Recently, significant research effort has been focused in the design of Wireless Local Area Networks (WLANs). WLANs are attractive because they offer important advantages, including interoperability, mobility and cost- effective deployment. However, the ever-increasing demand on very high data rates and coverage have boosted the expansion of IEEE 802.11 a/b/g WLANs to new standards; IEEE 802.11n. In this thesis, performance of the IEEE 802.11n is studied with focus on Medium Access Control (MAC) layer and Physical Layer (PHY) enhancements. The new features that IEEE 802.11n introduced in MAC layer are the frame aggregation schemes, the Block Ack and reverse direction. The new features aim in reducing the overhead of the MAC layer. While, in the PHY layer uses MIMO technology and channel bonding to improve throughput and coverage of WLANs. Many simulations were performed to provide a better knowledge of these new modules, in various network environments. The simulation tool used is the Network Simulator tool known as NS-3. Many improvements and implementations were implemented in NS-3 to support these new features that IEEE 802.11n introduced.