measuring traffic with iptables

i have a wireless network that is connected to internet over nat.there is ap that is connected to another ap in bridge mode, on ap is used for clients, and the other is connected to the machine that is doing masquerading. so i want to measure traffic of my clients and i thought about doing it with snmp, but ap`s that my clients are using doesn`t have snmp. how can i measure it using iptables?