Cisco 8961 Administration Manual page 116

Voice QoS in Wireless Network
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You need the following VLANs on the network switches and the APs that support voice connections on the
WLAN:
• Voice VLAN: Voice traffic to and from the wireless IP Phone
• Native VLAN: Data traffic to and from other wireless devices
Assign separate SSIDs to the voice and to the data VLANs. If you configure a separate management VLAN
in the WLAN, do not associate an SSID with the management VLAN.
By separating the phones into a voice VLAN and marking voice packets with higher QoS, you can ensure
that voice traffic gets priority treatment over data traffic, which results in lower packet delay and fewer lost
packets.
Unlike wired networks with dedicated bandwidths, wireless LANs consider traffic direction when implementing
QoS. Traffic is classified as upstream or downstream from the point of view of the AP as shown in the following
figure.
Figure 4: Voice Traffic in a Wireless Network
Beginning with Cisco IOS release 12.2(11)JA, Cisco Aironet APs support the contention-based channel access
mechanism called Enhanced Distributed Coordination Function (EDCF). The EDCF type of QoS has up to
eight queues for downstream (toward the 802.11b/g clients) QoS. You can allocate the queues based on these
options:
• QoS or Differentiated Services Code Point (DSCP) settings for the packets
• Layer 2 or Layer 3 access lists
• VLANs for specific traffic
• Dynamic registration of devices
Although up to eight queues on the AP can be set up, you should use only two queues for voice traffic so as
to ensure the best possible voice QoS. Place voice (RTP) and signaling (SCCP) traffic in the highest priority
queue, and place data traffic in a best-effort queue. Although 802.11b/g EDCF does not guarantee that voice
traffic is protected from data traffic, you should get the best statistical results by using this queuing model.
The Cisco Unified IP Phone marks the SCCP signaling packets with a DSCP value of 24 (CS3) and RTP
Note
packets with DSCP value of 46 (EF).
To improve reliability of voice transmissions in a nondeterministic environment, the Cisco Unified IP Phone
supports the IEEE 802.11e industry standard and is Wi-Fi Multimedia (WMM) capable. WMM enables
differentiated services for voice, video, best effort data and other traffic. However, in order for these
differentiated services to provide sufficient QoS for voice packets, only a certain amount of voice bandwidth
can be serviced or admitted on a channel at one time. If the network can handle "N" voice calls with reserved
Cisco Unified IP Phone 8961, 9951, and 9971 Administration Guide for Cisco Unified Communications Manager
10.0 (SIP)
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VoIP Wireless Network