QoS BSS: The QoS network

QoS BSS: The QoS network

The IEEE 802.11 QoS facility provides MAC enhancements to support LAN applications with QoS
requirements. The QoS enhancements are available to QoS STAs associated with a QoS access point in a
QoS BSS. A subset of the QoS enhancements is available for use between STAs that are members of the
same QoS IBSS. Similarly, a subset of the QoS enhancements is available for use between neighbor peer
mesh STAs. A mesh BSS is one type of QoS BSS and it is described in 4.3.15. Because a nonmesh QoS
STA implements a superset of STA functionality, as defined in this standard, the STA might associate with
a non-QoS access point in a non-QoS BSS, to provide non-QoS MAC data service when there is no QoS
BSS with which to associate. As a mesh STA does not implement the necessary service, the mesh STA does
not associate with any access point.
The enhancements that distinguish QoS STAs from non-QoS STAs and QoS APs from non-QoS APs are
collectively termed the QoS facility. Which of the QoS-specific mechanisms a QoS STA supports might
vary among QoS implementations, as well as between QoS STAs and QoS APs, over ranges specified in
subsequent clauses. All service primitives, frame formats, coordination function and frame exchange rules,
and management interface functions except for the Block Acknowledgment (Block Ack) function, directlink
setup (DLS), and automatic power save delivery (APSD) are part of the core QoS facilities. A QoS STA
or QoS AP implements those core QoS facilities necessary for its QoS functions to interoperate with other
QoS STAs. Functions such as the Block Ack, DLS, and APSD are separate from the core QoS facilities; and
the presence of these functions is indicated by STAs separately from the core QoS facilities.
For infrastructure BSS and IBSS, this standard provides two mechanisms for the support of applications
with QoS requirements.
The first mechanism, designated the enhanced distributed channel access (EDCA), delivers traffic based on
differentiating user priorities (UPs). This differentiation is achieved by varying the following for different
UP values:
— Amount of time a STA senses the channel to be idle before backoff or transmission, or

— The length of the contention window to be used for the backoff, or
— The duration a STA may transmit after it acquires the channel.
These transmissions may also be subject to certain channel access restrictions in the form of admission
control. Details of this mechanism are provided in 9.19.2.
The second mechanism, designated the hybrid coordination function (HCF) controlled channel access
(HCCA), allows for the reservation of transmission opportunities (TXOPs) with the hybrid coordinator
(HC). A STA based on its requirements requests the HC for TXOPs, both for its own transmissions as well
as for transmissions from the AP to itself.16 The request is initiated by the station management entity (SME)
of the STA. The HC, which is collocated at the AP, either accepts or rejects the request based on an
admission control policy. If the request is accepted, the HC schedules TXOPs for both STAs (both the AP
and the non-AP STA). For transmissions from the non-AP STA, the HC polls the STA based on the
parameters supplied by the STA at the time of its request. For transmissions to the STA, the AP directly
obtains TXOPs from the collocated HC and delivers the queued frames to the STA, again based on the
parameters supplied by the STA. Details of the mechanism are provided in 9.19.3 and 10.4. This mechanism
may be used for applications such as voice and video, which may need periodic service from the HC. If the
application constraints dictate the use of this mechanism, the application initiates this mechanism by using
the management service primitives.
Non-QoS STAs may associate in a QoS BSS, if allowed to associate by the AP. All individually addressed
frames that are sent to non-QoS STAs by an AP do not use the frame formats associated with the QoS
facility.
A QoS STA associated in a non-QoS BSS acts as a non-QoS STA.