http://www.vi1129.com/2009/06/some-mail-about-freeradius-sqlcounter-limit-the-octets/
I have successfully use this article to limit the bandwidth, the unit is bytes. hope it's helpful

On Tue, Jun 29, 2010 at 2:33 PM, Alan DeKok <aland@deployingradius.com> wrote:
Spacelee wrote:
> what's the use of acct-input-octets and acct-output-octets

 Read the link you posted to the list.  That is the official documentation.

 Alan DeKok.
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Spacelee