Abstract: For controlling the data traffic in an ATM network, in which a number of nodes are connected via a closed loop, and in which each node may be equipped with a number of access ports for sending and receiving ATM cells, the ATM cells are processed separately in each node based upon their assignment to a QoS (quality of service) class in a predefined QoS classification. In each access port, each QoS class is assigned a separate queue. The data traffic in the closed loop is monitored separately, based upon QoS class, by a cell monitor. If a performance characteristic of a QoS class is not fulfilled in the closed loop, then at least one back-pressure signal will be generated, which will serve to suppress at least one assigned queue of the access port. The monitoring of the data traffic can be implemented via a leaky-bucket system.