Design of the Proposed Mechanism

The proposed transmission mechanism considers the characteristics of voice packets, and differentiates voice and non-voice packets in the WLAN MAC transmission. The initial idea behind the proposed mechanism is to disable the MAC layer acknowledgement for voice packets, and to minimize the overheads for performing acknowledgements. However, to disable the MAC acknowledgement for voice packets, lost packets cannot be detected and retransmitted, and the voice quality may degrade. Therefore, the proposed mechanism is improved herein by dynamically turning the MAC-layer acknowledgement on and off for each transmission attempt of a voice packet, depending on the WLAN packet loss condition and the target voice quality. If packet error and loss rarely occur, then the MAC-layer acknowledgment for a voice packet is turned off, which improves the efficiency of network and energy. Conversely, while the packet loss rate increases, the MAC-layer acknowledgement for voice packets is turned on to ensure the voice quality.

By applying the PS-Poll transmission scheme shown in Figure 1, a VoWLAN STA transmits an uplink voice packet, and then uses a PS-Poll frame to retrieve a downlink voice packet on the AP. In addition to voice packets, an AP and STA may exchange non-voice packets such as SIP messages. The proposed design requires a WLAN AP to be upgraded to understand SIP

and session description protocol (SDP), so that an AP can distinguish voice packets and non-voice packets. For an uplink voice packet transmission such as Step 4 in Figure 1, a MAC-layer transmission counter, given by , is associated with each uplink voice packet, where denotes the maximum number of MAC-layer transmission attempts for an uplink voice packet. Similarly, is associated with downlink voice packets such as Step 6 in

ul

Figure 1. and are dynamically adjusted based on the current packet loss rate, and the target voice quality that a VoIP session sets. and are negotiated by an AP and STA, and are stored on both sides. For instance, if , then an uplink voice packet is sent only once, and does not need to be acknowledged. Since is stored on both the STA and AP, the STA knows that the packet will not be acknowledged, so the STA can return to sleep or perform the next packet transmission without waiting for the acknowledgement frame.

Therefore, the STA can save the time and energy to receive acknowledgement frames from the AP. Similarly, for a downlink packet transmission, the STA can conserve energy by not sending an acknowledgement frame for a downlink voice packet. Hence, the overheads to perform the MAC acknowledgement for voice packets are eliminated, and both the energy efficiency and WLAN utilization can be improved. If , then the uplink voice packet that is sent at the first time needs an acknowledgement from the AP. If the AP receives the packet, it acknowledges the uplink voice packet, and the transmission is complete. Otherwise, if the STA does not receive the acknowledgement from the AP, the STA detects the packet loss

ul

and resends the uplink voice packet again. Since , the resent voice packet does not need the MAC acknowledgement. The STA proceeds for the next transmission or returns to sleep after resending the voice packet. Restated, the STA sends the uplink voice packet at most times that require the MAC-layer acknowledgement. The packet resent at the

time needs not be acknowledged. Clearly, increasing the values of and generally improves the voice quality, but also increases the radio resource and energy consumption for a VoWLAN session. The method to determine and is discussed later in this section. Non-voice packets such as SIP messages and PS-Poll frames, which are important packets and need to be reliably delivered, are transmitted by the standard WLAN MAC mechanism.

=2

ul

Nr

−1

ul

Nr

ul

Nr N_r^ul N_r^dl

ul

Nr N_r^dl

Figure 4. The proposed mechanism applied to the PS-Poll scheme

Figure 4 shows a timing diagram of the proposed method applied to the PS-Poll transmission scheme, where both N_r^ul =1 and N_r^dl =1, i.e. voice packets are neither acknowledged nor

retransmitted. Comparing Figure 2 and Figure 4 reveals that sending packets without acknowledgement frames significantly reduces the length of a duty cycle. This is because the length of an acknowledgement frame is nearly equal to that of a small voice packet. Then, the energy consumption of a VoWLAN STA for an uplink and downlink transmission by applying the proposed mechanism to the PS-Poll scheme, denoted as and

respectively, become

The decrease in energy consumption per duty cycle from adopting the proposed mechanism for the PS-Poll scheme is thus obtained:

(A)

Assume that the PS-Poll scheme with and without the proposed mechanism suffers the same

dl

numbers of overhearing transmissions, i.e. . Additionally, the WLAN MAC contention windows for the PS-Poll transmission scheme with and without the proposed mechanism are assumed to be equal, i.e. . Then, Equation (A)

can be rewritten as

M

Equation (B) reveals that the performance improvement by applying the proposed mechanism comes not only from the elimination of acknowledgement frames for downlink and uplink voice packets, but also from reducing the length of the period that a VoWLAN STA overhears the other VoWLAN STAs’ transmissions.

This proposed mechanism can also be applied to the U-APSD transmission scheme, eliminating the acknowledgement frame for downlink voice packets. Notably, the uplink voice packet in the U-APSD scheme is used to trigger download packet transmissions.

Therefore, the proposed design categorizes the important uplink voice packets as non-voice packets in the U-APSD scheme, and processes them by using the standard MAC transmission scheme to prevent the packet loss.