Logic Design- Transfer Characteristics and Noise Margin

[gipc]

Hello,
I have the following question which I'm not quite sure about.

I have the following Transfer Characteristic for a buffer.
You can see the graph breaks at the points (2,0),(3.5,4),(5,5)

http://img521.imageshack.us/img521/5052/captureen.jpg

I need to set the appropriate values of VOH, VIH, VIL and VOL to get the maximum possible Noise Margins.

How can I do it?
Thanks.

 

[gipc]

Bumpity Bump :(