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 Powered by Billion BiPAC 7800N
Trouble with tbbmeter is that it only measures traffic on the computer it's installed on... if the OP wants to keep track of all internet traffic it might be better with PRTG, the 7800N is SNMP capable.
I used it for a while some years ago when I was running PCs, it's quite good- I just wish there was a Mac version