Cisco 7925G - Unified Wireless IP Phone VoIP Administration Manual page 43

Chapter 2 Overview of the VoIP Wireless Network
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
Figure 2-2
Downstream QoS only
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 you can have up to eight queues on the AP, you should use only two queues for voice traffic
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.
Note
To improve reliability of voice transmissions in a nondeterministic environment, the Cisco Unified
Wireless IP Phone 7925G 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 7925G can integrate layer 2 TSpec admission control with layer 3 Cisco Unified
Communications Manager admission control (RSVP). During times of network congestion, calling or
called parties receive a fast busy indication. The system maintains a small bandwidth reserve so wireless
phone clients can roam into a neighboring AP (AP), even when the AP is at "full capacity". After
reaching the voice bandwidth limit, the next call is load-balanced to a neighboring AP without affecting
the quality of the existing calls on the channel.
Cisco Unified Wireless IP Phone 7925G Administration Guide for Cisco Unified Communications Manager 7.0(1)
OL-15984-01
Figure 2-2.
Voice Traffic in a Wireless Network
Downstream
Upstream
Bi-Directional QoS
The Cisco Unified Wireless IP Phone 7925G marks the SCCP signaling packets with a DSCP
value of 24 and RTP packets with DSCP value of 46.
Downstream
Upstream
Components of the VoIP Wireless Network
Network
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