He quitado el zyxel y he puesto el comtrend, ademas he probado a forzar el SNR con los siguientes resultados:
snr 90
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 896
Downstream Upstream
SNR Margin (dB): 9.1 23.0
Attenuation (dB): 51.0 30.5
snr 80
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 896
Downstream Upstream
SNR Margin (dB): 9.0 24.0
Attenuation (dB): 51.0 30.5
Output Power (dBm): 14.2 12.5
snr 70
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 896
Downstream Upstream
SNR Margin (dB): 9.5 24.0
Attenuation (dB): 51.0 30.5
snr 60
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 896
Downstream Upstream
SNR Margin (dB): 9.5 23.0
Attenuation (dB): 51.0 30.5
Output Power (dBm): 14.1 12.3
snr 50
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 864
Downstream Upstream
SNR Margin (dB): 9.2 25.0
Attenuation (dB): 51.0 30.5
snr 40
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 896
Downstream Upstream
SNR Margin (dB): 8.9 25.0
Attenuation (dB): 51.0 30.5
snr 30
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 896
Downstream Upstream
SNR Margin (dB): 8.7 23.0
Attenuation (dB): 51.0 30.5
snr 20
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 864
Downstream Upstream
SNR Margin (dB): 9.6 24.0
Attenuation (dB): 51.0 30.5
snr 10
Line Rate - Upstream (Kbps): 512
Line Rate - Downstream (Kbps): 896
Downstream Upstream
SNR Margin (dB): 9.1 25.0
Attenuation (dB): 51.0 30.5