The impact of source traffic distribution on quality of service (QoS) in ATM networks

A principal attraction of ATM networks is that the key QoS parameters of every call are guaranteed by the network when appropriate cell-level traffic controls are imposed at the user network interface (UNI) on a per call basis, utilizing the peak cell rate (PCR) and sustainable cell rate (SCR) values for the traffic sources. The issue of providing QoS guarantees in the real world, through traffic controls applied on a per call basis, is rendered uncertain. This paper adopts a radically different, high level approach to the issue of QoS guarantees. It aims at uncovering through systematic experimentation a relationship, if any exists, between the key high level user traffic characteristics and the resulting QoS measures in a realistic operational environment. Based on the observation that the network provider cares for two factors, namely, to maximize the link utilization and to ensure the QoS guarantees for every user traffic, this paper proposes a two-phase strategy. Under the first phase, the average "link utilization" computed over all the links in a network is maintained within a range, specified by the underlying network provider, through high level call admission control, i.e. by limiting the volume of the incident traffic on the network, at any time. The second phase is based on the hypothesis that the number of traffic sources, their nature-audio, video, or data, and the bandwidth distribution of the source traffic, admitted subject to a specific chosen value of "link utilization" in the network, will exert a unique influence on the cumulative delay distribution at the buffers of the representative nodes and, hence, on the QoS guarantees of each call. Extensive simulations reveal that, for a given link utilization level held uniform throughout the network, while the QoS metrics are superior in the presence of many calls each with low bandwidth requirement, they are significantly worse when the network carries fewer calls of very high bandwidths. The findings demonstrate the feasibility of guaranteeing QoS for each and every call through high level traffic controls. The findings also underscore two additional important contributions of this paper. First, the network provider may guide and maintain the network at a desired network operating point (NOP) by exercising control over the input and system parameters including link utilization, call admittance etc. Second, the finding constitutes a vulnerability of ATM networks which a perpetrator may exploit to launch a performance attack.