Quality Of Service (Qos); 802.11E; Wmm™ (Wme™) - ASCOM VoWiFi System System Description

System Description
Ascom VoWiFi System
2.4

Quality of Service (QoS)

Without QoS all applications running on different devices have equal opportunity to
transmit data frames. This works well for data traffic but voice, and other multimedia in a
WiFi network, is highly sensitive to latency increases and throughput reductions and require
QoS functionality. When deploying a VoWiFi system, traffic prioritization is needed to
ensure the best possible voice quality.
QoS for a WLAN enables the AP to prioritize traffic and optimizes the way shared network
resources are allocated among different applications. It is defined as the control of
bandwidth, latency, jitter and traffic-loss.
2.4.1

802.11e

The 802.11e specification adds QoS to WLANs by defining how data, voice, video and audio
transmissions should be prioritized in the radio channels. The standard also specifies
enhanced error-correcting mechanisms in the Media Access Control (MAC) layer to improve
the voice traffic which is very delay-sensitive.
The amendment 802.11e is dealing with QoS issues and two different access mechanisms
are defined, EDCA and HCCA.
• Enhanced Distributed Channel Access (EDCA) is dealing with QoS by prioritization. This
means that some kind of data (voice) is given a higher priority, but does not guarantee
any quality of the call.
• Hybrid Controlled Channel Access (HCCA) makes use of a central coordinator that
allocates transmission opportunities to clients dynamically based on bandwidth needs
and the service interval required. This is an approach to make 802.11 functions like a
dynamic TDMA system.
2.4.2 WMM™ (WME™)
To meet the need of QoS in products until the ratification of the 802.11e amendment, the
Wi-Fi Alliance® developed the Wi-Fi Multimedia (WMM™), earlier called Wireless Multimedia
Enhancement (WME™), to provide QoS in products.
WMM prioritizes traffic according to four Access Categories (AC)—voice, video, best effort,
and background—and is based on the Enhanced Distributed Channel Access (EDCA)
method. It is important to note that WMM is not an alternative to 802.11e but rather a
subset of the standard, see section
functionality in a WiFi network, the AP, the client (device) that the application is running on,
and the source application must support WMM.
The standard differs from amendment IEEE 802.11e in some respects. The EDCA operation
and its default values are the same, but the coding of the information elements differs.
The following table shows some of the major differences between 802.11e and WMM:
Enhanced Distributed Channel Access (EDCA)
Hybrid Controlled Channel Access (HCCA)
Priorities
Packet-bursting
Automatic Power Save Delivery (APSD)
Block ACK
Direct Link Protocol
7 September 2011 / Ver. G
2.4.1 802.11e on page 15. To take advantage of the
802.11e
Yes
Yes
Yes
Yes
Optional
Optional
Optional
TD 92313EN
WMM
Yes
No
Yes
Yes
Optional
No
No
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