94 5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer

	using high	using DIFS, or
	priority with
		EIFS instead
	different
		of DIFS
	PF
	EDCF
	backoff
	entities
	DCF
	legacy
	receiving station stations

Figure 5.24: Scenario. All stations detect each other. If two or more stations transmit at the same time, a collision occurs.

The higher the number of legacy DCF stations, the lower is the achievable saturation throughput of the EDCF backoff entity. When operating in parallel to many legacy DCF stations, a smaller PF is helpful to increase the priority of EDCF over legacy DCF. In addition, the priority of EDCF over legacy DCF is significantly improved by forcing the legacy DCF station to operate with EIFS instead of DIFS.

5.1.4.38 EDCF Backoff Entities Against 8 DCF Stations

Figure 5.24 illustrates the scenario and Figure 5.25 shows the resulting throughput per AC vs. the offered traffic per backoff entity (a-c), and the distribution of backoff delays in saturation (d).

5.1.4.3.1Discussion

The PF and the interframe spaces are used similarly to the previous scenarios. It can be observed from Figure 5.25(a) that 8 EDCF backoff entities in total achieve a higher priority over 8 legacy DCF station than a single EDCF backoff entity. With an increased offered traffic, the influence of the PF becomes even more significant, as can be seen in Figure 5.25(b).

A clear priority of the 8 EDCF backoff entities over the 8 legacy DCF stations can be again achieved by forcing the 8 legacy DCF stations to operate with EIFS instead of DIFS, as can be seen in Figure 5.25(c). The analytical results in Figure 5.25(a-c) conform more precisely than before to the simulation results, because the assumption of geometrically distributed access probabilities per slot is more accurate with a larger number of backoff entities. Figure 5.25(d) illustrates the CCDFs of the backoff delay for all three scenarios. The delay increases again, since the number of contending backoff entities is now 16 in total. It must be highlighted that, when the legacy DCF stations use EIFS instead of DIFS, the 8 EDCF backoff entities now observe an increased backoff delay as well. The reason for this is as follows. Transmissions by EDCF backoff entities may collide more often than before because of the small contention window size.

5.1 HCF Contention-based Channel Access

95

[Mbit/s]	16	PF=2, AIFS=DIFS.					DCF sim.
	14	8 EDCF against 8 DCF stations.					EDCF sim.
							EDCF analyt.
AC	12						DCF analyt.
per	10
	8
thrp
	6
cummulative
	4
	2
	0
	0	2	4	6	8	10	12	14	16
			Offered traffic per station [Mbit/s]
			(a) standard configuration
[Mbit/s]	16
	14	PF=2, AIFS=DIFS, DCF uses EIFS.
		8 EDCF against 8 DCF stations.
AC	12
	10
per
	8
thrp							EDCF sim.
cummulative	6						EDCF analyt.
							DCF sim.
	4						DCF analyt.
	2
	0
	0	2	4	6	8	10	12	14	16
			Offered traffic per station [Mbit/s]

(c) DCF with extended interframe space

[Mbit/s]	16	PF=1.5, AIFS=DIFS.
							EDCF sim.
	14	8 EDCF against 8 DCF stations.
							EDCF analyt.
AC	12						DCF sim.
	10						DCF analyt.
per
	8
thrp
	6
cummulative
	4
	2
	0
	0	2	4	6	8	10	12	14	16
			Offered traffic per station [Mbit/s]

(b) EDCF with smaller PF

(d) backoff delays in saturation (simulation results)

lines w/o markers: analytical results (a-c), lines with markers: WARP2 simulation results

Figure 5.25: Throughput and backoff delay results for eight EDCF backoff entities contending with eight legacy DCF stations.

Therefore, the EDCF backoff entities have to use higher backoff stages, and more retransmissions. With higher backoff stages used by the EDCF backoff entities, there is a smaller influence of the legacy DCF stations using EIFS instead of DIFS.

5.1.4.3.2Summary

The results can be summarized as follows. The achievable saturation throughput of a number of EDCF backoff entities in contention with legacy DCF stations is supported with a smaller PF. As before, when operating in parallel to many legacy DCF stations, a smaller PF is helpful to increase the priority of EDCF over legacy DCF. In addition, the priority of EDCF over legacy DCF is improved by forcing legacy DCF stations to operate with EIFS instead of DIFS. Because of the small initial contention window size in the EDCF, collisions occur more often for transmissions initiated by EDCF backoff entities, which increases the resulting backoff delays even when the legacy DCF stations operate with EIFS instead of DIFS.