The pros and cons of a job buffer in a token-bank rate-control throttle

Rate-control throttles with token banks or leaky buckets have been used for overload control in telecommunication systems and have been recommended for traffic policing in broadband integrated services digital networks (B-ISDN). Enhancing the token-bank throttle with a buffer to shape the admitted traffic has been suggested. Researchers have shown that the presence of the buffer can dramatically reduce the squared coefficient of variation of the interadmission time. However, the authors show that the impact of the buffer on longer-time-scale characteristics of the admitted traffic is much less dramatic. In particular, they show (primarily through simulations) that the job buffer has much less impact on higher values of the index of dispersion for intervals and on small tail probabilities for the steady-state number in system at a downstream queue (with only this one arrival stream). Indeed, the smoothing benefit of the job buffer decreases as longer-time-scale characteristics become more important. However, if the downstream queue is fed by many sources with throttles, as would be the case in most applications, then the relevant time scale at the downstream queue indeed becomes relatively short. The simulation results show that the benefit of traffic shaping can be much greater. The benefit gained in reduced buffer requirements at the downstream queue, though, is typically significantly less than the sum of all job buffers added to the throttles. A full cost/benefit analysis depends on the relative cost of buffer space in the two places and on details of the relevant application