Cisco Unified Wireless Ip Phone 7925G, 7925G-Ex, And 7926G Administration Guide - Cisco 7925G Administration Manual

Voice QoS in Wireless Networks
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 3: 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:
• Differentiated Services Code Point (DSCP) settings for the packets
• Layer 2 or Layer 3 access lists
• VLANs for specific traffic
Different traffic types are sent to different priority queues. The following are the queues:
• Best Effort (BE) = 0, 3
• Background (BK) = 1, 2
• Video (VI) = 4, 5
• Video (VO) = 6, 7
Call Control (SCCP) is sent on UP4 (VI) and voice is sent as UP6 (VO).
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 Wireless IP Phone marks the call control packets with a DSCP value of 24 and voice
Note
packets with DSCP value of 46.
To improve reliability of voice transmissions in a nondeterministic environment, the Cisco Unified Wireless
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
bandwidth, when the amount of voice traffic is increased beyond this limit (to N+1 calls), the quality of all
calls suffers.
To help address the problems of VoIP stability and roaming, an initial Call Admission Control (CAC) scheme
is required. With CAC, QoS is maintained in a network overload scenario by ensuring that the number of
active voice calls does not exceed the configured limits on the AP. The Cisco Unified Wireless IP Phone can
integrate layer 2 TSpec admission control with layer 3 Cisco Unified Communications Manager admission

Cisco Unified Wireless IP Phone 7925G, 7925G-EX, and 7926G Administration Guide

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VoIP Wireless Network