Wireless Infrastructure

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Air Time: Understanding 802.11 Interference

Under conditions of heavy network utilization and co-channel interference, today's wireless LAN products behave unpredictably. Over time, these problems will become more critical.

Last November, Network Computing ran a feature story examining the performance of Wi-Fi products from Meru Networks and Cisco Systems. Their offerings differed fundamentally in how they manage contention. Meru markets its unique contention-management algorithms as its "Air Traffic Control" system. On most Wi-Fi networks, APs and wireless clients have equal rights to transmit. On Meru networks, APs and controllers exert more control, prioritizing traffic in ways it says offer significant advantages, particularly for converged voice and data environments.

Our initial testing confirmed Meru's value proposition. Meru bested Cisco in fairly managing contention between Vo-Fi phones and notebook computers. Cisco cried foul, asserting that Meru was playing fast and loose with standards, manipulating fields associated with 802.11's virtual carrier sense system. Packet traces confirmed that certain duration field values in Meru packets were out of spec. It was far short of proof, but it looked like a smoking gun.

Packet traces aside, we were more interested in real-world impact. We rigged up a test bed and threw a mix of 802.11b/g traffic at Cisco and Meru APs running on the same RF channel. Meru's performance dropped by 50 percent, as one might expect, but Cisco's cratered. Cisco pointed to this as proof that Meru was cheating. Meru blamed it on Cisco's poor software engineering.

We published what we knew at the time, reporting the favorable Meru test results as well as the coexistence results and Cisco's allegations of standards noncompliance. And we gave both vendors the opportunity to offer a formal response (see Cisco Vs. Meru: The Vendors Speak).

Cisco then brought its allegations to the Wi-Fi Alliance late last year, asserting that Meru was violating the alliance's good-neighbor policy. This was not the first time Meru had been challenged, but the result was the same: The alliance refused to rescind Meru's certification. Cisco to this day refuses to back down from its allegations.

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