112		5. Evaluation of	IEEE 802.11e with the IEEE 802.11a Physical Layer
	1	BPSK1/2 (6 Mbit/s)		1	BPSK1/2 (6 Mbit/s)
(norm.)	0.8	16QAM1/2 (24 Mbit/s)	(norm.)	0.8	16QAM1/2 (24 Mbit/s)
		64QAM3/4 (54 Mbit/s)			64QAM3/4 (54 Mbit/s)
	0.6	CW=0, AIFSN=1		0.6	CW=0, AIFSN=1
saturation thrp.			saturation thrp.
	0.4			0.4
	0.2			0.2
	0			0
				1	2	10
	1	2	10
					number of HCs allocating CAPs
		number of HCs allocating CAPs

(a) 48 byte frame body size.

(b) 512 byte frame body size.

	1	BPSK1/2 (6 Mbit/s)		1	BPSK1/2 (6 Mbit/s)
(norm.)	0.8	16QAM1/2 (24 Mbit/s)	(norm.)	0.8	16QAM1/2 (24 Mbit/s)
		64QAM3/4 (54 Mbit/s)			64QAM3/4 (54 Mbit/s)
thrp.	0.6	CW=0, AIFSN=1	thrp.	0.6	CW=0, AIFSN=1
	0.2			0.2
saturation			saturation
	0.4			0.4
	0			0
	1	2	10	1	2	10
		number of HCs allocating CAPs			number of HCs allocating CAPs

(c) 1514 byte frame body size.

(d) 2304 byte frame body size.

Figure 5.38: Saturation throughput for different frame body sizes, PHY modes, vs. the number of HCs in overlapping QBSSs.

5.4.3MSDU Delivery Delay in Coexistence Scenarios

The previous results were obtained with the analytical model, and show the saturation throughput. In addition to the saturation throughput analysis, it is worthwhile to look -by means of simulationat the MSDU Delivery delay that can be expected in realistic scenarios.

5.4.3.1Scenario

Two fully overlapping QBSSs as indicated in Figure 5.39 or one of the two QBSSs in an isolated environment are investigated in the following. Transmission powers and distances between stations are chosen in such a way that they are not hidden to each other with the selected PHY modes. All frames but the data frames are transmitted with the PHY mode BPSK1/2 (6 Mbit/s). Data frames are transmitted with the PHY mode 16QAM1/2 (24 Mbit/s). Each station generates the same mixture of offered traffic of three data streams, which are labeled “high (AC 6)”, “medium (AC 5)” and “low (AC 0),” according to their priorities. At the high priority AC, MSDUs with a frame body size of 80 byte arrive periodically to

5.4 HCF Controlled Channel Access, Coexistence of Overlapping QBSSs

113

	QBSS1
station 3.1
	station
station 4.1	1.1
	(AP)

3 DL streams per station with different priorities

6 streams (3DL + 3UL)

station

3.2

station 2.1

station 5.1

station 4.2

short distances, every station can detect any other station

QBSS2	6 streams
	(3DL + 3UL)
	station 2.2
station
1.2
(AP)	station 5.2

3 DL streams per station with different priorities

Figure 5.39: Coexistence scenario of overlapping QBSSs.

model isochronous traffic that is usually served with CAPs as part of the controlled channel access in the HCF. The repetition period depends on the offered traffic, and is 5 ms for the 128 kbit/s traffic stream. The medium and low priority ACs carry MSDUs with a constant frame body size of 200 byte with negative exponentially distributed inter-arrival times, each stream with 160 kbit/s. Table 5.7 shows the EDCF parameters used for the three priorities. If not stated otherwise, neither RTS/CTS nor fragmentation is used. The fragmentation threshold is set to 256 byte. The TXOPlimit is set to allow backoff entities to exchange a single data frame with the contention-based channel access of the HCF.

5.4.3.2Simulation Results and Discussion

Figure 5.40(a) shows the resulting MSDU Delivery delay distributions for an isolated QBSS. All backoff entities including the AP contend for medium access via EDCF. The HC resides in the AP. In addition, the AP carries one isochronous downstream (80 byte frame body size per MSDU, 128 kbit/s) which is delivered with the controlled channel access of the HCF by allocating CAPs with highest priority, higher than AC6, and therefore labeled as “AC7”. The data frames of this stream are immediately transmitted after PIFS when the channel is detected idle. As stated earlier, the controlled channel access of the HCF achieves its strict delay requirements by setting a maximum TXOP duration with the TXOPlimit for all other priority classes. The different QoS levels achieved by the priority classes are clearly visible: the higher the priority, the smaller the delay. The minimum delay in the figure results from the MSDU Delivery time, and depends on the duration of the respective frame. The delay is dependent on the offered traffic. With increased offered traffic, the delay of the three contention-based channel access AC 0, 5, 6 increase, whereas the maximum delay of the controlled AC 7 stays constant. The scenario is only slightly loaded. AC 6 is partially served better than AC 7 because there is only one AC 7 stream, whereas multiple streams in AC 6.

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

Figure 5.40(b) shows the resulting MSDU Delivery delay distributions for an isolated QBSS, but in comparison with Figure 5.40(a) with an increased offered traffic at the low and medium priority streams; the offered traffic is 320 kbit/s per stream instead of 160 kbit/s per stream. The line labeled as “AC7” corresponds to the stream that is served with CAPs. Only this stream stays within its maximum delay limit.

In contrast, if longer MSDUs are transmitted through the medium and lower priority streams, i.e., if the TXOPlimit is increased, then the maximum MSDU Delivery delay of the highest priority, served with CAPs, increases as well. This is shown in Figure 5.40(c). The same offered traffic as in the previous scenario (a) is simulated, but with longer data frames. The frame body size of MSDUs delivered in AC0 and AC5 now exceeds the fragmentation threshold. Since the HC has the control on the TXOPlimit duration it can avoid a high priority AC to exceed its delay by appropriate setting of the limit value.

A significantly increased MSDU Delivery delay occurs with coexisting QBSSs, as indicated in Figure 5.39. The delay is not under the control of the HC in this scenario. Here, two identical overlapping QBSSs are co-located to each other, so that they interfere. All traffic parameters remain as described for the isolated QBSSs. This coexistence scenario has been discussed in the previous section. The MSDU Delivery delay distributions in this scenario are shown in Figure 5.40(d). It can be seen that now the delay of the high priority AC 7 exceeds the TXOPlimit of 300µs defined by the HC to meet the needs of AC 7.

This example shows that a variety of delays and throughputs that are observed in overlapping QBSSs depend on the degree of overlap. One extreme example of this is when two HCs always attempt to allocate CAPs at the same time. Then all CAPs would collide, and the throughput of all streams in AC 7 would be significantly reduced. In this case the delay of AC 7 would be infinite.

Table 5.7: EDCF parameters used for the analysis of the overlapping QBSS scenarios.

AC (priority):	highest (AC 7)	higher (AC 6)	medium (AC 5)	lower (AC 0)
AIFSN[AC]:	1	2	4	7
CWmin[AC]:	0	7	10	15
CWmax[AC]:	0	7	31	255
PF[AC]:	N/A	2	2	2
RetryCnt[AC]:	1	7	7	7