Version 3.1.3-7 of Meru's software incorrectly calculated the duration value of nearly every packet. We confirmed the existence of this "bug" by analyzing traces on a production Meru network. Almost every packet we analyzed in a simple HTTP download had a duration value 16 microseconds too long. When confronted with these results, Meru asserted that it was a bug in its duration value calculation algorithm related to 802.11 rate adaptation, a bug that had been corrected.
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The 802.11 standard also lets contention be managed using the CTS-to-Self (Clear-to-Send-to-Self) mechanism. The function of this frame is to reserve the air for an impending frame by including a duration field long enough to cover the data frame, two SIFS intervals and the corresponding ACK frame. Upon analysis of packet captures taken during an 802.11b/g co-existence test on a Meru infrastructure running version 3.2.5.SR1-4, it became apparent that its CTS-to-Self duration values were inflated. The 802.11 standard defines the CTS-to-Self duration value as being long enough to cover one following data frame (often in the range of 300 microseconds), but Meru inflates this value outside of the 802.11 standard to as high as 2,500 microseconds, thereby protecting multiple sequential data frames and monopolizing airtime.
The effects of this duration field inflation are detrimental to adjacent networks. For those 2,500 microseconds being used by the Meru cell, no other client or AP within 802.11 listening range on that radio channel is allowed fair access to the medium.
To quantify the negative impact, we placed a Meru AP and a Cisco AP side by side on the same radio channel in an RF isolation chamber. For a baseline, we first ran the 802.11b/g co-existence test on two collocated Cisco APs and received 6.29 Mbps and 5.54 Mbps, respectively, exhibiting normal 802.11 airtime fairness, as expected. In contrast, the same test run with collocated Cisco and Meru APs yielded 2.74 Mbps for Cisco and 8.02 Mbps for Meru (see "Rude Neighbors," above right). Meru asserted that these results demonstrate their product's superior design.
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