WiMAX means Worldwide interoperability for Microwave Access. It is the wireless technology and enhancement of 802.16 standards. The forum describes WiMAX as “A standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL”. WiMAX has many salient advantages such as: high data rates, quality of service, scalability, security and mobility. In WiMAX, security is the major issue for transmission of data from end to end. In this paper, the authors discuss about NS2 used as simulation platform, MAC layer and protocol Architecture of WiMAX.