To counter the difficulties involved in measuring Wi-Fi client roaming performance, eWEEK Labs turned to Azimuth Systems Inc. for testing equipment that provides accurate and repeatable results.
The Azimuth equipment provides a closed, controlled environment for Wi-Fi testing. Each Wi-Fi device in the test is installed in its own isolated RF (radio frequency) chamber, which effectively filters radio emissions. Removing the vagaries associated with wireless testing, Azimuth tests instead rely on cabled connections among Wi-Fi devices—using each devices external antenna connector ports to facilitate connection to the WLAN (wireless LAN).
When testing voice-over-Wi-Fi roaming capabilities, the Azimuth system monitors voice traffic on each radio channel used in the test, identifying precisely when a phone starts transmitting through a different access point. To force the phone to roam, the Azimuth system slowly attenuates the radio signal between the phone and its associated access point. When the radio signal weakens sufficiently, the phone begins looking for an access point with a stronger signal and will eventually begin associating with it.
The full roam time is then calculated as the difference between the last data packet transmitted via the first access point and the first data packet sent through the second access point.
In this manner, we could effectively simulate, for example, a user pacing back and forth in a long hallway while talking on the phone, constantly roaming between access points.
Our test wireless network consisted of a pair of wireless access points from Cisco Systems Inc.—an IEEE 802.11b/g-compatible Aironet 1200 and an 802.11b-only Aironet 1100. For the purposes of this test, we did not use any encryption in our WLAN.
/zimages/1/28571.gifThe FCC recently approved Ciscos 802.11a radios as software-defined radios. Read morehere.
Because wireless phones are generally not designed to accommodate an external antenna, we instead connected each of the two phones we tested to the wireless network with a near-field antenna.
We physically strapped the near-field antenna as close as possible to the test devices built-in antenna, and the device transmitted communications to the Azimuth system. To account for variations in how well the near-field antenna received a signal from the test device, we calculated the additional path loss through the Azimuth system and accounted for this variability in our test configuration .
To provide call setup, we built a SIP (Session Initiation Protocol)-based voice server based on the open-source project Asterisk. The Asterisk server facilitated calls between the SIP phone under test and a PC running CounterPath Solutions Inc.s X-Lite SIP soft phone.
/zimages/1/28571.gifShoreTel 6 supports SIP.Click hereto read more.
In addition to the two Wi-Fi-enabled phones we tested, we evaluated a notebook PC configured with a SIP-based soft phone.
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