Method for scheduling data traffic in a network node, said data traffic being divided into a plurality of data traffic classes, the method comprising the steps of - detecting to which data traffic class an incoming data packet belongs, - storing said incoming data packet in a memory and adjusting a single queue associated with said memory to rank said incoming data packet in said single queue between a head of the queue and a tail of the queue, said single queue having for at least one data traffic class a number of reserved positions for arriving data packets of said at least-one data traffic class, said incoming data packet taking in said single queue a position corresponding to one of said reserved positions, and - taking a data packet referred to at the position closest to said head of the queue for transmission, characterised in that a step is performed of determining said number of reserved positions of said at least one data traffic class according to a given Quality-of-Service requirement relating to a queueing delay distribution of data packets of said at least one data traffic class having reserved positions in said single queue, said Quality-of-Service requirement expressed as a targeted delay quantile spacing between consecutive data traffic classes of said plurality, said targeted delay quantile spacing defined as a difference between per-class delay α-quantiles of said consecutive data traffic classes, whereby said number of reserved positions is determined as a function of said delay quantile spacing and the partial load of said data traffic classes, said partial load being the contributing portion of each data traffic class to the total load, and whereby a new reserved position is created at a free position towards the tail of said queue when a position corresponding to one of said reserved positions is taken if said incoming data packet belongs to said at least one traffic class having reserved positions, so that the number of reserved free positions for that class remains constant. - detecting to which data traffic class an incoming data packet belongs, - storing said incoming data packet in a memory and adjusting a single queue associated with said memory to rank said incoming data packet in said single queue between a head of the queue and a tail of the queue, said single queue having for at least one data traffic class a number of reserved positions for arriving data packets of said at least-one data traffic class, said incoming data packet taking in said single queue a position corresponding to one of said reserved positions, and - taking a data packet referred to at the position closest to said head of the queue for transmission, characterised in that a step is performed of determining said number of reserved positions of said at least one data traffic class according to a given Quality-of-Service requirement relating to a queueing delay distribution of data packets of said at least one data traffic class having reserved positions in said single queue, said Quality-of-Service requirement expressed as a targeted delay quantile spacing between consecutive data traffic classes of said plurality, said targeted delay quantile spacing defined as a difference between per-class delay α-quantiles of said consecutive data traffic classes, whereby said number of reserved positions is determined as a function of said delay quantile spacing and the partial load of said data traffic classes, said partial load being the contributing portion of each data traffic class to the total load, and whereby a new reserved position is created at a free position towards the tail of said queue when a position corresponding to one of said reserved positions is taken if said incoming data packet belongs to said at least one traffic class having reserved positions, so that the number of reserved free positions for that class remains constant.