RF Wireless World
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 WME/WMM
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 |
WiFi QoS WSM
• 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.
Wi-Fi — Quality of Service (QoS)
There are plans to incorporate quality of service (QoS) capabilities in WiFi technology with the adoption of the IEEE 802.11e standard. The 802.11e standard will include two operating modes, either of which can be used to improve service for voice −
WiFi Multimedia Extensions (WME)
WiFi Multimedia Extensions use a protocol called Enhanced Multimedia Distributed Control Access (EDCA), which is an extension of an enhanced version of the Distributed Control Function (DCF) defined in the original 802.11 MAC.
The enhanced part is that EDCA will define eight levels of access priority to the shared wireless channel. Like the original DCF, the EDCA access is a contention-based protocol that employs a set of waiting intervals and back-off timers designed to avoid collisions. However, with DCF all stations use the same values and hence have the same priority for transmitting on the channel.
With EDCA, each of the different access priorities is assigned a different range of waiting intervals and back-off counters. Transmissions with higher access priority are assigned shorter intervals. The standard also includes a packet-bursting mode that allows an access point or a mobile station to reserve the channel and send 3- to 5-packets in a sequence.
WiFi Scheduled Multimedia (WSM)
True consistent delay services can be provided with the optional WiFi Scheduled Multimedia (WSM). WSM operates like the little used Point Control Function (PCF) defined with the original 802.11 MAC.
In WSM, the access point periodically broadcasts a control message that forces all stations to treat the channel as busy and not attempt to transmit. During that period, the access point polls each station that is defined for time sensitive service.
To use the WSM option, devices need to send a traffic profile describing bandwidth, latency, and jitter requirements. If the access point does not have sufficient resources to meet the traffic profile, it will return a busy signal.