This is from the 7921 Deployment Guide:
The Cisco Access Point can support up to 27 active RTP streams for both 802.11g and 802.11a at a data rate of 24 Mbps or
greater. To achieve this capacity, you must use U-APSD and have minimal WLAN background traffic and radio frequency (RF)
utilization.
The number of calls may vary depending on the data rate, initial channel utilization, and the environment.
At 6 Mbps, the Cisco Access Point can support up to 13 active RTP streams.
Using U-APSD instead of PS-POLL provides higher call capacity because U-APSD is more efficient and has limited
management overhead.
An RTP stream is bi-directional audio to and from a phone
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7921g/6_0/english/deployment/guide/7921dply.pdf
-Scott
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