What happens when voice meets MIMO?

* Cisco expert says 11n's MIMO benefits voice performance upstream

Earlier this month, I passed along antenna advice to a reader for a Cisco 802.11n network, much of which came from Cisco technical marketing engineer Frederick Niehaus. I’d like to share a bit more of my conversation with Niehaus, which pertains to the multiple input/multiple output (MIMO) technology inherent to 802.11n and its effect on Voice over WLAN (VoWLAN) deployments.

Observes Niehaus:

“The radio capabilities of the voice client are half of the equation, and the voice client usually has just one antenna. Typically, that antenna is shared with the Bluetooth radio.... So, three or two spatial streams [from an 11n access point] down to the voice client radio will not be totally utilized.

“The voice client typically makes connection and roaming decisions based on channel utilization, AP client load and signal strength. The voice client keeps an active table of these conditions by AP MAC address, so that it can execute a faster roam when needed. [In this way], voice [in an 802.11n network] will behave in the same fashion as in earlier technology.

“The benefit of MIMO for the voice client is upstream. Because there are three receivers on the [Cisco] 802.11n AP, the likelihood of the reception of a weak signal or a reflected signal is significantly increased. Testing shows a 12% reduction in upstream retries in coverage areas with high multipath conditions. This relates directly to improved MOS [mean opinion score] values and call capacity in that channel and coverage area.”

Note that in the next newsletter, I’ll balance out this MIMO discussion and my earlier Cisco 802.11n antenna discussion with some commentary from Wi-Fi system and smart-antenna maker Ruckus Wireless, which sheds a different perspective on antenna positioning and MIMO altogether.

And finally, while we’re on the topic of VoWLAN, it seems pertinent that Cisco partner AirMagnet has updated its portable AirMagnet Survey tool to allow IT departments to automatically verify that their WLANs meets Cisco and Vocera Wi-Fi handset requirements for voice. The Cisco and Vocera spec verifications are “pre-built,” says Dilip Advani, AirMagnet product manager, and users can customize the settings for other wireless handsets, as well.

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