5.5 Summary and Conclusion

(a) isolated QBSS

115

(b) isolated QBSS with higher offered traffic

(c) isolated QBSS with longer EDCF-TXOPs

(d) overlapping QBSSs

Figure 5.40: (Mangold et al., 2002a) MSDU Delivery delay for the four ACs in isolated and overlapping scenarios.

5.4.4Conclusions about the HCF Controlled Channel Access

The controlled channel access of the HCF allows allocating resources with highest priority, and to guarantee a maximum MSDU Delivery delay only if the QBSS operates in isolation. If backoff entities of overlapping QBSSs allocate TXOPs that are longer than the specified TXOPlimit, the MSDU Delivery delay of the controlled channel access of the HCF increases unpredictably.

Further, if HCs of overlapping BSSs allocate CAPs as well, allocation attempts may often fail due to colliding frames, and MSDU Delivery delays may increase dramatically. As a result, QoS guarantees are not possible then.

5.5Summary and Conclusion

The upcoming 802.11e standard is an efficient means for QoS support in wireless LANs for a wide variety of applications, although open problems such as the overlapping QBSSs remain to be solved. This chapter provides an in-depth analy-

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

sis of potential 802.11e enhancements. The maximum achievable throughput as well as the saturation throughput for the HCF contention-based channel access (EDCF) have been calculated analytically, and validated by simulation, see Section 5.1.2.

A new model to approximate the resulting share of capacity per AC, i.e., the relative priority between different ACs when backoff entities operate with different EDCF parameters, has been developed and validated by simulation in Section 5.1.3. The model sufficiently approximates the simulation results for nearly all studied load scenarios.

The specific problem of providing EDCF backoff entities a higher priority in channel access over the legacy DCF stations has been discussed, and means based on the Persistence Factor and the EIFS to support EDCF in congestion with DCF were proposed and evaluated, see Section 5.1.4.

The importance of applying contention free bursts together with radio resource control schemes in coexistence scenarios have been highlighted in Section 5.2. Radio resource capture scenarios have been investigated in Section 5.3 and have been shown to be sensitive in that they might affect the ability of 802.11e to guarantee QoS support.

The controlled channel access of the HCF has been analyzed in Section 5.4. It provides in an isolated QBSS the means for delivering time-bounded traffic, and requires all backoff entities within the range of the HC to follow its coordination. QoS support appears to be problematic when multiple QBSSs overlap.

Chapter 6

COEXISTENCE AND INTERWORKING BETWEEN

802.11 AND HIPERLAN/2

6.1 ETSI BRAN HiperLAN/2................................................

118

6.2Interworking Control of ETSI BRAN HiperLAN/2

and IEEE 802.11.................................................................	122
6.3 Coexistence ........................................................................	128

WIRELESS networks are able to coexist if they operate with the same radio resources at the same time and location, i.e., in an overlapping scenario (Mangold et al., 2001h). Wireless networks that coexist with

each other operate in the same coverage area without harmful interference. With successful coexistence, individual wireless networks that overlap with each other generally see a reduced capacity of the radio resources but still have full access to the available radio resources. Without a successful coexistence strategy, wireless networks are not able to successfully control their access to radio resources. It is worth mentioning that coexisting wireless networks not necessarily must follow the same standard. Further, the capability to exchange information between overlapping wireless networks is not a required characteristic for establishing successful coexistence, as will be shown in this chapter. Wireless networks are able to interwork rather than to coexist if they are able to exchange information across the different networks. When interworking, wireless networks are able to coordinate the usage of radio resources. This is achieved by exchanging spectrum coordination information. To be capable of exchanging information, it is required that overlapping wireless networks operate with the same transmission scheme through a protocol that is common between these wireless networks.