Impact of each 802.11e parameter

In Section 5.6.1, we saw that using AC VO and AC VI for VoIP traffic does not increase the capacity due to the TXOP problem. Then, what if we disable the TXOP in AC VO? Also, in Section 5.6.2, we saw that the impact of TCP traffic using AC BE on VoIP traffic is the same as that using AC BK. It could mean that QoS for VoIP traffic can be protected against TCP traffic only by assigning different TXOP values, because the difference between the two ACs is only their AIFSN values (AC BE uses 3, and AC BK uses 7). In order to investigate the problems, the impact of each 802.11e parameter was identified

Table 5.3: Experimental configuration

Traffic VoIP traffic TCP traffic Effective

Params CWmin/CWmax AIFS TXOP CWmin/CWmax AIFS TXOP parameter

Config 1 7/15 2 0 31/1024 2 0 CW

Config 2 7/15 2 0 7/15 7 0 AIFS

Config 3 7/15 2 0 31/1024 7 0 CW + AIFS

Config 4 7/15 2 3264 31/1024 2 0 TXOP

via additional experiments. The delay of VoIP traffic (64 kb/s and 20 ms packetization interval) and TCP traffic throughput were measured by setting different values to each parameter in each experiment (Table.

5.3), and Fig. 5.23 and Fig. 5.24 show the results.

We can see that the capacity decreases from 15 calls to 13 calls when VoIP traffic is prioritized with only either CW or AIFS (Fig. 5.23(a) and Fig. 5.23(b)), and when both are applied, the capacity still decreases to 14 calls (Fig. 5.23(c)); the delays with 14 calls in Figs. 5.23(a) and 5.23(b) are below 60 ms, but the packet loss rate is about 5% according to Figs. 5.24(a) and 5.24(b), which does not meet the requirements of the QoS for VoIP traffic explained in Section 5.5.8. In the same way, while the delay with 15 calls in Fig. 5.23(c) is under 60 ms, we can see that the packet loss rate is not acceptable from Fig.

5.24(c). However, prioritizing VoIP traffic using TXOP only can protect the QoS of VoIP traffic against TCP traffic, keeping the capacity to 15 calls, even though the delay slightly increases (Fig. 5.23(d)).

Instead, the throughput of TCP traffic decreased; with 15 VoIP sources, TCP throughput decreased from 1.2 Mb/s in other three cases to 0.8 Mb/s by 0.4 Mb/s. However, the total throughput decreases by only 0.3 Mb/s according to Fig. 5.24. This is because the downlink throughput of VoIP traffic using TXOP only is slightly bigger than other cases due to the lower packet loss rate.

5.7 Related work

Hole et al. [22] provides an analytical upper bound on the capacity for VoIP applications in IEEE 802.11b networks, evaluating a wide range of scenarios including different delay constraints, channel conditions and voice encoding schemes using analysis, assuming only the long preamble. The capacity of 64 kb/s CBR VoIP traffic with the low bit error rate was the same as my experimental results.

Veeraraghavan et al. [83] analyzed the capacity of a system that uses PCF, where clients can transmit data frames only when they are polled by the AP, for CBR and VBR voice traffic, using Brady’s model [6] for VBR voice traffic. In their analysis, they used values of 75 ms and 90 ms as the CFP interval, which causes a delay that is not acceptable for VoIP traffic. The capacity for VoIP traffic with a 90 ms CFP interval was 26 voice calls, but the maximum delay was 303 ms.

Chen et al. [9] evaluated the capacity of VoIP traffic, via simulations with IEEE 802.11e Enhanced DCF (EDCF) and Enhanced PCF (EPCF), which are called EDCA and HCCA in the 802.11e standard.

They used G.711, G.729 and G.723.1 as voice codecs and assumed CBR traffic. IEEE 802.11e provides low end-to-end delay for voice packets even if mixed with best effort traffic.

In [18] and [41], the capacity for VoIP traffic was measured experimentally. However, most of those factors mentioned in the previous section were not taken into account, and no comparison with simulation results was provided.

Sachin et. al. [18] experimentally measured the capacity for VoIP traffic with a 10 ms packetiza-tion interval and the effect of VoIP traffic on UDP data traffic in 802.11b. They found that the capacity of

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Figure 5.23: The effect of each 802.11e parameter; delay is 90th percentile (refer to Table 5.3 for the experimental parameters in each case.)

0 Std-Dev of packet loss rate of downlink VoIP Average packet loss rate of downlink VoIP

(a) Impact of CW (config 1) Std-Dev of packet loss rate of downlink VoIP Average packet loss rate of downlink VoIP

(b) Impact of AIFS (config 2) Std-Dev of packet loss rate of downlink VoIP Average packet loss rate of downlink VoIP

(c) Impact of CW+AIFS (config 3) Std-Dev of packet loss rate of downlink VoIP Average packet loss rate of downlink VoIP

(d) Impact of TXOP (config 4)

Figure 5.24: The effect of each 802.11e parameter on total throughput and packet loss rate

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such VoIP traffic is six calls and the effective available bandwidth is reduced by ongoing VoIP connections.

Anjum et. al. [41] also measured the capacity and the performance of their new scheme, Backoff Control and Priority Queuing (BC-PQ) experimentally. However, in order to determine the capacity for VoIP traffic, they used the packet loss rate, which depends on the network buffer size of the AP in DCF, unless the wireless link is unreliable, as shown in Section 5.5.8. They found that the capacity with 20 ms packetization interval is 10 calls, which differs from our results. We believe that the difference is due to the effect of the Auto Rate Fallback (ARF) and preamble size, but such parameters are not mentioned in the paper.

5.8 Conclusion

In this chapter, the capacity for VoIP traffic was measured via experiments with actual wireless clients in the ORBIT test-bed, and compared it with the theoretical capacity and simulation results. Also, some factors were identified that are commonly ignored in simulations and experiments but affect the capacity significantly, and the effect was analyzed in detail with additional experiments and simulations.

Also, it was confirmed that after considering all those factors, we can achieve the same capacity among simulation, experiments, and theoretical analysis, resulting in 15 calls for CBR and 38 calls for VBR VoIP traffic.

The capacity with the 802.11e standard was measured including the effect of TCP traffic on VoIP traffic. From the experiments, it was found that when using 802.11e, the QoS for VoIP traffic is protected well, but the capacity is not improved even with a few milliseconds of TXOP due to significantly increased retransmissions during TXOP.

Even though the effect of those factors on the VoIP capacity was analyzed in this study, those factors affect any experiment and simulation with 802.11 WLANs, and this study can be utilized in their analysis and comparison.

Chapter 6