Motorola WiNG 5, Справочное руководство системы

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Wireless Configuration 6 - 61
6.3 Radio QoS Policy
Without a dedicated QoS policy, a network operates on a best-effort delivery basis, meaning all traffic has equal priority
and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an equal chance of
being dropped!
When configuring a QoS policy for a radio, select specific network traffic, prioritize it, and use congestion-management
and congestion-avoidance techniques to provide deployment cusomizations best suited to each QoS policy’s intended
wireless client base.
Motorola Solutions 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, Email 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 Motorola Solutions 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) specifing 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.