Nowadays IEEE 802.11 family is the most popular technology amongst wireless protocols including Bluetooth, UWB and WiMAX for wireless services, among which media streaming such as voice over Internet Protocol (VoIP) and IPTV are most demanded with high challenge. To accommodate the QoS issues, EDCA and HCCA functions are defined in IEEE 802.11e. However, EDCA provides QoS based on probability, so it is still unable to fulfill the requirements of time sensitive applications such as VoIP. In this study, we focus on HCCA polling scheme. We design a new time-based polling scheduler called Adaptive Time-Stamp Polling (ATSP) scheme for QoS improvement, as well as design a mechanism that detects silence mode of a VoIP conversation so as to reduce the frequency of polling silent stations. When the station comes back to talk-spurt mode, we revert to the original frequency to poll. Simulation results show that our scheme improves the QoS on throughput, jitter, and access delay significantly.