Cisco 8961 Administration Manual page 106

Components of the VoIP 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
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 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 IP
Phone 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.
Implementing QoS in the connected Ethernet switch is highly desirable to maintain good voice quality.
The COS and DSCP values that the Cisco Unified IP Phone sets do not need to be modified.
The DSCP, COS and UP (WMM) markings correctly for the optimum transmission of video frames.
The Cisco Unified IP Phone 9971 does not support Video CAC; however, Voice CAC is supported for
Note
WLANs.
Related Topics
Authentication Methods, page 6-11
•
Interacting with Cisco Unified Communications Manager, page 6-11
•
VoIP WLAN Configuration, page 6-15
•
Cisco Unified IP Phone 8961, 9951, and 9971 Administration Guide for Cisco Unified Communications Manager 8.6 (SIP)
6-10
The Cisco Unified IP Phone marks the SCCP signaling packets with a DSCP value of 24 (CS3)
and RTP packets with DSCP value of 46 (EF).
Chapter 6 Understanding the VoIP Wireless Network