MIMO takes advantage of multipath to transmit and/or receive on multiple antennas. Through signal processing and other types of gain, a signal transmitted at the same output power can reach farther than before to allow for higher link rates and better coverage.
Speaking of 802.11n, we haven't signed on to the "everyone will need to replace their switches immediately" line. Wireless network usage is growing, yes, but even a year from now, we don't think we'll see enterprises replacing their controllers for capacity reasons. For incremental growth, enterprise WLAN vendors will be happy to sell you an additional controller to spread the load.
Bottom line, don't jump the gun here: No enterprise WLAN vendor has thus far suggested a software-upgradable path, and only Xirrus has put into place a hardware-based upgrade program with fixed pricing. Enterprises aren't clamoring for MIMO or pre-802.11n APs from WLAN vendors, confirming that we aren't yet in desperate need. So far, better AP placement, antennas and DAS are helping IT hold its own. Make Room For Microcells
Rather than building a wireless network for coverage, where every AP is transmitting at its highest output power and overlapping coverage is at a minimum, established WLAN vendors Cisco, Aruba and Trapeze tout microcells, which require APs to be more densely deployed with power levels turned down. This does address some capacity concerns: More APs lower the client-to-AP ratio, and the average link rate will go up because clients are more likely to be near an AP.
But microcells introduce their own problems. AP placement, channel design and planning become crucial. Rather than focusing on coverage, areas of higher usage require more APs, which leads to challenges in assigning three nonoverlapping channels in the 2.4-GHz range with respect to the three dimensions. Fortunately, there are third-party tools from Motorola (the former Wireless Valley product), Ekahau, AirMagnet and AirTight, as well as from select enterprise WLAN vendors, to help in this process; see our wireless site survey workshop at Crash Course: Wireless Site Surveys.
