4.2 Hybrid Coordination Function, Contention-based Channel Access					51
CWmin[higher pr.]
busy	higher			time
channel
	priority AC
				CWmin[medium
AIFS[higher pr.]				(legacy) pr.]
	medium
	priority AC
AIFS
[medium pr.]			lower
			priority AC
AIFS[lower pr.]			CWmin[lower pr.]
X1	X2	X3	X4	X5
exclusive higher pr.				exclusive lower pr.
high and medium pr. content				medium and lower pr. content
				all three priorities content

Figure 4.6: Overlapping contention windows of three ACs, higher, medium, lower priority. The initial contention windows of higher and lower priority overlap.

4.2.2.4Maximum TXOP Duration as Parameter per Access Category

In addition to the backoff parameters, the TXOPlimit [AC ] is defined per AC as part of the EDCF parameter set. The larger TXOPlimit [AC ], the larger the share of capacity for this AC.

4.2.3Collisions of Frames

As described above, four backoff entities reside inside an 802.11e station with different EDCF parameters. During contention, when the counters of two or more backoff entities in the same station reach zero at the same time, a collision is avoided as explained in the following. Upon access to the same slot by more than one backoff entity, the backoff entity with the higher priority will transmit, whereas all other backoff entities will act as if a collision occurred on the channel. See Figure 4.2 for an illustration. There is still a chance that the frame transmitted by the backoff entity with the higher probability will collide with another frame transmitted by backoff entities of other stations.

4.2.4Other EDCF Parameters per AC that are not Part of 802.11e

4.2.4.1Retry Counters as Parameter per Access Category

The retry counter RetryCnt [AC ] defined per AC is a candidate to be included in the EDCF parameter set. The larger RetryCnt [AC ], the more often a backoff entity of this AC will attempt to deliver MSDUs. In general, also the maximum

52	4. IEEE 802.11e Hybrid Coordination Function

size of the contention window is determined by the RetryCnt [AC ]. Since the influence of the retry counter can be mitigated by setting the CWmax [AC ] accordingly, a differentiation of the retry counter per AC, i.e., RetryCnt [AC ], is not included in the 802.11e standard.

4.2.4.2Persistence Factor as Parameter per Access Category

In legacy 802.11, a new contention window is used after any unsuccessful transmission attempt. This contention window is twice as large as the previously used contention window. A new backoff counter out of this enlarged contention window is drawn to reduce the probability of a new collision.

In 802.11e, instead of increasing the contention window by the factor 2, a Persistence Factor ( PF [AC ]), can be used to support priorities between ACs. Whereas in legacy 802.11, the contention window is always doubled after any unsuccessful transmission attempt (equivalent to PF=2), 802.11e may use PF [AC ] to increase the contention window differently for each AC. The contention window size is calculated as

CW (i )= min (CWmax [AC ], (CWmin [AC ]+1) PF [AC ]i −1−1),

where i indicates the backoff stage. This calculation of the contention window upon unsuccessful transmission attempts is illustrated in Figure 4.7.

first attempt of backoff entitiy [AC]

second attempt of backoff entitiy [AC]

3rd attempt of backoff entitiy [AC]

		random start	smaller contention windows allow a backoff entity to
previous	AIFS	new frame	access the channel with priority higher than legacy stations			backoff
	[AC]*
frame exchange		exchange				stage 1
	CW = CWmin[AC]*** -1			*) here, AIFS[AC] remains
	AIFS	random start		constant, independent	time
previous		repeated frame		of the backoff stage		backoff
frame exchange	[AC]*	exchange				stage 2
CW = (PF[AC]**)2-1 x CWmin[AC]*** -1				**) An AC-specific “persistence
			random start
				factor“, PF[AC], can be used to
	AIFS
previous			repeated frame	control the growth of the CW		backoff
	[AC]*
frame exchange			exchange			stage 3

	CW = (PF[AC]**)3-1 x CWmin[AC]*** -1			***) minimum and
	(upon unsuccessful attempts, the CW continues			maximum size of CW
				can be defined per AC
	to grow, up to AC-specific CWmax[AC])
			random start

i-th and	previous		AIFS		backoff
following			[AC]*	repeated frame exchange
	frame exchange				stage i
attempts
(until AC-specific retry		CW = (PF[AC]**) i -1		x CWmin[AC]*** -1 = CWmax[AC]***
				NOTE: NOT ALL ENHANCEMENTS SHOWN
counter exceeds limit, or,
				IN THIS FIGURE ARE PART OF 802.11e
until ACK reception)

Figure 4.7: Increase of contention window size in EDCF, after unsuccessful frame exchange attempts. The growth of the contention window per attempt depends on many parameters. Not all parameters shown in this figure will be part of the 802.11e standard.

4.2 Hybrid Coordination Function, Contention-based Channel Access

53

How the PF [AC ] may help to support QoS in 802.11e is illustrated in Figure 4.8 and Figure 4.9, for a scenario with three priorities labeled with “higher,” “legacy” and “lower”. In Figure 4.8, all ACs use the same PF [AC ] as the legacy priority. i.e., PF [AC ]= 2 for all ACs. In contrast, in Figure 4.9, the higher priority AC uses a smaller and the lower priority uses a larger PF [AC ].

The figures indicate that PF [AC ] is a help to support QoS especially in scenarios with a high number of collisions. For example, in case of collisions, a high priority backoff entity does not need to increase its contention window by factor 2. This is helpful to give this high priority backoff entity the chance to access the channel earlier than other backoff entities, to retransmit the collided frame.

The PF [AC ] is not included in the 802.11e standard, but may be an important means to support QoS against legacy stations, as will be shown in Chapter 5, where 802.11e is evaluated.

backoff stage

1

CW=31

higher pr.

CW=15

legacy pr.

CW=7

lower pr.

2

CW=63

CW=15

CW=31

CW=127

3

CW=63

PF=2

CW=31

4

CW=255

CW=127

CW=63

5

CW=511

CW=255

CW=127

6

CW=1023

CW=511

7

CW=255

CW=1023

8

CW=511

CW=1023

CW=512

2

4

15

31

63

127

255

511

1023

position and size of contention windows of three parallel backoff entities (log.)

Figure 4.8: Change of contention window size with increasing backoff stage, PF=2 for all ACs.

backoff stage

1

2

3

4

5

6

7

8

CW=31

higher pr.

CW=15

legacy pr.

CW=7

lower pr.

CW=82

CW=9

CW=31

CW=215

CW=63

PF[AC]=1.2; 2; 2.6

CW=11

CW=561

CW=127

CW=13

CW=1023

CW=255

CW=16

CW=1023

CW=511

CW=19

CW=1023

CW=23

CW=1023

CW=28

2

4

15

31

63

127

255

511

1023

position and size of contention windows of three parallel backoff entities (log.)

Figure 4.9: Change of contention window size with increasing backoff stage. Here, different PFs are used for the 3 priorities (ACs): PF[AC=high, legacy, low]=1.2, 2, 2.6.