Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter Design Guide
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Enterprise Mobility 4.1 Design Guide
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Chapter 5 Cisco Unified Wireless QoS
802.11 DCF
•
Network downstream—Traffic leaving the WLC traveling to the AP. QoS can be applied at this point
to prioritize and rate-limit traffic to the AP. Configuration of Ethernet downstream QoS is not
covered in this chapter.
to prioritize and rate-limit traffic to the AP. Configuration of Ethernet downstream QoS is not
covered in this chapter.
•
Network upstream—Traffic leaving the AP, traveling to the WLC. The AP classifies traffic from the
AP to the upstream network according to the traffic classification rules of the AP.
AP to the upstream network according to the traffic classification rules of the AP.
QoS and Network Performance
The application of QoS features might not be easily detected on a lightly loaded network. If latency,
jitter, and loss are noticeable when the media is lightly loaded, it indicates either a system fault, poor
network design, or that the latency, jitter, and loss requirements of the application are not a good match
for the network. QoS features start to be applied to application performance as the load on the network
increases. QoS works to keep latency, jitter, and loss for selected traffic types within acceptable
boundaries. When providing only radio downstream QoS from the AP, radio upstream client traffic is
treated as best-effort. A client must compete with other clients for upstream transmission as well as
competing with best-effort transmission from the AP. Under certain load conditions, a client can
experience upstream congestion, and the performance of QoS-sensitive applications might be
unacceptable despite the QoS features on the AP. Ideally, upstream and downstream QoS can be operated
either by using WMM on both the AP and WLAN client, or by using WMM and a client proprietary
implementation.
jitter, and loss are noticeable when the media is lightly loaded, it indicates either a system fault, poor
network design, or that the latency, jitter, and loss requirements of the application are not a good match
for the network. QoS features start to be applied to application performance as the load on the network
increases. QoS works to keep latency, jitter, and loss for selected traffic types within acceptable
boundaries. When providing only radio downstream QoS from the AP, radio upstream client traffic is
treated as best-effort. A client must compete with other clients for upstream transmission as well as
competing with best-effort transmission from the AP. Under certain load conditions, a client can
experience upstream congestion, and the performance of QoS-sensitive applications might be
unacceptable despite the QoS features on the AP. Ideally, upstream and downstream QoS can be operated
either by using WMM on both the AP and WLAN client, or by using WMM and a client proprietary
implementation.
Note
Even without WMM support on the WLAN client, the Cisco Unified Wireless solution is able to provide
network prioritization in both network upstream and network downstream situations.
network prioritization in both network upstream and network downstream situations.
Note
WLAN client support for WMM does not mean that the client traffic automatically benefits from WMM.
The applications looking for the benefits of WMM assign an appropriate priority classification to their
traffic, and the operating system needs to pass that classification to the WLAN interface. In purpose-built
devices, such as VoWLAN handsets, this is done as part of the design. However, if implementing on a
general purpose platform such as a PC, application traffic classification and OS support must be
implemented before the WMM features can be used to good effect.
The applications looking for the benefits of WMM assign an appropriate priority classification to their
traffic, and the operating system needs to pass that classification to the WLAN interface. In purpose-built
devices, such as VoWLAN handsets, this is done as part of the design. However, if implementing on a
general purpose platform such as a PC, application traffic classification and OS support must be
implemented before the WMM features can be used to good effect.
802.11 DCF
Data frames in 802.11 are sent using the Distributed Coordination Function (DCF), which is composed
of the following two main components:
of the following two main components:
•
Interframe spaces (SIFS, PIFS, and DIFS).
•
Random backoff (contention window) DCF is used in 802.11 networks to manage access to the RF
medium.
medium.
A baseline understanding of DCF is necessary to deploy 802.11e-based enhanced distributed channel
access (EDCA). For more information on DCF, see the IEEE 802.11 specification at the following URL:
access (EDCA). For more information on DCF, see the IEEE 802.11 specification at the following URL:
.