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MU-MIMO routers


It's still early days for MU-MIMO routers. Buyers of first-generation AC2350 / AC2400 class 4x4 "MU-MIMO ready" routers using Quantenna 5 GHz radios are still waiting for firmware to enable MU-MIMO. But most consumer router makers have moved on to produce a second round of 4x4 routers, using chipsets from Qualcomm Atheros (QCA) and more recently, Broadcom.

QCA-based AC2600 class routers have all shipped with working MU-MIMO from the get-go. Broadcom's 4x4 routers are shipping as "MU-MIMO capable", with new firmware to turn on MU-MIMO promised by the end of this year. (Déjà vu, anyone?)

Not that all this really matters, because MU-MIMO devices are scarce as hen's teeth. There are no MU-enabled tablets and only Acer has built it into some of its notebooks you can actually buy in the U.S. MU-enabled smartphones, which hold the most potential to drive MU-MIMO adoption, are China only for now and require a new phone in any event.

MU-MIMO In A Nutshell

  • MU-MIMO uses a special form of beamforming that is part of the 802.11ac standard. It works in 5 GHz only.
  • MU-MIMO enables up to four devices to share the same Wi-Fi airtime. Current 4x4 based designs support three simultaneous devices
  • You need both MU-MIMO enabled routers and devices to benefit from MU-MIMO
  • You need at least two MU-MIMO devices to get any benefit from MU-MIMO
  • MU-MIMO works only for downlink data (moving from router to device). It provides no benefit for uplink
  • MU-MIMO works best with strong to medium strength signals
  • MU-MIMO does not increase range

Despite limited availability and other gotchas and caveats listed in the sidebar above, eager-beaver early adopters are snapping up MU-MIMO routers of all flavors anyway. (It's a tribute to consumer networking companies' marketing troops that they've been so effective in getting people to buy (much) more expensive routers that provide little to no benefit to most users. Such is the power of those bigger numbers on the box!)

The Method

Anyway, since router makers are going to make 'em and gullible consumers are gonna buy 'em, SmallNetBuilder is on the job to test them. Our test process hasn't been as good as we'd like it to be, since it has depended on manufacturers to loan MU-MIMO devices to test with.

Our first MU-MIMO router review of Linksys' EA8500 used a trio of Dell Inspiron 13 7000 series laptops specially equipped with QCA reference design MU-MIMO adapters.

Since then, we've received three China-only Xiaomi Mi Note Pro smartphones for testing the TP-LINK Archer 2600, Amped Wireless RTA2600 and NETGEAR R7500v2 (review coming soon). But the Xiaomi phones are long overdue to be returned and we need a more permanent solution with better test capability.

This test marks the first appearance of VeriWave as a SmallNetBuilder test partner. VeriWave is a division of Ixia, the company that has long provided its IxChariot network performance software test tool to us. IxChariot forms the basis of virtually all our router and wireless performance testing, acting as our primary network traffic generation and measurement tool.

Ixia VeriWave is a precision hardware / software tool for Wi-Fi testing, fundamentally different from software only test tools like IxChariot. VeriWave uses hardware-based traffic generation and analysis, with the ability to track every packet end-to-end and provide fine control of transmission parameters. Specifically, Ixia has provided a WaveTest 20 three slot chassis loaded with RF46014 WaveBlade Wi-Fi, WBE1601 WaveBlade Ethernet and Management modules.

The Wi-Fi blade acts as the Wi-Fi device (STA), supporting 802.11a/b/g/n/ac. It has four RF ports, so can emulate any STA from an ancient 802.11b 1 Mbps slowpoke to a full 4x4 MU-MIMO 1733 Mbps speedster. Actually, the Wi-Fi blade can create up to 500 (!) fully independent stateful clients.

The Management blade is the controller for the two instrumentation blades, allowing the chassis to sit on a LAN and be accessed via a web interface for simple management chores. The Ethernet blade is a single-port Gigabit wire-speed traffic generator with full packet-capture capability.

The Wi-Fi blade is a complete VeriWave custom design; no store-bought radios from any of the usual suspects are used. The block diagram below from the WaveTest data sheet, shows the client stack architecture that provides the load with multiple independent stateful clients. Download the data sheet to dig into the blade specs.

Veriwave WaveTest Client Stack

VeriWave WaveTest Client Stack

The heavy lifting for testing is done by a suite of Windows applications. Each application takes the raw capability of the blades and configures it into easier-to-configure test scenarios.

The Test

With all this power at hand, I wanted to go beyond what I've been doing with three MU-MIMO devices. So, working with VeriWave, we came up with a MU-MIMO scaling test. Since up to 500 MU-MIMO STAs (or mix of SU, MU and any other type) can be created, we thought it would be interesting to see what happens when today's MU-MIMO routers are loaded with four or more devices.

The custom test script VeriWave developed starts with a single MU 1x1 STA. The STA is configured to connect at the maximum MCS 9 index rate, 80 MHz bandwidth and short guard interval to achieve the maximum 433 Mbps link rate. Although the Wi-Fi WaveBlade can be configured to use any of the MIMO channel models (A-F), we kept things simple and used bypass mode.

The test connects the STA, then runs a TCP Goodput test, with MSS, Window and Frame size set so the link runs as fast as the device will allow. Each test runs for 30 seconds and throughput is recorded at the end of the test. We used a single TCP/IP session (connection) for each test.

NOTE: The TCP Goodput script is capable of much more. But we configured it so that it hits the device with traffic similar to how we use IxChariot's throughput script for our other network throughput testing.

The test then adds another MU STA, verifies connection, repeats the 30 second test and records throughput for each STA. Tests were run by VeriWave with up to 100 STAs during development. But to keep test time manageable, I tested only up to 16 STAs. The results were still plenty interesting!

The test is repeated with the STA mode set to SU, to see how total throughput behaves with non MU devices.

The Wi-Fi WaveBlade normally can be either directly cabled to the device under test or use an antenna connection in an RF chamber. For MU-MIMO testing, however, the Wi-Fi WaveBlade and router must be cabled. This is needed so the WaveBlade can precisely control beamforming to ensure test consistency. Because the device was cabled and I have a completely quiet RF environment, I was able to simply connect the VeriWave chassis and router under test as shown in the photo below.

That's a NETGEAR R7500v2 connected and ready to rock. 50 dB attenuators were needed between the router and Wi-Fi WaveBlade because it has a limited input range and would have been overloaded otherwise. Attenuators are usually required with direct-cabled testing because signal levels are abnormally high.

Veriwave chassis and router under test

VeriWave chassis and router under test

I tested all four routers you can buy today with working MU-MIMO, shown in Table 1. All use QCA radios. I would have loved to include the Broadcom-based NETGEAR R8500 and ASUS RT-AC88U and RT-AC5300 in this test. Don't think they dodged a bullet; when they ship working MU-MIMO, they'll be tested!

Router Firmware Class Price
Amped Wireless RTA2600 1.65 AC2600 $253
Linksys EA8500 AC2600 $220
NETGEAR R7500v2 AC2350 $198
TP-LINK Archer C2600 1.1.2 Build 20150924 Rel. 66045 AC2600 $200
Table 1: Routers and firmware

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