For the purposes of estimating quality-of-service parameters, it is enough to know the large deviation rate-function of an ATM traffic stream; modeling procedures can be bypassed if we can estimate the rate-function directly, exploiting the analogy between the rate-function and thermodynamic entropy. We show that this proposal is soundly based on statistical sampling theory. Experiments on the Fairisle ATM network at the University of Cambridge have established that it is feasible to collect the required data in real time.>