The present invention discloses a method and device for identifying unusable data blocks (bad frames) during communication in a network. The method comprises the steps of determining (S1) a classifier (CL_C) in a current data block, calculating (S3) a mean (M) of classifiers (CL_—1 to CL_N) of a predetermined number (N) of data blocks, calculating (S4) a standard deviation () of the classifiers (CL_—1 to CL_N), calculating (S5) a threshold for identifying bad frames on the basis of the mean (M) and the standard deviation () of the classifiers (CL_—1 to CL_N), and distinguishing (S6) whether the current data block is usable or not on the basis of a comparison of the threshold (TH) and the current classifier (CL_C). By the method and the device, changing conditions of a radio channel can be taken into account quickly.