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What wireless QoS mechanisms does the Cisco 7920 Wireless IP Phone support?
The Cisco 7920 IP Phones support Layer 2 (L2) Enhanced Distributed Coordination Function (EDCF) such as Quality of Service (QoS) for downstream (toward the access point) traffic. In addition, the Cisco 7920 dynamically announces its presence with the Cisco Aironet access point to ensure that its downstream traffic is placed into the High Priority queue on the access point. This dynamic announcement is done using Cisco Discovery Protocol. The Cisco Discovery Protocol packets are sent from the Cisco 7920 to the access point and identify the phone so that the access point can place all traffic to the phone in the High Priority queue.
The Cisco 7920 marks Real Time Protocol (RTP) voice packets with DiffServ EF. The Cisco 7920 Skinny Client Control Protocol (SCCP) signaling messages are marked with DiffServ AF31 (Type of Service [ToS] 3). This matches the DiffServ markings of Cisco wired Ethernet IP phones, and allows the QoS settings to be consistent from LAN to Wireless LAN (WLAN) environments.
In addition to setting the DiffServ markings correctly and supporting EDCF-like QoS, the Cisco 7920 also supports an intelligent mechanism to determine the QoS that can be provided by a given access point. It does this based on an algorithm that takes into consideration Relative Signal Strength (RSSI) and Radio Frequency (RF) Channel Utilization (CU) based on updates received by the Cisco access point in beacon messages using the QoS Basis Service Set (QBSS) element. Based on this information, the Cisco 7920 can determine if the load on a given access point is excessive, and if it should attempt to associate with a less-congested access point in order to preserve QoS of an IP Telephony call.