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This paper presents results from performance measurements in a test network that can be operated as an ordinary best effort or as a Differentiated Services (DiffServ) capable network. The target of the measurements was to analyse the effects that the DS mechanism has on the behaviour of the TCP flow control mechanism. The network implements service differentiation by using eight drop precedence levels and separate classes for real time and non-real time traffic. Performed tests verified that our QoS (quality of service) implementation allows QoS-controlled TCP senders to recover faster from congestion than what is possible for TCP sessions in a purely best-effort network. When QoS is enabled, the access node helps smooth traffic bursts flowing into the DiffServ core network. As a consequence, the congestion window oscillates moderately and keeps TCP operating in the congestion avoidance phase, making the rate adjustment scheme more controllable.