Traffic management with Queues

HI all,
I have used mikrotik since long time. It is awesome Router OS. Now in my current situation, I want to manage b/w of user in such a way that they will be sure for thier allocated b/w but when there is b/w free in total queue then they can get more b/w. I tried with burst but it didn’t work. Ok lets take one scenario. I have to class A and B. abc is client under A class and def is client under B class. Both abc and def client is resticted to 64k/64k but their Parent have unlimited b/w. Now is there any method in mikrotik so that when client abc under A class is not using his 64k/64k b/w I can assign these b/w to client def under B class. can u explain anyone that how can I manage burst b/w to any user.

Use pppoe and set two different profiles with different speeds