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6.8 IEEE 802.11e and WMM – Quality of Service

Within a few years, WLAN has revolutionized networking in offices and homes.
Originally, these networks were mainly used for applications such as web browsing
and access to files on a local server. Here, high bandwidths are required to transfer
data quickly. Other aspects such as a guaranteed bandwidth and jitter were less
important.
Today, applications such as VoIP and video streaming have additional requirements.
Video streaming, for example, requires, in addition to a high bandwidth, a guaranteed
bandwidth and maximum latency to ensure a smooth user experience. VoIP applica-
tions have similar requirements. While there is sufficient capacity on the network for all
applications using it, such applications will function properly even without additional
measures being taken. If, however, a multimedia transmission already requires a significant
amount of the available bandwidth while other applications, potentially on other
devices, start a file transfer or other bandwidth‐intensive operation, it is likely that this
transmission will interfere with the multimedia streaming. To prevent such issues, QoS
measures were added with IEEE 802.11e. As with other extensions of the standards,
there are some parts which must be supported by all devices while the support of others
is optional.
To speed up the introduction of the QoS extensions, the Wi‐Fi Alliance has created
the Wi‐Fi Multimedia (WMM) specification, which is based on 802.11e. If an AP or
mobile device is WMM‐certified, it contains all of the features that are declared as man-
datory in the WMM specification and will be able to communicate with WMM devices
of other vendors. To ensure that the QoS extensions are implemented in as many devices
as possible, the Wi‐Fi Alliance requires in its certification program that 802.11n devices also
implement the WMM extensions. The following section now describes the 802.11e
functionalities used by WMM. Subsequently, a number of additional features which are
defined as optional are described.
The core of the QoS enhancements is an extension of the DCF that controls access
to the air interface as described in Section 6.5.1. DCF requires that devices wait for
a random time before starting their transmission to prevent collisions when several
devices have data waiting in their transmission buffers simultaneously. This delay
has been specified to be up to 31 slots of 20 microseconds in 802.11b and g. The
value used by a device is determined by generating a random number between 1 and
31. In case the transmission fails, for example, because of a collision, the delay is
increased to 63, 127, and so on up to a maximum of 1023 slots, which equals 20
milliseconds.
802.11e extends this channel allocation method with the Hybrid Coordination
Function (HCF). HCF describes two channel access methods – Enhanced Distributed
Channel Access (EDCA) and Hybrid Coordination Function Controlled Channel
Access (HCCA). HCF is backward compatible with DCF, which means that both
HCF‐capable and non‐HCF‐capable devices can be used in the network simultane-
ously. The following section describes EDCA, which is the basis for the WMM
specification.
Instead of using the same window length for the random number generator, EDCA
specifies four QoS classes with queues. Each QoS queue is then assigned a different
window length before the air interface can be accessed. WMM defines queues for voice,