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Cisco Unified Wireless IP Phone 7920 Design and Deployment Guide
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Preface
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Overview of Cisco Wireless IP Telephony
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Radio Frequency and Site Survey
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Security
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Wireless Network Infrastructure
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Layer 2 and Layer 3 Roaming
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Quality of Service
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Deployments and Configuration
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Wireless IP Telephony Verification
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Troubleshooting the Cisco 7920 Phone
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Advanced Cisco 7920 Commands
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Site Survey RF Recommendations
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Example Configurations for AP and RADIUS Server
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Example Port Configurations for Voice Operations
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Site Information Help Request Form
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User Support Help Request Form
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Using Cisco Emergency Responder for E911 Calls with the Cisco 7920 Phone
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Guidelines and Limitations
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Maximum Throughput Calculations for 802.11b WLAN
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Index
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Table Of Contents
Distributed Coordination Function (DCF)
Quality of Service
Quality of Service (QoS) is essential to ensure that voice traffic receives timely and reliable treatment with low delay, low jitter, and little or no packet loss on the network. QoS ensures that voice traffic receives priority treatment when traveling across the network. It is required for both wired and wireless VoIP networks.
The following sections discuss QoS for wireless voice traffic:
QoS for Voice Traffic
Because WLANs operate as a shared medium, QoS is more difficult to achieve on wireless networks than on wired networks because the endpoints on a wireless network do not have dedicated bandwidth for sending and receiving traffic. The following characteristics apply to QoS for voice traffic on a wired network:
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Dedicated access per user or device (switched ethernet, point-to-point WAN)
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The following characteristics apply to QoS for voice traffic on a wireless network:
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Unlike wired networks with dedicated bandwidth, WLAN networks have to consider traffic direction when implementing QoS. Traffic is considered as either upstream or downstream from the point of view of the AP, as shown in Figure 6-1.
Figure 6-1 Upstream and Downstream Traffic
WLAN Traffic Transmission
This section describes the following topics pertaining to traffic transmission over a WLAN:
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Collision Avoidance
Similar to wired Ethernet networks, 802.11b WLANs employ Carrier Sense Multiple Access (CSMA). However, instead of using collision detection (CD), WLANs use collision avoidance (CA). Instead of each station trying to transmit as soon as the medium is free, WLAN devices use a CA mechanism to prevent multiple stations from transmitting at the same time.
Distributed Coordination Function (DCF)
The model used for WLAN data transmission is called Distributed Coordination Function (DCF), illustrated in Figure 6-2.
Figure 6-2 802.11b DCF Model
DCF ensures that the following events occur when the endpoints try to transmit data:
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Figure 6-3 shows the growth in the random backoff range with retries, as outlined in the preceding steps.
Figure 6-3 Growth in Random Backoff Range with Retries
Enhanced DCF (EDCF)
For WLAN QoS, Cisco APs and 7920 Wireless IP Phones use a technique similar to IEEE 802.11e, called enhanced DCF (EDCF). EDCF enables endpoint devices that have delay-sensitive multimedia traffic to modify their CWmin and CWmax values to allow for statically greater (and more frequent) access to the medium. In practice, EDCF on Cisco WLAN devices works as illustrated in Figure 6-4.
Figure 6-4 802.11b EDCF Model
The default values in Cisco IOS for CWmin and CWmax have been determined as the best possible values for a voice deployment. (Figure 7-7 shows an example of these default values.)
Queues
Beginning with Cisco IOS Release 12.2(11)JA, Cisco Aironet APs support EDCF-type QoS, with up to eight queues for downstream (toward the 802.11b clients) QoS. These queues can be allocated in any of the following ways:
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Although eight queues are supported on the AP, Cisco recommend that you have only two queues for traffic on the AP to ensure the best possible voice QoS. Voice (RTP) and signaling (SCCP) traffic should be placed into the highest priority queue, and all data traffic should be placed into a best-effort queue. While 802.11b EDCF does not guarantee that voice traffic will be protected from data traffic, using this queuing model should provide the best statistical results for voice QoS.
The Cisco 7920 phones support EDCF-type QoS for upstream (toward the AP) traffic. In addition, the Cisco 7920 phone dynamically announces its presence to the Cisco Aironet AP to ensure that its downstream traffic is placed into the high-priority queue on the AP. This dynamic announcement is done via Cisco Discovery Protocol (CDP). The CDP packets are sent from the Cisco 7920 phone to the AP, and they identify the phone so that the AP can place all traffic to the phone in the high-priority queue.
DSCP Marking
The SCCP signaling messages are marked with DSCP 26 or Per-Hop Behavior (PHB) AF31, and RTP packets are marked with DSCP 46 (PHB EF). These markings match the DSCP markings of Cisco wired Ethernet IP phones and make the QoS settings consistent for both LAN and WLAN environments.
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QoS Basis Service Set (QBSS)
In addition to setting the DSCP or PHB markings correctly and supporting EDCF-type QoS, the Cisco 7920 phone also supports an intelligent mechanism to determine the QoS that can be provides by a given AP. This mechanism uses an algorithm that takes into consideration the Received Signal Strength Indicator (RSSI) and the RF channel utilization (CU) based on updates received by the Cisco AP in beacon messages using the QBSS element. Based on this information, the Cisco 7920 phone can determine if the load on a given AP is excessive and if it should attempt to associate with a less congested AP in order to preserve QoS for an IP Telephony call.
Additional Considerations
Beyond providing proper queuing and DSCP (or PHB) markings for the voice packets, you must consider delay and jitter. These factors are especially important for upstream traffic because there is no queuing among clients on the 802.11b side of the AP. The simplest way to reduce delay and jitter is to stay within the guidelines for the number of WLAN clients per AP. Exceeding these guidelines creates additional opportunities to introduce packet delay and jitter.
For more details about deploying QoS in WLAN networks, refer to the Cisco Aironet Wireless Quality-of-Service Deployment Guide, available at
