This read-only value reports the downstream signal-to-noise ratio margin, the safety buffer the xDSL line has above the noise floor in the receive direction. A low or shrinking margin warns an admin that the downstream link is close to losing sync or having to retrain at a lower rate due to noise or attenuation. For example, if this margin drops from a healthy value toward zero during a storm, an admin would expect the line to renegotiate to a lower downstream rate or drop.