Call Admission Control; Quality Of Service And Differentiated Services Code Point Marking - Cisco Mesh Access Points Deployment Manual

Connecting the Cisco 1500 Series Mesh Access Points to the Network
Voice is supported only on indoor mesh networks. Voice is supported on a best-effort basis in the outdoors
Note
in a mesh network.

Call Admission Control

Call Admission Control (CAC) enables a mesh access point to maintain controlled quality of service (QoS)
when the wireless LAN is experiencing congestion. The Wi-Fi Multimedia (WMM) protocol deployed in
CCXv3 ensures sufficient QoS as long as the wireless LAN is not congested. However, to maintain QoS under
differing network loads, CAC in CCXv4 or later is required.
CAC is supported in Cisco Compatible Extensions (CCX) v4 or later. See Chapter 6 of the Cisco Wireless
Note
LAN Controller Configuration Guide at
configuration/guide/c70sol.html
Two types of CAC are available for access points: bandwidth-based CAC and load-based CAC. All calls on
a mesh network are bandwidth-based, so mesh access points use only bandwidth-based CAC.
Bandwidth-based, or static CAC enables the client to specify how much bandwidth or shared medium time
is required to accept a new call. Each access point determines whether it is capable of accommodating a
particular call by looking at the bandwidth available and compares it against the bandwidth required for the
call. If there is not enough bandwidth available to maintain the maximum allowed number of calls with
acceptable quality, the mesh access point rejects the call.

Quality of Service and Differentiated Services Code Point Marking

Cisco supports 802.11e on the local access and on the backhaul. Mesh access points prioritize user traffic
based on classification, and therefore all user traffic is treated on a best-effort basis.
Resources available to users of the mesh vary, according to the location within the mesh, and a configuration
that provides a bandwidth limitation in one point of the network can result in an oversubscription in other
parts of the network.
Similarly, limiting clients on their percentage of RF is not suitable for mesh clients. The limiting resource is
not the client WLAN, but the resources available on the mesh backhaul.
Similar to wired Ethernet networks, 802.11 WLANs employ Carrier Sense Multiple Access (CSMA), but
instead of using collision detection (CD), WLANs use collision avoidance (CA), which means that instead
of each station trying to transmit as soon as the medium is free, WLAN devices will use a collision avoidance
mechanism to prevent multiple stations from transmitting at the same time.
The collision avoidance mechanism uses two values called CWmin and CWmax. CW stands for contention
window. The CW determines what additional amount of time an endpoint should wait, after the interframe
space (IFS), to attend to transmit a packet. Enhanced distributed coordination function (EDCF) is a model
that allows end devices that have delay-sensitive multimedia traffic to modify their CWmin and CWmax
values to allow for statically greater (and more frequent) access to the medium.
Cisco access points support EDCF-like QoS. This provides up to eight queues for QoS.
These queues can be allocated in several different ways, as follows:
OL-27593-01
Configuring Voice Parameters in Indoor Mesh Networks
http://www.cisco.com/en/US/docs/wireless/controller/7.0/
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