Simulations were conducted to evaluate the WLAN performance and the energy consumption of a VoWLAN STA for different packet transmission schemes. The VoWLAN simulation environment comprised an AP and several VoWLAN STAs, which established VoIP sessions and exchanged voice packets with wired-line nodes in the same subnet. Table 1 summarizes the simulation parameters.
Table 1. Simulation parameters Parameters of Voice codec
GSM G.711
Bit rate 13.2 kbps 64 kbps
Framing interval 20 ms 20 ms
Payload 33 bytes 160 bytes
Parameters of the WLAN MAC accesses
802.11b 802.11g
SIFS/DIFS/Slot time 10 μs/50 μs/20 μs 10 μs/28 μs/9 μs
PHY preamble + header 192 μs 20 μs
PS-Poll/ACK 80 μs/56 μs 27 μs/19 μs
CWmin/CWmax 32/1024 16/1024
Parameters of the power consumption of a WLAN interface (ORiNOCO PC Gold, IEEE 802.11b) [14]
P /tx Prx 1400 mW/950 mW
P dz 60 mW
Bit Error Rates (BERs)
WLAN channel conditions 1.0e-5/1.0e-4
The length of a duty cycle, energy consumption, packet loss rate, and packet delay for a
VoWLAN STA by adopting the PS-Poll, U-APSD, PS-Poll with the proposed mechanism (denoted as PS-Poll-M) and U-ASD with the proposed mechanism (denoted as U-APSD-M) were first evaluated. The first simulation used the IEEE 802.11b and GSM voice codec, and assumed a good WLAN channel condition, i.e. BER = 1.0e-5. Figure 5(a) shows the average duty cycle per VoWLAN STA under different numbers of concurrent VoWLAN STAs in an AP. The y-axis of Figure 5(a) shows the length of a duty cycle over a voice framing interval which indicates the percentage of time that a VoWLAN STA must stay awake to transmit one uplink and one downlink voice packet every voice framing interval. A larger percentage of a duty cycle per voice framing interval implies that more energy is consumed for a VoWLAN STA. Figure 5(a) indicates increasing the number of concurrent VoWLAN STAs in an AP also increases the average duty cycle per VoWLAN STA, because a VoWLAN STA needs to spend more time contending the WLAN channel when more concurrent VoWLAN STAs are served by an AP. The figure illustrates that while the number of VoWLAN STAs in an AP is 1, the PS-Poll and U-APSD transmission schemes with the proposed mechanism decrease by about 10% length of a duty cycle than the transmission schemes without the proposed mechanism.
This performance improvement is gained only by eliminating acknowledgement frames. The performance improvement from applying the proposed mechanism grows as the number of concurrent VoWLAN STAs in an AP increases. This is because that a VoWLAN STA usually has to wait for other VoWLAN STAs’ transmissions while the AP is serving many VoWLAN
STAs. The proposed mechanism reduces the transmission time of voice packets, and hence, significantly reduces the average length of an overhearing period while the WLAN load becomes heavy. For instance, the average duty cycle per VoWLAN STA for the PS-Poll and PS-Poll-M schemes when supporting 9 concurrent VoWLAN STAs are 4.48 ms and 3.16 ms respectively. The proposed mechanism reduces the duty cycle by about 29%. The average duty cycles for the U-APSD and U-APSD-M schemes are 1.96 ms and 1.48 ms respectively, indicating a reduction of about 24% when adopting the proposed mechanism for the case that 9 VoWLAN STAs are served by an AP. The simulation results reveal that the proposed method removes the acknowledgement frames for voice packets, thus shortening each duty cycle, which also reduces the WLAN channel waiting time while a VoWLAN STA tries to transmit a voice packet. To evaluate the improvement of energy consumption, the energy consumption of a VoWLAN STA using various transmission schemes was investigated.
ORiNOCO PC Gold Card is used for this simulation [14]. Figure 5(b) shows the corresponding energy consumption of Figure 5(a). The proposed mechanism reduced the average energy consumption of PS-Poll and U-APSD by about 23% and 17%, respectively, while an AP is serving 9 concurrent VoWLAN STAs.
