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Adding 'Quality' to Wireless LANs: Page 2 of 7

Consider the Options

WLAN infrastructure vendors, including Airespace, Cisco Systems and Colubris Networks, offer proprietary wireless QoS. They link traffic prioritization to users and groups by assigning priority to either the user's authenticated identity or the 802.11 ESSID (Extended Service Set Identifier).

We recently tested technology from Colubris in Network Computing's Syracuse University Real-World Labs® that provides QoS by ESSID, using what Colubris calls "service-aware" Wi-Fi QoS. We connected three Dell Latitude notebooks with embedded 802.11b NICs to our lab Ethernet LAN over a Colubris CN320 access point. We used NetIQ Chariot to transfer files to each of the clients. Not surprisingly, each client shared the available bandwidth and achieved comparable throughput of about 1.6 Mbps. We then reconfigured the AP with multiple 802.11 ESSIDs and gave each a different priority assignment, resulting in higher performance for prioritized users (the chart on page 83 shows how each client performed).





Directing Traffic By Priority



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Although this approach doesn't address all QoS problems, it's easy to implement. It's simple to assign ESSIDs and associated security policies to phone devices so that they get priority over conventional data traffic. The catch is that though AP-centric prioritization schemes let you prioritize traffic from the AP to the client and from the AP to the wired segment, they don't do QoS over the airwaves. So if the wireless network experiences severe congestion, VoIP clients may not gain predictable access to the network for upstream traffic. That's because all WLAN clients adhere to the same rules for gaining access to the medium--those of CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)--which are designed for fairness rather than prioritized access.