Just had a look in the 7800N user manual and couldn't find anything about bandwidth monitoring/usage, though you can assign bandwidth priority to different protocols. If its really important perhaps its worth using tbbmeter?
TalkTalk Plus LLU ADSL2+ connected @ 17612/1019 kbps
Attenuation 27db down / 9db up
Noise Margin 3.3 db down / 12db up
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