[altq:1896] How to use CBQ ( or any other discpline) dynamically ?

["newbie" <dynks@polbox.com>]

I got 256Kbps link.
What I want to accomplish is splitting the bandwidth dynamically using CBQ.
Is it possible to borrow the desired bandwidth only on demand ?
I want to guarantee 30 % or 80 kbps from 256K connection to ftp but  :
"borrow pbandwidth 30 " makes these 30 % reserved all the time - no matter
if the filter matches the rule for ftp or not.
Is there any other way to do it? It does not have to be CBQ :-)

Regards.