Total number of frames received from this subscriber port
that were discarded. Possible reasons for discard are:
Frame contained HDLC abort (7 or more ones in a row)
Frame had a CRC error
Frame had a non-integral number of bytes
Frame was too short (less than 5 bytes)
Frame was too long (more than 8192 bytes)
Frame had illegal DLCI (not 16-23)
Frame was discarded due to lack of buffer space or
due to congestion control