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WiFi QoS | WLAN QoS | IEEE 802.11e Quality of Service

This page describes WiFi QoS. This WiFi Quality of Service(QoS) requirements are defined in WLAN IEEE 802.11e standard. WME (WiFi Multimedia Extenstions) or WMM (WiFi MultiMedia) and WSM (WiFi Scheduled Maintenance) concepts of WiFi QoS are described.

In 802.11e WLAN network, QoS has been incorporated for following reasons:
➨To have minimum collision in comparison with older 802.11 based WLAN system.
➨To achieve higher performance in delivering voice streaming, video streaming, Best Effort (e.g. web browsing), Background (e.g. email) applications over WiFi link.

In order to enhance and introduce QoS features in IEEE 802.11e MAC layer has been modified. In 802.11e following protocols have been added:
• WME (WiFi Multimedia Extensions) or WMM (WiFi Multimedia), it is enhancement to DCF (Distributed Co-ordinated Function defined in 802.11 WLAN)
• WSM (WiFi Scheduled Maintenance), it is enhancement to PCF (Point Co-ordinated Function) defined in 802.11 WLAN)


WiFi QoS WMM Queue System

Figure-1 depicts WiFi QoS WMM Queue System. Following are the features of WME/WMM.
• It uses EDCA (Enhanced Distributed Channel Access ) protocol. It is mandatory feature to be introduced in 802.11e compliant devices.
• Traffic carried over WLAN link is categorized into four categories viz. Voice, Video, Best Effort, Background applications.

• Priorities have been defined for each of the traffic type. As per this high priority traffic waits less time compare to low priority traffic.
➨ Implementation: Each traffic priority has different backoff value. High priority will use shorter backoff value.

• Transmit opportunity have been created for each type of traffic. When station gets the access of channel, it can transmit packet during its assigned TXOP (Transmit Opportunity).
➨Implementation: The TXOP is different for each priority level, higher the priority, longer is the TXOP.

Following table-1 mentions Access Caterogy for different traffic types and their priorities.

Priority Code Point (PCP) Traffic Type Access Category
1 Background AC_BK (BACKGROUND)
0 Best Effort AC_BE (Best Effort)
2 Excellent Effort AC_BE (Best Effort)
3 Critical Applications AC_VI (Video)
4 Video AC_VI (Video)
5 Voice AC_VO (voice)
6 Internetwork Control AC_VO (voice)
7 Network Control AC_VO (voice)

Following table-2 mentions EDCA parameters (CWmin, CWmax, AIFSN, Max. TxOp) for different Access Caterogies.

AC CWmin CWmax AIFSN MAx. TxOp
Background (AC_BK) 15 1023 7 0
Best Effort(AC_BE) 15 1023 3 0
Video (AC_VI) 7 15 2 3.008 ms
Voice (AC_VO) 3 7 2 1.504ms
Legacy DCF 15 (non QoS) 1023 2 0


• It uses HCCA (Hybrid Controlled Channel Access ) protocol. It is optional feature defined in IEEE 802.11e standard to be implemented on 11e compliant devices.
• In this protocol, station(STA) or client negotiates traffic profile with WLAN Access Point (AP) or Router. Traffic profile includes bandwidth, latency and jitter requirement to be met over WiFi link.
• Once AP satisfies or agrees with the STA requirement, it sends OK signal otherwise it sends busy signal to the STA.
• Once the conditions are satisfied, they can communicate over the wifi link.

Other WiFi/WLAN QoS features introduced

Following are additional WiFi QoS (or WLAN QoS) features:
➨Automatic Power Save Delivery: It introduces advanced power management for VOIP phone. The VOIP phone transmits and receives data from AP instantaneously and enters into sleep state until there is further data to be transmitted.
➨Block Acknowledgments: This feature allows entire TXOP to be acked in one single frame.
➨No ACK: This feature allows receiver not to send ack for any frame. This is useful for delivery of highly time critical data frame.
➨Direct link setup: This feature allows wireless wifi devices which are associated with same AP (Access Point) or router to directly communicate with each other by-passing AP.

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