This reports the ONU's received downstream optical power in dBm, read via its transceiver's DDM. A weak reading here is the field an admin checks to catch a dirty connector or a bent fiber before it causes an outright link loss, since it degrades gradually rather than failing all at once. For example, a reading near -28dBm against a typical -8 to -27dBm acceptable range points to excess loss on that fiber run that warrants a connector cleaning or inspection.