New differential mode excitation photoacoustic scheme for near-infrared water vapour sensing
Abstract A new gas sensor based on the differential mode excitation photoacoustic (DME-PA) technique is presented. The DME-PA method utilizes the selective excitation of two different modes in a resonant photoacoustic cell and the gas concentration is derived from the amplitude ratio of these acoustic modes. The presented device has only one cell that acts as both sample and photoacoustic cell and uses a current-modulated near-infrared light emitting diode as excitation source. No power-meter is required for PA signal normalization. The new DME-PA sensor was tested with water vapour yielding a ±250 ppm uncertainty for the water vapour content of ambient air.
In this paper, we study the behavior of voice over IP (VoIP) traffic over IEEE 802.11 wireless networks. Specifically, we propose a QoS provisioning mechanism for VoIP traffic, and suggest a practical solution to configuring the 802.lie enhanced distributed control access (EDCA) parameter sets for different types of traffic. With our mechanism, the EDCA parameter sets can be easily configured via software interface in off-the-shelf WiFi phone products and it is not required to modify the operations of access points or 802.11 MAC layer protocols. The performance of our mechanism is evaluated via ns-2 simulations and via laboratory experiments over Quanta's 02 dual-mode handsets. The results show our mechanism can provide effective and efficient QoS provisioning for VoIP traffic over IEEE 802.11 WLANs.