Cisco 7925G Administration Guide - Page 43

Chapter 2 Overview of the VoIP Wireless Network Components 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. Figure 2-2 Voice Traffic in a Wireless Network Downstream Downstream Upstream Upstream Network 119160 Downstream QoS only Bi-Directional QoS 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 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. 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. OL-15984-01 Cisco Unified Wireless IP Phone 7925G Administration Guide for Cisco Unified Communications Manager 7.0(1) 2-13