The IEC 60870-5-104 quality descriptors OV (OVERFLOW/NO OVERFLOW), BL (BLOCKED/NOT BLOCKED), SB (SUBSTITUTED/NOT SUBSTITUTED), NT (NOT TOPICAL/TOPICAL) and IV (INVALID/VALID) are not set by the protocol implementation and therefore do not reflect the quality of the signal as seen through other protocols. Invalid signals are indicated by setting the value to default. The connected application should verify the validity of signals where applicable.