As a “wide-deployed” wireless local area networks (WLANs) standard, the IEEE 802.11n was designed to support a high data transmission rate (up to 600 Mbps) and maintain a compatibility with the previous versions. Due to a substantial demand of real-time data services, such as VoIP and video conference, an enhanced quality of service (QoS) mechanism become crucially important. The IEEE 802.11n standard inherits the enhanced distributed channel access (EDCA) mechanism from the IEEE 802.11e, as the MAC layer improvement. It classifies the traffic into four different classes of data, called access categories (ACs). However, EDCA still suffers from a data packet collision, caused by hidden nodes problems. The packet loss ratio and end-to-end delay increase along with the escalation of real-time traffic. To address the issue, we conduct a simulation in Riverbed Modeler 17.5 environment with a 128, 256, 512, and 1,024 Bytes of request to send (RTS) threshold variation value to overcome a collision caused by hidden nodes problem. The result shows that the 512 Bytes of RTS threshold value gives the best QoS output among them all.