The invention relates to a method and arrangement for detecting congestion in a communications network (120). A receiving node (130) receives (710) data packets from a sending node (110) over a communications link (140). First and second parts (320, 330) of a frequency spectrum (310) of a time delay of the data packets are determined (720). The first part comprises non-congestion related low frequency components. The second part comprises congestion related and non-congestion related low frequency components when a load level of the communications network is above a threshold value (220) but substantially no congestion related low frequency components when the load level is below the threshold value. Then reference jitter level (yr) and congestion jitter level (yg) corresponding to the first and second parts respectively are estimated (740). Finally, congestion state information is determined (750) by comparing said congestion jitter level to said reference jitter level.