802.11e - Wi-Fi Multimedia (WMM)
Background
802.11e was published in 2005 and
provides basic QoS for Wireless LANs. Stations that support 802.11e are called QSTAs whilst stations that do not support
802.11e are called nQSTAs.
802.11e improves on the original PCF and DCF by way of the Hybrid Coordination Function (HCF), a new mode that allows new QoS
features to operate within a cell whilst maintaining operability of nQSTAs. With HCF the AP takes the role of
Hybrid Coordinator (HC) and the cell becomes the QoS Basic Service Set (QBSS).
The two methods of channel access are an improvement on DCF called Enhanced Distributed Channel Access (EDCA) and
an improvement on PCF called HCF Controlled Channel Access (HCCA).
Although bandwidth is not guaranteed there are four Access Classes (AC) defined as outlined in the table below:
| 802.1p |
802.11e Traffic Class (User Priority) |
Access Category |
Designation |
| 7 |
NC |
AC_VO |
Voice |
| 6 (Cisco 5) |
VO (Platinum) |
|
|
| 5 (Cisco 4) |
VI (Gold) |
AC_VI |
Video |
| 4 |
CL |
|
|
| 3 (Cisco 0) |
EE (Silver) |
AC_BE |
Best Effort |
| 0 |
BE |
|
|
| 2 |
|
AC_BK |
Background |
| 1 (Cisco 1) |
BK (Bronze) |
|
|
Enhanced Distributed Channel Access (EDCA)
In DCF, before sending, a station waits a DIFS before counting down. In EDCA the station waits for an Arbitrated
Interframe Space (AIFS) (used by QSTAs) where the AIFS has to be equal or greater than the DIFS (used by nQSTAs).
The length of the AIFS can vary depending on packet loss or the TC.
A mechanism called Controlled Contention Mechanism provides a way for stations to obtain a
Transmit Opportunity (TXOP) which is used to provide varying levels of priority.
Different types of traffic are given different TXOP values depending on their priority. The TXOP
is a specific interval of time during which a station is given the right to send frames. If there is a large frame that would take
longer than the assigned TXOP (a cell-wide limit indicated in beacon frames),
then this frame has to be fragmented. A TXOP of value 0 means only one frame
may be sent, this could be a MSDU or a MPDU.
EDCA also includes for Admission Control e.g. bandwidth available for voip calls. Bandwidth availability is
transmitted by the AP in beacons. The QBSS Information Element sits within the beacon and the probe responses
and allows a station to decide which AP offers the best access. The QBSS contains the following:
- Station Count - the number of stations that are associated to that AP
- Channel Utilisation - percentage of time that the medium is busy
- Available Admission Capacity - the number of 32us units that are available when admission control is enabled
The QBSS is fine as far as the AP giving availability information but for a station to indicate its own requirements it needs
to use Traffic Specification (TSPEC) whereby the station asks to use a TC or AC e.g. AC_VO for a voice call.
The station uses the Add Traffic Stream (ADDTS) action frame to perform an admission request because this contains
the TSPEC. The station can also go through an association and reassociation process which would thereby also send TSPECs.
HCF Controlled Channel Access (HCCA)
During a CP the AP (acting as Hybrid Coordinator (HC)) can send a Controlled Access Phase (CAP). With PCF,
the CFP is a type of CAP. The CAP may be sent at any time that the AP wishes to send a frame to a station. During the CP
whilst a CAP is not being sent, the network works in EDCA mode.
The AP can also use Traffic Classes (TC) (or User Priorities (UP)) and Traffic Streams (TS) to further define traffic from just
per station queuing decisions to per-session decisions. Stations send information on the length of their
queues for each TC.
Other 802.11e additions
- Block Acknowledgements (BA) - allows multiple frames to be sent in a stream with only one global ACK, thereby saving frame overhead.
The station must first check the TXOP to see how many frames it has space for. The station then sends an Add Block Acknowledgment (ADDBA)
to the recipient to ask permission. At the end of the stream the recipient sends a BlockACK containing the list of frames received.
- Direct Link Setup (DLS) - allows station-to-station frame transfer within a BSS.
- Unscheduled Automatic Power Safe Delivery (U-APSD) - provides more efficient power savings than PS-Poll. With U-APSD,
any application can wake up the station rather than the station having first to listen to the TIM, thus the AP can send buffered packets
whilst a station communicates before the station falls back to sleep mode. U-APSD is only available in EDCA.
- Scheduled Automatic Power Safe Delivery (S-APSD) - stations have sheduled times to sleep. The station
uses the ADDTS frames to inform the AP if it wants to use S-APSD. The AP sends back the Service Start Time (SST)
and Service Interval (SI) based on the TSPEC. The station stays awake until it receives a frame with the EOSP set to
1. S-APSD is good for heavily loaded networks.
- NoAck - There are two service class values, these are QoSAck and QosNoAck. Frames with QosNoAck
are not acknowledged thereby saving retransmissions of time-sensitive traffic.
The Wi-Fi Alliance have based Wi-Fi Multimedia (WMM) on the EDCA part of 802.11e. WMM takes the eight Traffic Categories (TC),
also called User Priorities (UP)
and just uses four; platinum, gold, silver and bronze. BA, APSD and NoAcks are all maintained.
Wi-Fi QoS
For a detailed look at Quality of Service go to the link QoS. Here we shall look at QoS as it applies to Wi-Fi.
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