I found papers on this issue, but I think these two will help (especially the second link):
http://www.ericsson.com/hr/etk/dogadjanja/mipro_2009/12_1112_F.pdf
http://iosrjournals.org/iosr-jece/papers/Vol.%209%20Issue%203/Version-1/N09318387.pdf
Cheers!