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

proximating the system saturation throughput per Access Category (AC), when multiple backoff entities of different ACs operate in parallel, is introduced. In Section 5.2 a new concept, the CFB, is discussed, and its importance in scenarios of overlapping QoS supporting Basic Service Sets (QBSSs) in unlicensed bands is evaluated. In Section 5.3 some known capture effects in 802.11 are analyzed. In Section 5.4 a performance evaluation of the HCF contention-based channel access and a discussion of coexistence of overlapping QBSSs is provided. This chapter ends up with a conclusion in Section 5.5.

5.1HCF Contention-based Channel Access

Four different approaches are taken in this section to evaluate the HCF conten- tion-based channel access. In Section 5.1.1 the maximum achievable throughput is calculated. In Section 5.1.2, the achievable throughput (here referred to as system saturation throughput) for an arbitrary number of contending backoff entities all operating with the same EDCF parameter set is discussed. When all backoff entities operate with the same EDCF parameter set, the same AC is used by all backoff entities. For the analysis, a known model for the legacy 802.11 is modified to capture all relevant effects of the new MAC enhancements. With the modified model, the saturation throughput in 802.11e can be calculated. In Section 5.1.3, this modified model is extended to approximate the saturation throughput per AC in scenarios where an arbitrary number of backoff entities operate in parallel, but with different EDCF parameter sets. When backoff entities operate with different EDCF parameter sets, multiple ACs are used and the resources are shared among backoff entities of different ACs according to their relative priorities. Finally, in Section 5.1.4 the specific problem of QoS support under contention with legacy 802.11 stations is discussed.

5.1.1Maximum Achievable Throughput

The maximum achievable throughput depends on a large number of parameters when all parameters such as channel errors with respect to the PHY modes, frame body sizes, fragmentation, the use of RTS/CTS, and many more are considered. However, it is of interest to know the maximum achievable throughput with the EDCF, in case all protocol parameters are adjusted such that the achievable throughput in a single point to point downstream is maximized. This throughput is discussed in this section for the three different PHY modes BPSK1/2 (6 Mbit/s), 16QAM1/2 (24 Mbit/s), and 64QAM3/4 (54 Mbit/s). An ideal environment without transmission errors is assumed. The simple scenario is illustrated in Figure 5.1.

5.1 HCF Contention-based Channel Access							63
			1m
			station 1.2
station			one single DL stream in
1.1			an isolated QBSS, with
			perfect channel conditions

Figure 5.1: A single stream is simulated to measure the achievable throughput with EDCF using the EDCF parameters given in Table 5.1.

The maximum achievable throughput is calculated as

	Thrp [Mbit s ]=	frame body size [Mbit]	.
		expected backoff time[s] + frame exchange time[s]

With one single transmitting backoff entity, collisions never happen, and the expected backoff time is given byCWmin/2 . The frame exchange time includes the PHY Protocol Data Unit (PPDU) durations including the headers and PHY overheads according to the applied PHY mode, and all interframe spaces, including AIFS. The frame exchange time increases when RTS/CTS is used.

In the following, results for three different priorities with three different EDCF parameter settings are compared with each other. EDCF parameters as defined in Table 5.1 are used.

Figure 5.2, left, shows the resulting throughput as a function of the MPDU frame body size for the most optimistic scenario, where RTS/CTS is not used, and neither the optional encryption Wired Equivalent Privacy (WEP) nor the address 4 is used (see p. 65). Note that without encryption, and without address 4, an MPDU can be transmitted with the smallest overhead. In terms of maximum throughput, RTS/CTS can be considered as overhead as well. Therefore, it is not used here.

Table 5.1: Used EDCF parameters with legacy backoff.

AC (priority):	highest	legacy	lower
AIFSN[AC]:	2	2	4
AIFS[AC]:	DIFS (34us)	DIFS (34us)	52us
CWmin[AC]:	0	14	31
CWmax[AC]:	N/A	N/A	N/A
PF[AC]:	N/A	N/A	N/A
RetryCnt[AC]:	N/A	N/A	N/A

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

The legacy priority uses AIFS=DIFS (AIFSN=2)5, and an initial contention window size known from the legacy standard, that means CWmin=15 (valid for 802.11a). The results are similar to what a legacy DCF backoff entity may theoretically achieve. The lower priority uses AIFSN=4 and CWmin=31, and shows a smaller throughput than the legacy priority due to larger AIFSN and longer backoff durations. In contrast, the highest priority shows clearly the largest throughput. With AIFSN=2, and CWmin=0, the backoff entity always transmits with highest priority. Without contention, the maximum achievable throughput can therefore be increased, as long as no other contending backoff entity operates in the near environment. In this scenario it is assumed that the receiving station is not allowed to contend for medium access.

Results of a stochastic simulation study are also shown and prove the analysis to be sufficiently accurate. The right hand graph in Figure 5.2 shows results for the same parameters, but now including RTS/CTS, WEP encryption, and the address 4 in the MAC header of the directed MPDU are necessary. The resulting achievable throughput is smaller, but show qualitatively the same characteristics: with small AIFSN and small CWmin, the maximum achievable throughput can be increased as long as there is only one transmitting backoff entity.

To understand the overhead that results from the 802.11 MAC protocol, it is worth to discuss the maximum achievable throughput for the 802.11a physical layer in scenarios assuming PHY modes with higher modulations than defined in 802.11. The higher the modulation of a transmitted PPDU, the shorter the transmission duration of a PPDU. A frame is also referred to as PPDU, see Figure 3.4, p. 25. A PPDU consists of the preamble, the PHY header, and the MPDU as payload. Parts of the PPDU are transmitted with the basic PHY mode (BPSK1/2). However, the interframe spaces SIFS and AIFS are independent of the used PHY mode. Therefore, and because of the limited frame body size, the maximum achievable throughput converges to a finite limit when using higher modulations of up to an infinite amount of bits per second. This is indicated in Figure 5.3. It can be seen that the maximum throughput with improved PHY modes is around 111 Mbit/s with RTS/CTS and 188 Mbit/s in the most optimistic case. Obviously, assuming an error free channel is not realistic with such unre-

5In this analysis, the legacy DCF backoff is used for the EDCF discussion. Specifically, the earliest channel access time of the EDCF backoff entities is DIFS, which is here interpreted as AIFS=DIFS and AIFSN=2. This is consistent with the interpretation of the legacy DCF backoff, but not with the interpretation of the EDCF backoff. However, since comparisons of DCF and EDCF are presented in this chapter, only the legacy interpretation is used.