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Three-stream Wi-Fi hits the mark

In 3-stream vs 2-stream tests, 7 MIMO-based access points deliver impressive performance increases

By Craig Mathias, Network World
March 26, 2012 12:07 AM ET

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How we tested three-stream 802.11n access points

Testing was conducted in freespace, but in a residential environment so as to provide a greater degree of control over the airwaves by minimizing the impact of any potential interference. It's almost impossible today to find a commercial facility free of a good deal of arbitrary Wi-Fi traffic, evidence of the status that Wi-Fi has achieved.

All testing was monitored with a spectrum analyzer (in this case, AirMagnet's Spectrum XT and Fluke Networks' AirCheck WiFi Tester) to assure a level spectral playing field. The server end of this test was an Ixia VeriWave WaveTest90 chassis with a WBE1604 Ethernet Board, providing a consistent and highly instrumented environment with respect to load and the recording of results.

The client software was Ixia's VeriWave WaveInsite and WaveTest running on a Console PC connected to the back of the chassis, and the company's WaveAgent on the Test notebook PCs equipped with wireless client adapters.

The only exceptions were in the case of Cisco, in which a Cisco Catalyst 3750-X PoE switch and 5500 WLAN controller were also used, and Meraki, which requires a connection to the Internet for control and management plane implementation. This was accomplished via reconfiguring IP addresses and connecting to the Web via a switch and our office router.

Each series of tests with each subject access point was conducted with both two- and three-stream client devices so as to fully evaluate the performance of the access point. The two-stream client was Linksys' popular WUSB600N, a USB 2.0 device using the Ralink RT2870/2850 chipset, and the three-stream client was a TRENDnet TEW-680MB media bridge, connected to the Test PC's gigabit Ethernet port, and based on the Ralink RT3883F chip.

While three-stream USB adapters are available, it was felt that the 450Mbps peak of the wireless device was too close to the 480Mbps limit of USB 2.0, and that gigabit Ethernet would provide more headroom just in case. All device settings were left at their defaults other than being configured to automatically connect to the test SSID.

All of the access points under test, except the Xirrus Array, were configured to use a single 40-MHz. channel (149/153) in the 5GHz band. By virtue of its need to manage eight separate radios, the Xirrus instead auto-configured channels across all of its radios. Apart from the radio channel, all that was changed from the default in all access point configurations was the IP address, SSID, and WPA2 key. The 2.4GHz radio, if present, was disabled if possible and otherwise left unconfigured, so the only variables for each test run were the particular client adapter used and the distance involved. The real variable under study, then, was the performance of the access point under test. WaveInsite was configured to generate both upstream and downstream HTTP traffic at a nominal 1Gbps rate, thus saturating the link.

Each 30-second test run was repeated three times, with the results averaged so as to factor out any otherwise-undetected anomalies, over both "Near" (both endpoints in the same room with nominally four meters of distance between them) and "Far" (straight up from the basement of the structure to the second floor, a linear distance of about seven meters but through numerous solid objects) geometries. The Near test was designed to evaluate performance that might be seen in a typical dense-access point deployment, while Far is more characteristic of a typical access point deployment. We thus had a pretty good look at the contribution of three-stream MIMO across all of the access points. Just to be sure, we also baselined the test using a direct wired gigabit Ethernet connection, which showed that gigabit speeds were indeed possible.

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