A first field of a data unit is analyzed to determine whether the data unit is marked as a particular type of data unit. If the data unit is marked as a particular type of data unit, a second field of the data unit is analyzed to determine whether characteristics of the second data field correspond to the particular type of the data unit. Determining whether the characteristics of the second field correspond to the type of data unit ensures that the data unit is properly marked with respect to type. The first field may indicate a particular priority or that the data unit is carrying voice data. The second field may carry voice data. The analysis of the fields may be done on a data unit by data unit basis, or on a stream basis.