I Just like to know what is a VPN concentrator and its pupose.
Any device that acts as a VPN Server is called a VPN Concentrator. VPN is a very generic term can mean many technologies such as PPTP, L2TP, IPSEC, GRE, etc. But, most of the times when someone refers VPN Concentrator, it means IPSEC Server. You can use a VPN3000, Pix, ASA , IPSEC SPA Module on 6500 and Cisco Routers as a VPN Concentrator. Below is a link that has information on the VPN Concentrator devices:
*Pls rate if it helps*