[ns] How does Back-off procedure work in NS? (802.11)

[Bahman Kalantari Sabet]

Dear all,

I was wondering if any of you knows how the back-off procedure works in NS.

In NS, as far as I understand, in terms of signaling, we have the followings 
respectively:

(DIFS), RTS, (SIFS), CTS, (SIFS), tcp, (SIFS), ACK   and then,    (DIFS + 
Back-off timer), RTS,..... etc.

How and where does NS calculate this Back-off time?

Thanks,

Bahman.