I want to make system in which clients connect to lan can get internet access on username and password basis so:-

clients can be (20 to 30)

1)users have username and password.
2)users get bandwidth on their user quota basis
3)users have variable speed for example if a user have 1mbps speed till 200mb and above 200mb he get 512kbps so, when a user uses internet more than 200mb than he get 512kbps.
4)users connect to internet by giving username and password.

what servers are required for this and any tutorial on it.
Thanks for help in advance.

probably you need a proxy server. Check this one: http://www.squid-cache.org/

I have looked in squid squid allow user based internet sharing
but i don't think it allow user quota based internet sharing.
thanks for reply.

have you seen this: http://tldp.org/HOWTO/Bandwidth-Limiting-HOWTO/ ?