Chapter 7 SSID Screen
On APs without WMM QoS, all traffic streams are given the same access priority to the wireless network. If the introduction of another traffic stream creates a data transmission demand that exceeds the current network capacity, then the new traffic stream reduces the throughput of the other traffic streams.
The NWA uses WMM QoS to prioritize traffic streams according to the IEEE 802.1q or DSCP information in each packet’s header. The NWA automatically determines the priority to use for an individual traffic stream. This prevents reductions in data transmission for applications that are sensitive to latency and jitter (variations in delay).
7.3.1.1 WMM QoS Priorities
The following table describes the WMM QoS priority levels that the NWA uses.
Table 19 WMM QoS Priorities
Priority Level | description | |
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voice | Typically used for traffic that is especially sensitive to jitter. Use this | |
(WMM_VOICE) | priority to reduce latency for improved voice quality. | |
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video | Typically used for traffic which has some tolerance for jitter but needs to | |
(WMM_VIDEO) | be prioritized over other data traffic. | |
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best effort | Typically used for traffic from applications or devices that lack QoS | |
(WMM_BEST_EFFORT) | capabilities. Use best effort priority for traffic that is less sensitive to | |
latency, but is affected by long delays, such as Internet surfing. | ||
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background | This is typically used for | |
(WMM_BACKGROUND) | print jobs that are allowed but that should not affect other applications | |
and users. Use background priority for applications that do not have strict | ||
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| latency and throughput requirements. | |
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7.3.2 Type Of Service (ToS)
Network traffic can be classified by setting the ToS (Type Of Service) values at the data source (for example, at the NWA) so a server can decide the best method of delivery, that is the least cost, fastest route and so on.
7.3.2.1 ToS (Type of Service) and WMM QoS
The DSCP value of outgoing packets is between 0 and 255. 0 is the default priority. WMM QoS checks the DSCP value in the header of data packets. It gives the traffic a priority according to this number.
In order to control which priority level is given to traffic, the device sending the traffic must set the DSCP value in the header. If the DSCP value is not specified, then the traffic is treated as best- effort. This means the wireless clients and the devices with which they are communicating must both set the DSCP value in order to make the best use of WMM QoS. A Voice over IP (VoIP) device for example may allow you to define the DSCP value.
The following table lists which WMM QoS priority level the NWA uses for specific DSCP values.
Table 20 ToS and IEEE 802.1d to WMM QoS Priority Level Mapping
Dscp Value | WMM qos Priority Level |
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224, 192 | voice |
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160, 128 | video |
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| 83 |
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