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

legacy priority

variable	low
priority	priority

receiving station

Figure 5.18: Scenario. One backoff entity per station. All stations detect each other. If two or more stations transmit at the same time, a collision occurs.

The analytical results overestimate the share of the legacy stations for the parameter combinations shown in the left hand side of the figure. This is a result of the assumption that the binary exponential backoff of 802.11 can be approximated by the σ -persistent CSMA, even for such a small number of backoff entities. With reduced priority in the more right hand side parameter combinations, the 10 dominating legacy backoff entities obtain the largest throughput and all other backoff entities are suppressed entirely.

5.1.4QoS Support with EDCF Contending with Legacy DCF

The earliest access time of EDCF backoff entities cannot be smaller than DIFS. Thus, the question arises if EDCF backoff entities do really have a chance to achieve a higher channel access priority than legacy DCF stations.

	0.7	← higher priority	← legacy priority	lower priority →
	0.6			512 bytes frame body, no RTS/CTS
(norm.)			analyt.	2+10+4 backoff entities
	0.5
AC	0.4		sim.
per				variable pr. (high → low) sim.
(=thrp.)	0.3			variable pr. (high → low) apprx.
				legacy pr. sim.
	0.2			legacy pr. apprx.
share				lower pr. sim.
				lower pr. apprx.
	0.1		sim.

0									analyt.
AIFS: 2	2	2	2	2	2	2	2	2	2	2	2	3	4	5	6	7	8	9	9	9	9	9	9	9	9	9	9

CWmin:	7	7	7	7	7	8	9	10	12	13	14	15	15	15	15	15	15	15	15	17	19	21	23	25	27	29	31	31
16 × PF:	24	26	28	30	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	32	40

lines w/o markers: analytical approx.; lines with markers: WARP2 sim. results

Figure 5.19: Saturation throughput per AC with 2 backoff entities with varying EDCF parameters, contending with 10 legacy and 4 lower priority backoff entities.