High-capacity Wi-Fi coverage

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We left off last time with a discussion of challenges that crop up when trying to design a WLAN for a high-density environment. Using a traditional multi-cell architecture, the main challenge is tuning the power of the APs appropriately so that there can be many small coverage cells. The trick is making sure users have plenty of aggregate capacity to share while avoiding the co-channel interference that lurks when lots of APs occupy a relatively small space. Now, what about single-channel and array architectures?

You likely know that Meru offers a single-channel architecture approach. The upside of this setup, whereby all APs in a shared domain are tuned to a single channel, is that the WLAN controller coordinates client-to-AP associations. It determines which client transmits when and for how long and when the AP transmits and for how long. With this kind of “big picture” control, it’s easier to avoid the interference problem.

However, Meru APs run at full power, which means that fewer APs are used in a manufacturer-recommended network design than with a multi-cell design. Thus a high degree of uplink and downlink transmission coordination is necessary in order to deal with a high-density client environment.

Devin Akin, CTO at Wi-Fi training and certification company CWNP, recommends appropriately spacing out APs on the same channel to create multiple collision domains so that the aggregate system bandwidth shared by users remains abundant.

Finally, there’s the “array” approach to covering high-density environments. Xirrus makes a 16-radio controller with an integrated directional antenna array, which provides a ton of capacity. It avoids co-channel interference by the fact that the antennas concentrate and focus their energy in wedged directions of about 20 to 25 degrees each. Each wedge can get the full 54Mbps connect rate of 802.11g or 300Mbps connect rate of 802.11n.

Meru makes some arrays, too, the AP440 (802.11n) and the RS4000 (802.11a/b/g) in which radio 1 represents one layer tuned to one channel, radio 2 represents a second layer tuned to another channel, and so forth. However, these arrays use omnidirectional antennas. As such, they emit RF energy that overlaps, so you need to test the systems to see whether interference arises simply from the physical proximity of the radios, antennas and power.

A similar issue would likely arise if you attempted to deploy more than one Xirrus 16-radio array within a given coverage area, because at that point, those radios, too, would start to overlap in the directions that they send RF energy.