I have a 256kbps unlimited connection from BSNL. From its dataone site tariff you will find they do provide for LIMITED plans(where there is cap on the download/upload) they provide a bandwidth (wherever technically feasible)
256 Kbps/ upto 2 Mbps. So the speed will vary between those speeds, and this does not affect bsnl because this type of system is for limited plans where incase the user goes over the cap limit, he will be charged so in a way its in the interest of bsnl if the speed is increased lets say when the traffic is less which in effect will load the pages at the customer end faster and the customer may be tempted to open more pages thus generate more revenue for bsnl if the customer goes over the limit.