On the basis of a reference model for ADSL traffic on an IP backbone link, we show that it is possible to infer the characteristics of long flows by performing a deterministic $1/N$ packet sampling. By using the fact that the number of active long flows can be represented by means of the number of customers in an $M/G/\infty$ queue with Weibullian service times, we derive some probabilitic properties of sampled data. These properties are then used to infer the characteristics of original flows. The method is illustrated by considering an actual traffic trace captured on the France Telecom IP backbone network. Experimental data show that the method proves quite efficient.