3.4 The 802.11 Standards

37

3.4The 802.11 Standards

To conclude this chapter on 802.11, this section outlines briefly all supplement standards (amendments) under the roof of 802.11 that have been developed or are being developed at the time this thesis is written. General information about the structure of the standards can be found in Mittag (2002), Gast (2002), and Geier (2002).

3.4.1IEEE 802.11

The 802.11 standard is considered as the root standard, defining operation and interfaces at MAC and PHY for data networks such as the popular TCP/IP network. Three PHY layer interfaces are defined that are not compatible with each other. One is based on Infrared (IR) communications, and the other two use the 2.4 GHz unlicensed band, which is more or less, harmonized over the world. One is based on Frequency-Hopping Spread Spectrum (FHSS) and the other uses Direct Sequence Spread Spectrum (DSSS).

802.11 was published in 1997, an updated version is available since 1999, see IEEE 802.11 WG (1999c).

3.4.2IEEE 802.11a

This extension defines the PHY that allows up to 54 Mbit/s by operating in the 5 GHz unlicensed band, making use of the Orthogonal Frequency Division Multiplexing (OFDM), equivalent to HiperLAN/2. This PHY is mainly considered in this thesis. IEEE 802.11a is also sometimes referred to as Wi-Fi5, to highlight that 802.11a networks operate in the 5 GHz band, and to reduce the apparent confusions of the many abbreviations.

3.4.3IEEE 802.11b

This extension is defined in the IEEE 802.11 supplement standard "HigherSpeed Physical Layer Extension in the 2.4 GHz Band", known as IEEE Standard 802.11b-1999. IEEE 802.11b defines the High Rate Direct Sequence Spread Spectrum (HR/DSSS) transmission mode with a chip rate of 11 Mchip/s, providing the same occupied channel bandwidth and channelization scheme as DSSS. The higher data rate is achieved through a transmission mode based on 8-chip Complementary Code Keying (CCK) modulation. The code set of complementary codes is richer than the set of Walsh codes. At 11 Mbit/s, the spreading code length is 8 and the symbol duration is 8 instead of 11 chips, as it was with the DSSS. Data bits encode the symbols with Quaternary Phase Shift Keying (QPSK) and Differential

38	3. IEEE 802.11
QPSK	(DQPSK). IEEE 802.11b is known under the acronym Wireless Fidel-

ity (Wi-Fi).

3.4.4IEEE 802.11c

This task group of the 802.11 working group finished its work by not developing an additional supplement standard, but providing information for changes in other standards. The results of 802.11c were modifications of other standards, not a separate document. The 802.11c task group defined protocols for what is referred to as AP bridging. 802.11 APs can communicate which each other across networks within relatively short distances.

3.4.5IEEE 802.11d

This standard is related to radio regulation in an international context. The use of the frequency spectrum is regulated by nations and is different from one nation to another. 802.11d provides procedures and protocols to let 802.11 networks operate compliantly to what is regulated, by introducing regulatory domains. If a station does not comply with the rules defined for a specific regulatory domain, it will not initiate transmissions, and not associate with a network. The domains are identified by information elements that are broadcasted by the AP.

3.4.6IEEE 802.11e

The 802.11e task group is defining enhancements to 802.11 to allow QoS support. It is described in detail in Chapter 4 of this thesis. 802.11e will work with any PHY extension.

3.4.7IEEE 802.11f

AP handovers are supported by the Inter AP Protocol (IAPP), defined by 802.11f. A constant operation while the station is actually moving is supported when the IAPP is used. The concept of handovers is familiar to cellular networks, and will need to be standardized, as APs and stations will be provided by different vendors.

3.4.8IEEE 802.11g

IEEE 802.11g combines the advantages of 802.11b (relatively large coverage) and 802.11a (higher throughputs) by defining the application of the multi carrier 802.11a OFDM transmission scheme in the 2.4 GHz band, in which originally 802.11b stations are operating. Therefore, 802.11g will provide up to 54 Mbit/s at

3.4 The 802.11 Standards

39

the air interface. There is also an extended rate PHY mode based on DSSS single carrier (802.11b), which allows up to 33Mbit/s. Further, because 802.11g and 802.11b stations are likely to operate at the same time in many scenarios, it is possible to use the DSSS based preambles and headers together with the remainder of a frame being transmitted with extended rate PHY modes (single carrier or multi carrier). With this multi-mode operation, 802.11g stations will be able to interwork and coexist with 802.11b networks, which makes 802.11g attractive for increasing the capacity of already rolled-out 802.11b networks.

3.4.9IEEE 802.11h

Dynamic Frequency Selection (DFS) and Transmitter Power Control (TPC) are defined by this group, with focus on 802.11a and the 5 GHz band. The reasons for applying these schemes are spectrum sharing and efficiency, QoS support, and energy consumption.

To select the frequency channel to operate its BSS, an AP needs to know the status of all frequency channels. While the status of the current channel is available to the AP, the AP needs to collect the information about other channels as well, in order to initiate a channel selection. This will be performed via the standardized channel measurements by other station and the AP itself. The channel measurement by the AP does not need to be standardized, as it does not need to report the measurement results to other stations. However, the AP measurement should be performed in such a way that the service disruption is minimized. Any other measurements must be standardized in the context of 802.11h. The channel measurements by stations will be (1) detection of other BSSs (2) measurement of Clear Channel Assessment (CCA) busy periods, and (3) measurement of received signal strength statistics.

TPC is a difficult task in 802.11 networks, since as part of the DCF; every station needs to detect all transmissions of frames within its BSS. Thus, there are no peer links between two stations that are subject to TPC. However, to meet future regulatory requirements, and for increased spectrum efficiency, and in order to reduce interference imposed on other networks, TPC is standardized in 802.11h.

3.4.10IEEE 802.11i

Security and privacy becomes increasingly important with the growing popularity of 802.11. There are problems in the algorithms for providing security defined in the legacy 802.11 protocol. 802.11i is tasked with improving the security by enhancing the Wired Equivalent Privacy (WEP) protocol.

Chapter 4

IEEE 802.11E HYBRID COORDINATION

FUNCTION

4.1

Overview..............................................................................

42

4.2Hybrid Coordination Function, Contention-based

Channel Access ...................................................................

45

4.3Hybrid Coordination Function, Controlled Channel

	Access ..................................................................................	55
4.4	Improved Efficiency ...........................................................	57

QUALITY OF SERVICE (QoS) is a synonym for service characteristics that are provided by a MAC layer service to higher layer applications, within a layered data transport system. QoS may or may not be required by an application, and may or may not be provided by the service. The service of

interest here and in the rest of this thesis is the MSDU Delivery, which is an 802.11 station service available at the MAC Service Access Point (MAC-SAP) at the Logical Link Control (LLC)/MAC boundary. For this service, QoS involves achieving at least a minimum MSDU Delivery throughput as well as achieving MSDU Delivery delays not exceeding a maximum limit. Additionally, MSDU Delivery delay variation and MSDU loss rate are often considered as part of QoS.

The legacy 802.11 DCF is a distributed contention-based channel access protocol and cannot deliver MSDUs under QoS constraints.

The legacy 802.11 PCF is centralized and provides contention-free channel access. It is inefficient and provides only limited QoS for MSDU Delivery.

For these reasons, IEEE 802.11 Task Group E (TGe) defines enhancements of the legacy 802.11 MAC. The enhancements are defined in the supplement stan-