A New Look at Bandwidth Latency Tradeoffs

Concerns about propagation delay have dominated the discussion of latency, bandwidth and their effect on distributed applications. In this paper, we argue that the relevant latency measure for applications is the Application Data Unit (ADU) Latency, defined as the time between the sending of an ADU and its receipt. Since ADUs are often large, ADU latency is influenced by throughput as well as propagation delay. We investigated the effects the effects of ADU latency with an experimental study of several applications. The applications used Distributed Shared Memory as an interprocess communications mechanism, constraining the ADUs to page sized units. The applications were run on an Ethernet, an experimental ATM LAN, and using ATM on an experimental high-speed WAN. The measured results were used to normalize results gathered by inserting an experimental ATM switch output port controller in the network to create tunable delays. The results conclusively demonstrate the effect of ADU latency on distributed application response time. The experiments give a precise characterization of the effect of varying bandwidth and propagation delay on a real system, and suggest promising directions for further improving application performance in future networks.