5.1 HCF Contention-based Channel Access

65

alistically high modulation values. The results in this section are obtained through analytical calculations and stochastic simulation, without taking the capacity loss through regular beacon transmissions into account. The small variations of the throughput curves in Figure 5.2 are the results of the bit padding into OFDM symbols. A perfect radio channel is assumed.

5.1.2System Saturation Throughput

The system saturation throughput Thrpsat is defined as expected sum of all MSDU throughputs of contending backoff entities that are saturated with traffic load so that all entities have always MSDUs to deliver, queues are never empty.

In Appendix D, an approximate analysis is presented based on a Markov model to calculate the saturation throughput of a number of contending backoff entities. The approximation is based on Bianchi (1998a, 1998b, 2000) and in this thesis referred to as Bianchi’s legacy 802.11 model. Hettich (2001) uses Bianchi’s legacy 802.11 model and extends it for the analysis of not only the throughput, but also the backoff delay. To evaluate the concepts of the EDCF contention window, Bianchi’s legacy 802.11 model is modified in the following. The focus of the discussion is the throughput approximation.

5.1.2.1Modifications of Bianchi’s Legacy 802.11 Model

To model the saturation throughput of an EDCF backoff entity instead of a legacy station, some modifications of Bianchi’s legacy 802.11 model are required.

	50	highest pr.		w/o address 4, w/o WEP,
(Mbit/s)		legacy pr.		w/o RTS/CTS
		lower pr.
	40
thrp.						64QAM3/4
	30
achievable
	20					16QAM1/2
max.	10
						BPSK1/2
	0	500	1000	1514	2000	2304
	100

frame body size (bytes)

	50	highest pr.		w/o address 4, w/o WEP,
thrp. (Mbit/s)		legacy pr.		with RTS/CTS
		lower pr.
	40
	30					64QAM3/4
achievable
	20					16QAM1/2
max.	10
						BPSK1/2
	0	500	1000	1514	2000	2304
	100

frame body size (bytes)

lines: analytics, markers: WARP2 simulation results

Figure 5.2: Maximum achievable throughput for three PHY modes, and three EDCF parameter settings. Left: most optimistic situation. Right: realistic situation with RTS/CTS, WEP encryption and use of optional address 4. Analytical and simulation results.

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

max. achievable thrp. (Mbit/s)

200 legacy pr.

w/o address 4, w/o WEP,
w/o RTS/CTS					oo−QAM
111
100
50					1024QAM3/4
					256QAM3/4
					64QAM3/4
0	500	1000	1514	2000	2304
100

	200					oo−QAM
(Mbit/s)	188
		highest pr.
thrp.	w/o address 4, w/o WEP,
	w/o RTS/CTS
achievable
	100
						1024QAM3/4
	50					256QAM3/4
max.
						64QAM3/4
	0	500	1000	1514	2000	2304
	100

frame body size (bytes)

frame body size (bytes)

lines: analytical results

Figure 5.3: Maximum achievable throughput and its upper limit for higher PHY modes, which are not part of the 802.11 standard. Left: DCF (legacy priority) configuration. Right: EDCF (highest pr.) configuration. The results are obtained with the analytical model only.

In Equation (D.1), the parameter i is the backoff stage, and m is the maximum value of the backoff stage. The contention window sizes Wi , i = 0…m and the maximum number of backoff stages m are dependent on the EDCF parameter set, individually defined per AC. Further, since the Persistence Factor (PF) can also be included in the modified model as well -although it is not part of 802.11ethis parameter has to be considered in the equation. The modifications are as follows. The size of the contention window in 802.11e is calculated by

W	[AC ]= PF [AC ]min(i ,m[AC ]) W , i 0,1,…m [AC ].	(5.1)
i	0

The probability that transmission attempts of a single backoff entity at a particular slot are unsuccessful due to collision is denoted by p. As in the approach to model the legacy 802.11, it is in the following assumed that this probability is independent of the contention window size. ForWi ≥1 , the persistence factor is incorporated in Equation (D.4) by considering Equation (5.1):

m

Wi −1 W −k

1 = ∑

bi ,0

∑

i

, Wi ≥1

0 ≤ i ≤ m, m ≥ 0

W

i =0

k=0

i

1

m

1 =

∑bi ,0 (Wi

+1)

2

(5.2)

i =0

b0,0

1

(PF [AC ] p)

m

=

W0 m∑−1

(PF [AC ] p)i +

+W0

,

−p

1 −p

2

i =0

1

with m ≥ 0,W0 ≥1.