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

5.1.4.1.2Summary

The results can be summarized as follows. The PF is not a helpful measure to increase the priority of EDCF over legacy DCF, as long as a small number of backoff entities operate at the same time, i.e., as long as the collision probability is relatively small. However, the priority of EDCF over legacy DCF is significantly improved by forcing the legacy DCF station to operate with EIFS instead of DIFS.

5.1.4.21 EDCF Backoff Entity Against 8 DCF Stations

Figure 5.22 illustrates the scenario where the EDCF backoff entity now operates in parallel to 8 legacy DCF stations. As before, Figure 5.23 shows the resulting throughput per AC vs. offered traffic per backoff entity (a-c), and the distribution of backoff delays in saturation (d).

5.1.4.2.1Discussion

The PF and the interframe spaces are used similarly to the previous scenarios. Figure 5.23(a) shows the results for the standard configuration with PF=2. Figure 5.23(b) shows the results for scenarios where the EDCF backoff entity operates with PF=1.5. Figure 5.23(c) shows the results for scenarios where all 8 legacy DCF stations operate with EIFS instead of DIFS at any time. Finally, as before, Figure 5.23(d) illustrates the CCDFs of the backoff delays for the three scenarios.

It can be observed from Figure 5.23(a) that in contention with 8 instead of 1 legacy DCF station, the achievable saturation throughput for the EDCF backoff entity with standard configuration is considerably lower than in contention with 1 legacy DCF station.

However, the single EDCF backoff entity is still achieving a higher throughput than a single legacy DCF station (the total sum of the results of all 8 legacy DCF stations are shown in the figure), but this throughput is lower than what was achieved before, when contending against 1 legacy DCF station (see Figure 5.23(a) and Figure 5.21(a)). Figure 5.23(b) shows the results for scenarios where the EDCF backoff entity operates with the high priority EDCF parameters, now including PF=1.5. This has now more impact than before, as there are more contending backoff entities in total, and the number of collisions is higher. Thus, Figure 5.23(b) indicates saturation throughput improvements for the EDCF backoff entity as a result of the usage of the smaller PF. However, comparing Figure 5.23(b) with Figure 5.21(b), it can be seen that the 8 legacy DCF stations still have an undesirable effect on the throughput results for the single EDCF backoff entity.

5.1 HCF Contention-based Channel Access																			93
[Mbit/s]	16	PF=2, AIFS=DIFS.					DCF sim.			[Mbit/s]	16	PF=1.5, AIFS=DIFS.					DCF sim.
	14	1 EDCF against 8 DCF stations.					EDCF sim.				14	1 EDCF against 8 DCF stations.					EDCF sim.
							EDCF analyt.										EDCF analyt.
AC	12						DCF analyt.			AC	12						DCF analyt.
	10										10
per										per
	8										8
thrp										thrp
	6										6
cummulative										cummulative
	4										4
	2										2
	0										0
	0	2	4	6	8	10	12	14	16		0	2	4	6	8	10	12	14	16
			Offered traffic per station [Mbit/s]										Offered traffic per station [Mbit/s]

(a) standard configuration

(b) EDCF with smaller PF

cummulative thrp per AC [Mbit/s]

16
14 1 EDCF against 8 DCF stations.
12	PF=2, AIFS=DIFS, DCF uses EIFS.
10
8						EDCF sim.
6						EDCF analyt.
4						DCF sim.
						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	(d) backoff delays in saturation (simulation
	results)

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

Figure 5.23: Throughput and backoff delay results for one EDCF backoff entity contending with eight legacy DCF stations.

A clear priority over the 8 legacy DCF stations can be achieved only by forcing the 8 legacy DCF stations to operate with EIFS instead of DIFS all the time, as can be seen in Figure 5.23(c). As before, the analytical results in Figure 5.23(a-c) show only the same trends as the simulation results due to the assumption of geometrically distributed access probabilities per slot.

Figure 5.23(d) illustrates the CCDFs of the backoff delays for all three scenarios. The delays are in general larger than before, due to the higher number of backoff entities. As before, when the legacy DCF stations use EIFS instead of DIFS, the EDCF backoff entity observes very small backoff delays.

5.1.4.2.2Summary

The results can be summarized as follows. The achievable saturation throughput of an EDCF backoff entity in contention with legacy DCF stations depends considerably on the number of legacy DCF stations that operate in parallel.