Brocade Mobility Access Point System Reference Guide (Supporting software release 5.5.0.0 and later) - Manual (Page 462)

452 Brocade Mobility Access Point System Reference Guide
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Brocade access point radios and wireless clients support several Quality of Service (QoS)
techniques enabling real-time applications (such as voice and video) to co-exist simultaneously
with lower priority background applications (such as web, E-mail and file transfers). A well designed
QoS policy should:
• Classify and mark data traffic to accurately prioritize and segregate it (by access category)
throughout the network.
• Minimize the network delay and jitter for latency sensitive traffic.
• Ensure high priority traffic has a better likelihood of delivery in the event of network
congestion.
• Prevent the ineffective utilization of access points degrading session quality by configuring
admission control mechanisms within each radio QoS policy.
Within a Brocade wireless network, wireless clients supporting low and high priority traffic contend
with one another for data resources. The IEEE 802.11e amendment has defined Enhanced
Distributed Channel Access (EDCA) mechanisms stating high priority traffic can access the network
sooner then lower priority traffic. The EDCA defines four traffic classes (or access categories); voice
(highest), video (next highest), best effort and background (lowest).The EDCA has defined a time
interval for each traffic class, known as the Transmit Opportunity (TXOP). The TXOP prevents traffic
of a higher priority from completely dominating the wireless medium, thus ensuring lower priority
traffic is still supported by connected radios.
IEEE 802.11e includes an advanced power saving technique called Unscheduled Automatic Power
Save Delivery (U-APSD) that provides a mechanism for wireless clients to retrieve packets buffered
by an access point. U-APSD reduces the amount of signaling frames sent from a client to retrieve
buffered data from an access point. U-APSD also allows access points to deliver buffered data
frames as bursts, without backing-off between data frames. These improvements are useful for
voice clients, as they improve battery life and call quality.
The Wi-Fi alliance has created Wireless Multimedia (WMM) and WMM Power Save (WMM-PS)
certification programs to ensure interoperability between 802.11e WLAN infrastructure
implementations and wireless clients. An access point managed wireless network supports both
WMM and WMM-Power Save techniques. WMM and WMM-PS (U-APSD) are enabled by default in
each WLAN profile.
Enabling WMM support on a WLAN just advertises the WLAN’s WMM capability and radio
configuration to wireless clients. The wireless clients must be also able to support WMM and use
the values correctly while accessing WLAN to benefit.
WMM includes advanced parameters (CWMin, CWMax, AIFSN and TXOP) specifying back-off
duration and inter-frame spacing when accessing the network. These parameters are relevant to
both connected access point radios and their wireless clients. Parameters impacting access point
transmissions to their clients are controlled using per radio WMM settings, while parameters used
by wireless clients are controlled by a WLAN’s WMM settings.
Access points support static QoS mechanisms per WLAN to provide prioritization of WLAN traffic
when legacy (non WMM) clients are deployed. An access point allows flexible WLAN mapping with a
static WMM access control value. When enabled on a WLAN, traffic forwarded from to a client is
prioritized and forwarded based on the WLAN’s WMM access control setting.
NOTE
Statically setting a WLAN WMM access category value only prioritizes traffic to the client.