Hi Imran,
In short, FR allows a cost effective WAN solution rather dedicated leased lines.
Per se lets assume your traffic pattern varies by time in a day and your peak hrs. be 10.00 am to 12.00 pm.
Commited burst rate allows you to provide maximum bandwidth during this period as you wish. You are expected to define such parameters as SLAs with the service provider.
Yes DLCIs has only local significance. In otherwords, a DLCI value assigned to the router has significance only with the FR switch to which it is connected. Apparently, packet delivery beyond the FR-Switch to which router is connected must be service provider's concern.So there may be same DLCI number defined to other end of the router in a PVC.
Yes you could use a router as FR Switch.This URL may help you out.
http://www.cisco.com/warp/public/125/fr_switching.pdf
Hope this helps.
Regds,
Dinesh