Network Monitoring

Hi,
I am using a Cisco 1800 Series Router with NAT enabled. Our network consists of a Domain Server & Call Centre Server for Telephony. Our network is simply designed with 2 swicthes and 4 Access Points and 1 PoE switch for Access Points.
Our WAN connection is terminated on router and hence the gateway is router for all WAN traffic.
Now,I just want to ask that how can I monitor the bandwidth usage on the router, ie currently, per day, per month etc….

Can you please help me which tool should I use…. There is software called DU Meter to monitor bandwidth,but u know it can only be installed on PC …. I want to monitor router ….

Thanks

Zeeshan