Monitor Cisco 837 ADSL Usage

Monitor Cisco 837 ADSL Usage

I have setup Op Manager and configured it to monitor a Cisco ADSL 837 router that has a permanant VPN to our site.
It seems to be monitoring cpu, memory and the interfaces ok, except it isn't showing me what I expect for my usage of my DSL line?
Which interface should I look at to monitor the DSL connection? ATM0? Should I change it so it also reflects the correct speed that it has negoatiated? Op Manager shows it as 1mb up and 1mb down whereas I see it has negotiated 8mb down and 1mb up.

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