A read-only integer specifying the noise margin of this HDSL modem's receiver, indicating how much extra signal-to-noise headroom the line currently has. A low or declining noise margin is an early warning of a marginal or degrading copper line, letting an admin proactively address line quality issues before the link actually drops. If cmHDSLModemNoiseMargin trends steadily downward over several weeks on one line, the admin would schedule a physical line inspection before it eventually fails to train.