This paper gives an overview about the various PHY and MAC layer specification which define the IEEE 802.16 standard and which define the Fixed WiMAX (Ver. 802.16d-2004) and Mobile WiMAX (Ver. 802.16e-2005) network scenarios. The authors also discuss about the various updates in the Advanced Air Interface update (802.16m - 2011), which aims at fulfilling the 4G requirements as put forward by the ITU IMT-Advanced requirements. 4G is the term used to refer to the next wave of high-speed mobile technologies that will be used to replace current 3G networks. As per the standards of 4G under ITU (International Telecommunication Union), the new technology must provide peak speed for Internet communication at 100 Mbps for high mobility users and 1 Gbps for low mobility users.