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This paper evaluates and compares six rate-based congestion control protocols for the ABR (Available Bit h:ate) trafic over the ATM (Asynchronous Transfer Mode) networks. They include: the EFCI-bit setting, the EFCI-bit setting with separate RM queues, the CI-bit setting in the backward direction, the CIbit setting in the backward direction with separate RM queues, the CAPC2 ER (Explicit Rate) scheme, and the EFCI-bit setting with utilization-based congestion indication. Each scheme is simulated and compared in the LAN, WAN, and GFC (General Fairness Configuration) environments specified b y the ATM Forum. Effects of varying VC (Virtual Circuits) number and changing endsystem-switch distance has been investigated. Their fairness is also compared using the GFC confi:guration. For each simulation run, we measure average queueing delay, maximum queue length, and network utilization. Traces of ACR (Allowed Cell Rate) and buffer queue length are also examined. We have found that the ER control scheme performs significantly better than the other five binary control schemes by its faster response to congestion, smoother regulation of bit rates, lower queueing delay, shorter buffer queue length, and fairness. Among the other five schemes, the CI-bit setting scheme performs better than the EFCI bit setting scheme. Providing separate RM queues has significmtly improved the EFCI scheme in the WAN environment, but has little effect on the CI scheme. Link utilizlation-based congestion detection has sufFered from either low utilization or an excess cell loss which is unacceptable in most data applications. I11 IV IhtroductionIn providing integrated service for multimedia and heterlogeneous traffic, the ATM Forum traffic management specification has categorized five traffic and service classes [l]: constant bit rate (CBR), real-time variable bit rate (rt-VBR), non-real-time variable bit rate CI-bit setting in the backward direction CI-bit setting with separate RM-cell queues

SUMMARYCongestion control is very important for e!ective and stable operation of ATM (Asynchronous Transfer Mode) networks. Owing to the bursty and unpredictable characteristic of data network tra$c, its congestion control is particularly a challenge for network researchers and designers. The ATM Forum has recently adopted rate-based congestion control for ABR (Available Bit-Rate) tra$c which is the service class de"ned for data network applications. However, there is a number of congestion control schemes prevalent. ATM Forum has decided not to specify switch behaviour for ABR tra$c; this has further introduced additional ambiguity. Consequently, an evaluation and comparison of the existing protocols would provide valuable guidance for network designers and engineers; it would also give insight for researchers to explore the essence of di!erent congestion control schemes. In the "rst part of this paper, we investigate the e!ectiveness of ABR congestion control in the presence of bursty source tra$c and the relationship between the burst time scale and the ABR control time scale. Two ABR congestion control schemes, the ABR Explicit Forward Congestion Indication (EFCI) and ABR Congestion Indication (CI) schemes, are compared with Unspeci-"ed Bit Rate (UBR) transport which makes no e!ort to control congestion. Tra$c sources of various burst lengths of 100, 1000, 10 000, and an equal mix of 100 and 10 000 ATM cells are used in simulations. It is found that ABR congestion control schemes e!ectively control low frequency, medium to long-term tra$c load transients. This is further supported by the result of integrating TCP over ATM congestion control schemes included in the paper. ABR control schemes do not control high frequency, short-term load transients well, but ABR control is not necessary in such cases since short-term transients do not require large amount of bu!ering. In the second part of this paper, we evaluate and compare six rate-based congestion control protocols including Scheme I: EFCI, Scheme II: EFCI with separate RM queues, Scheme III: CI, Scheme IV: CI with separate RM queues, Scheme V: the CAPC2 ER (Explicit Rate), and Scheme VI: the EFCI with utilization-based congestion indication. Each scheme is simulated and compared in the LAN, WAN, and GFC (General Fairness Con"guration) environments speci"ed by the ATM Forum. E!ects of varying VC (Virtual Circuits) number and changing endsystem}switch distance has been investigated. Their fairness is also compared using the GFC con"guration. We have found that ER control scheme performs signi"cantly better than the other "ve binary control schemes by its faster response to congestion, smoother regulation of bit-rates, lower queueing delay, shorter bu!er queue length, and fairness. Among the other "ve schemes, the CI scheme performs better than the EFCI scheme. Providing separate RM queues has signi"cantly improved the EFCI scheme in the WAN environment, but has little e!ect on the CI scheme.

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