0
Number of VoWLAN STAs in an AP
Average duty cycle per VoWLAN STA (%) PS-Poll
U-APSD
Number of VoWLAN STAs in an AP Average power consumption per VoWLAN STA (mW)
PS-Poll U-APSD PS-Poll-M U-APSD-M
(b)
Figure 5. Aveage duty cycle and enegy consumption for a VoWLAN STA
Voice packets for which acknowledgement frames are not sent might be lost due to packet error or collision, degrading the voice quality. Therefore, the proposed mechanism dynamically adjusts the maximum number of transmission attempts, i.e. and , for each voice packet based on a target packet loss rate. In the simulations, the maximum permissible packet loss rate was set to 2%.
ul dl
Nt Nt
Figure 6 shows the average loss rates for uplink and downlink voice packets by applying different transmission schemes. The simulation results shows that although the packet loss rates of PS-Poll-M and U-APSD-M were slightly higher than those of PS-Poll and U-APSD, the packet loss rates for the proposed transmission mechanism are still less than 1%. The packet delays of different transmission schemes were then investigated. Figure 7 shows the average delay for uplink and downlink voice packets.
The figure indicates that the voice packets encounter longer delays as the number of VoWLAN STAs served by an AP increases. This is because that increasing the WLAN load causes more packets to be queued on the AP, increasing the packet delay. Since the proposed mechanism reduces the length of a duty cycle, it increases the maximal number of VoWLAN STAs that can be supported by a WLAN. Simulation results indicate that PS-Poll-M can support two more VoWLAN STAs than the PS-Poll scheme under a 50 ms delay and 2% loss rate constraints for voice packets. U-APSD-M was found to support four more VoWLAN STAs than the U-APSD scheme. Simulation results depicted in Figure 6 and Figure 7 reveal that the proposed mechanism improves the WLAN utilizations of the PS-Poll and U-APSD
transmission schemes by about 18% and 24%, respectively and also provides a similar voice
Number of VoWLAN STAs in an AP
Average uplink packet loss rate (%)
PS-Poll
Number of VoWLAN STAs in an AP
Average downlink packet lost rate (%)
PS-Poll U-APSD PS-Poll-M U-APSD-M
(b)
Figure 6. Average loss rates for uplink and downlink voice packets
0
Number of VoWLAN STAs in an AP
Average uplink packet delay (ms
200
Number of VoWLAN STAs in an AP
Average downlink packet delay (ms)
PS-Poll U-APSD PS-Poll-M U-APSD-M
(b)
Figure 7. Average delay for uplink and downlink packets
The performance improvement of the proposed mechanism by using the IEEE 802.11b,
different voice codecs and BERs was further evaluated. Figure 8 illustrates the length reduction of a duty cycle per VoWLAN STA from applying the proposed mechanism to the PS-Poll and U-APSD schemes. Simulation results indicate that the PS-Poll-M scheme using GSM codec performed better than that using G.711 codec, because the acknowledgement frame introduces relatively more overhead for low bitrate codecs such as GSM than high bit-rate codecs such as G.711. Additionally, the simulation results show that PS-Poll-M performs better at a BER of 1.0e-5 than 10e-4. This is because a poor WLAN channel quality leads to a high packet loss, implying that the proposed methods must increase the maximum number of transmission for voice packets to maintain an acceptable voice quality.
0 5 10 15 20 25 30
1 2 3 4 5 6 7 8
Number of VoWLAN STAs in an AP The length reduction of duty cycle per VoWLAN STA (%)
PS-Poll-M (GSM, 1.0e-5) PS-Poll-M (G.711, 1.0e-5) PS-Poll-M (GSM, 1.0e-4)
Figure 8. The length reduction of a duty cycle per STA under different codec and BERs
Finally, the performance improvement under different WLAN standards was investigated.
Figure 9 shows the average duty cycle per VoWLAN STA when using GSM codec in the IEEE 802.11g with BER = 1.0e-5. While the number of concurrent VoWLAN STAs that can be served by an AP increases, the probability to overhear other STAs’ transmission also increases. Therefore, the proposed mechanism that reduces the length of overhearing period efficiently reduces the length of a duty cycle for a VoWLAN STA for high speed WLAN standards. For instance, while 51 concurrent VoWLAN STAs are supported by an IEEE 802.11g AP, the proposed mechanism reduces the average duty cycle of PS-Poll and U-APSD by about 40% and 28%, respectively.
0 20 40 60 80 100
1 11 21 31 41 51 61 71 81 91 101 111 121 131 Number of VoWLAN STAs in an AP
Average duty cycle per VoWLAN STA (%) PS-Poll
U-APSD PS-Poll-M U-APSD-M
Figure 9. Average duty cycle per VoWALN STA in the IEEE 802.11g environment