Please help me out, i want to do two things:
1> to limit the bandwidth for a user (i.e. i am getting 12mbps of downlink speed and want to give only 2mbps to any user out of this)
2> i also want to limit the download quota (i.e. using 2mbps downlink speed an user cannot download more than 2gb)

i also studied that using qos we can do teh first one but is it also possible to do the same using nodogsplash. second can be done using some iptables magic. but for first which i should use or if i am getting wrong mean of qos and nodogsplash then please move me on right track.
any help will be greatly appreciated. 
thanks.

Didn't see your post. Nodogsplash will probably suit your needs and is pretty simple to setup. Just look out for the problem I'm having here,

https://forum.openwrt.org/viewtopic.php?id=23164

Also the latest trunk, actually around revision 16000 or so the ipt_IMQ was removed and replaced. I have no idea how to get Nodog running with that and have focused on an older build. I just need some iptables expert to help me out.