Chapter 3

IEEE 802.11

3.1	IEEE 802.11 Reference Model ...........................................	20
3.2	IEEE 802.11 Architecture and Services .............................	21
3.3	Medium Access Control .....................................................	25
3.4	The 802.11 Standards...........................................................	37

THE IEEE 802 COMMITTEE has established standards for communication systems that have been major contributions to the communications industry, for exampled IEEE 802.3 Ethernet and IEEE 802.5 Token Ring. The most successful IEEE standard for wirless communication is

IEEE 802.11. The IEEE published IEEE 802.11 in 1997 as a standard for wireless LANs, and published a revised version in 1999 (IEEE 802.11 WG, 1999c). At the same time, the IEEE 802.11b supplement standard for higher data rates than originally defined has been published (IEEE 802.11 WG, 1999b). This is the best-known wireless LAN today and referred to as IEEE 802.11b. IEEE 802.11b is known under the acronym Wireless Fidelity (Wi-Fi). This standard is widely accepted as a wireless LAN that meets the current requirements; see Heegard et al. (2001), Henry and Luo (2002). Wireless LAN IEEE 802.11 is described and analyzed in detail in Hettich (2001) and Walke (2002).

In this chapter, the MAC protocol of 802.11 is briefly described. Problems in QoS support that motivated the 802.11 working group to develop new enhancements for the support of QoS are summarized. This chapter is outlined as follows. In the next section, the reference model is described, and in Section 3.2, the architecture and provided services are discussed. Section 3.3 highlights the details of the 802.11 protocol. For clarification, all supplement standards developed so far are briefly summarized in Section 3.4, at the end of this chapter.

20	3. IEEE 802.11

3.1IEEE 802.11 Reference Model

Like IEEE 802.3 (Ethernet) and IEEE 802.5 (Token Ring), 802.11 is restricted to the lower two layers of the Open System Interconnection (OSI) reference model, as indicated in Figure 3.1. In 802.11, the Data Link Control (DLC) layer is divided into Logical Link Control (LLC) and Medium Access Control (MAC) sublayers. 802.11 defines Physical layer (PHY) transmission schemes, and the MAC protocol, but no LLC functionality. For LLC, 802.11 relies on already defined protocols that are available for all systems in the 802 context. As the LLC layer is the same for all 802.x LANs, it does not address the specific characteristics of the wireless channel with its typical error characteristics. Therefore, management functions to address the needs of wireless communication are incorporated into the 802.11 MAC. To consider for example radio range aspects, the 802.11 standard contains functions for the management and maintenance of the radio network, which exceed the normal tasks of the MAC sublayer. The LLC layer does not provide association aspects of mobility; consequently, they are also handled in the MAC layer.

802.11 in its original form defines three	different types	of PHYs, namely
2.4 GHz Frequency Hopping Spread Spectrum	(FHSS), Direct	Sequence Spread Spec-

trum (DSSS) and Infrared (IR). Until the time this thesis is written, there are three more PHYs defined in the supplement standards 802.11a, 802.11b, and 802.11g. With FHSS, a set of communicating stations operate with certain center frequency only for short times, before selecting another center frequency (“hopping”) for continuing the communication. Which center frequency is used is defined by a pseudo-random list of frequencies, known to all stations.

Different sets of communicating stations use different lists of frequencies, which reduces the probability that they operate with the same radio resources, i.e., at the same center frequency. In contrast to FHSS, in DSSS all stations operate at the same center frequency. In DSSS, different spreading codes allow different sets of communicating stations to reduce the mutual interference.

See Figure 3.1 for the illustration of the 802.11 reference model. The Physical Medium Dependent (PMD) sublayer is responsible for sending and receiving data via the wireless channel and defines the transmission scheme, which is different for the different PHYs, whereas the Physical Layer Convergence Protocol (PLCP) sublayer adapts the requests of the common MAC to the different PHYs into a format specific to the applied PMD. The MAC user plane is fed with data frames via the MAC Service Access Point (MAC-SAP) at the MAC/LLC boundary.