Please go through links you will get some idea.
on page 27,
http://www.ceenet.org/workshops/lectures2004/Richard_Perlman/additional_materials_(wimax)/802_16B-Wireless-MAN.ppt
http://www.intel.com/technology/itj/2004/volume08issue03/art04_ieee80216mac/p05_implchallenges.htm
Thanks,
srinivas.