By default the available bandwith is the 75% bandwith of the Bandwidth configured in the interface.That is if your interface bandwidth is 1.544Mbps , Your available bandwith will be 1.544 x 0.75 = 1.158
By default weighted fair queuing is applied to interface for input and output packets. In case if you configure for QOS in the interface, the default value of available bandwidth is taken into account for the queing mechanism (Priority, custom queueing) you are configuring. This is done because of mission critical mechanisms (like keepalive, routing updates) not to be affected.
Suppose you want to configure QOS for voice traffic using Priority queuing and the bandwidth configured is 1.544 Mbps, You can use upto 75% of the availble bandwidth that is 1.158 Mbps for the voice tarffic.
To modify the Available bandwith use the command : max-reserved-bandwidth xx
xx - % of bandwidth say 90 for 90%
And you can view the modified Availbale bandwidth in Show interface serial
Hope this helps
