Bit_Rate measurement

Hi,

I'm trying to measure bit rate of a source by using* Counter *element's
handler but it's not working (return 0).
Can anybody explain me why? and what should be the solution? Thank you in
advance...

C:: Counter

Source -> C -> Discard;

DriverManager(wait 3s, printn "Bit Rate: ", save C.bit_rate -, stop)

ELEMENT HANDLERS

*count (read-only)*Returns the number of packets that have passed through
since the last reset.*byte_count (read-only)*Returns the number of bytes
that have passed through since the last reset.*rate (read-only)*Returns the
recent arrival rate, measured by exponential weighted moving average, in
packets per second.*bit_rate (read-only)*Returns the recent arrival rate,
measured by exponential weighted moving average, in bits per second.

-- 
Ruetee C.