mb
Mac/802_11 set dataRate_ 2Mb

the value of dataRate_ is still 1Mb

What's wrong with this ?
thanx