We define the Lyapunov function as the scalar measue of the congestion in both the Zg(t) and Qg(t) queues: L(Θ(t)) =12PGg=1[(Zg(t)2+Qg(t)2] and the conditional Lyapunov drift as ∆(Θ(t)) = E{L(Θ(t + 1)) − L(Θ(t))|Θ(t)}. Considering both the charging cost (7) and queue backlog growth, our objective is then to minimize the following function in each timeslot t, (12).