An Active Queue Management (AQM) robust control strategy for Traffic Control Protocol (TCP) data transfer is proposed. To this purpose, the TCP behaviour is first approximated by a second-order model with delayed input obtained from the linearization of an efficient and commonly used nonlinear fluid-based model. The adopted feedback control structure uses a fractional-order PI controller. To ensure the desired robustness, the parameter regions where such a controller guarantees a given modulus margin (inverse of the H∞ norm of the sensitivity function) are derived. An example commonly used in the literature is worked out to show that the suggested graphically-based design technique is simple to apply while it limits the effects of disturbances and of the unmodelled dynamics.
