Preparing for voice on WLANs

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A number of recent newsletters have discussed emerging technology approaches to taming the radio frequency air space in wireless LAN deployments. Among them are self-calibrating access points, spectrum management tools and automated site survey capabilities.

And, as noted in the last newsletter, an emerging WLAN architecture from start-up AirFlow Networks purports to do away with co-channel interference between APs altogether by removing the media access control (MAC) portion of an 802.11 radio from distributed APs and placing it in a centralized wireless controller.

Removing the requirement that mobile users re-associate and re-authenticate with MACs in distributed APs virtually eliminates latency, a likely boon to voice over WLANs. Latency in an AirFlow network is less than 20 milliseconds, says vice president of product marketing Brian Jenkins.

He says that instead of using channels 1, 6 and 11 – the three, nonoverlapping channels in 802.11b’s 2.4 GHz band – to overcome interference, you can use them to scale capacity.  He suggests dedicating one channel to data, one to voice and another to video, for example.

He acknowledges, however, that the still-emerging 802.11e standard for quality of service (QoS) will be needed to support integrated voice-data traffic running on a single channel.

Other vendors attack the voice-over-WLAN latency challenge in different ways. Some architectures let users “preauthenticate” to multiple APs at once, against a centralized database of allowable MAC addresses, for example.

Another approach is the Fast Secure Roaming feature in Cisco IOS Release 12.2(11)JA, which became available for Cisco Aironet 1100 and 1200 Series APs in June. The feature reduces latency to under 150 milliseconds, says Ron Seide, wireless product line manager.

The Cisco feature works by designating the local Cisco wired switch or router directly connected to APs as a so-called Wireless Domain Service (WDS) device. Users initially authenticate with a central Remote Authentication Dial-In User Service server, and the local WDS handles roaming and re-authentication using a “master key” downloaded from the central RADIUS server.