Roll over charges enquiry

hello sir i need some clarification regarding roll over …first what is roll over (i have still confuse)…suppose usdinr feb contract trading at 62.30(margin of 1 lot 1200) will expires on 26 feb,usdinr march contract is at 62.60 (margin of 1 lot 1220))so here is my doubt on feb 26 if i buy usdinr march month contract at 62.60 means is there any additional charges included for me by my broker or nse …what is cost ,its in magin or its charged in my account please anybody know clarify now i am developing one stragedy based on this …help me

Hi Naveen,
Your questions have been answered here already:
To know what a rollover is:
To your question on margins: