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Technical Report

Title

SP-EPRCA: an ATM Rate Based Congestion Control Scheme based on a Smith Predictor

Abstract

This report presents a feedback control algorithm for ATM congestion control in which ABR source rates are adjusted according to VC queue levels at intermediate nodes along the path. The goal is to fully and promptly utilize the available bandwidth left by QOS constrained trac (CBR+VBR) for transmitting the less QOS stringent ABR trac, without incurring in queue over ow and consequently cell loss. In order to obtain this, we propose a simple and classical proportional controller, plus a Smith Predictor to overcome instabilities due to large propagation delays. In the SP-EPRCA model, each queue ideally behaves as a simple first order dynamic system with a delay in cascade. The delay is out of the feedback loop, and therefore does not affect stabilitity. Moreover, since the system dynamic is a first order one, it is not only stable but it has no damped oscil lations as well. The real SP-EPRCA implementation, however, may behave as a higher order system due to innacuracies in estimating propagation delays. We show in which conditions system stability is guaranteed under this more realistic scenario. Finally, we propose an e ective SP-EPRCA implementation in which each source computes its input rate based on the maximum queue level along the path, which is compatible with the current EPRCA type UNI specications. Theoretical and experimental results show max-min fairness of the proposed control scheme, its eciency under the constraints of the EPRCA implementation, as well as its cell loss free property. Moreover, the relationship between throughput, bu er size, and round trip delay is established. Finally, various queue services disciplines are considered in conjunction with the basic scheme.

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Information & Date

Technical Report #960001, February, 1996, UCLA CSD, February. 1996

Authors

Dirceu Cavendish
Saverio Mascolo
Mario Gerla