The WRT1900AC is Wi-Fi Certified for 802.11a,b,g,n and ac. It defaulted to Auto channel mode and width on both 2.4 and 5 GHz radios upon power-up. The router comes with unique 2.4 and 5 GHz SSIDs set, so you don't have to change anything to connect dual-band clients to the desired band.
WPS was enabled on both bands and a WPS pushbutton session completed quickly on both resulting in WPA2/AES connections.
For throughput testing, the router was first reset to factory defaults. Then Channel 6 and 20 MHz bandwidth mode was set for 2.4 GHz and Channel 153 and 80 MHz mode set for 5 GHz. The test client was connected using WPA2/AES encryption.
The router's four antenna cluster was centered on the test chamber turntable with all antennas vertical as shown in the photo below. The 0° position for the router had the front facing the chamber antennas.
Linksys WRT1900ACS in test chamber
The Benchmark Summary below shows the average of throughput measurements made in all test locations.
Linksys WRT1900ACS Benchmark Summary
The charts below, filtered to show AC1900 class routers and current test process only, show the WRT1900ACS' average 2.4 GHz throughput performance over the entire measured range is near the bottom of all AC1900 class routers tested for both down and uplink.
Linksys WRT1900ACS 2.4 GHz Average throughput comparison
The story is very different for average 5 GHz performance, where the ACS sits at the top of both charts.
Linksys WRT1900ACS 5 GHz Average throughput comparison
The 2.4 GHz downlink profile shows the why the ACS did poorly in the average throughput comparison. Although it starts out at the same throughput as the other two routers, throughput starts to fall off 12 dB earlier at 18 dB attenuation.
The ACS' throughput falloff is a little less steep than the other two routers from 18 to 36 dB of attenuation, where its falloff rate then increases to match the other two routers'.
2.4 GHz Downlink Throughput vs. Attenuation
The 2.4 GHz uplink plot shows the ACS more like the EA6900, although again falling off sooner. The AC's throughput starts out exceptionally high, but then starts to fall off pretty early. The high start keeps its curve above the others for most of the tested range.