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Another Approach

Broadcom, which has not yet announced a 4x4 MU-MIMO capable chipset, has taken a different approach to improving bandwidth use with its XStream architecture. I've been skeptical of XStream since first hearing about it because of the way Broadcom has been marketing it. Individual link rates and processor cores have been combined into larger numbers to impress and maybe mislead gullible Wi-Fi buyers who have been trained to care mostly about big numbers.

After spending time with Broadcom, however, I better understand what XStream has the potential to provide in real benefits to any Wi-Fi clients, not just those implementing MU-MIMO. The block diagram below shows the 2.96 GHz "Penta Core" CPU is composed of the two 1 GHz ARM Cortex A9 cores in the BCM4709 CPU (same as used in NETGEAR's R7000 Nighthawk) and the 320 MHz ARM Cortex R4 cores in the three BCM43602 3x3 MIMO 802.11ac SoCs.

Broadcom XStream Block Diagram

Broadcom XStream Block Diagram
Image credit: Broadcom


The BCM43602's take the BCM4360 3x3 802.11ac MAC/PHY present in the majority of AC1750 and AC1900 routers today and add a local processor and 960 KB of SRAM to make a complete standalone radio. At boot, the CPU downloads code to the radios so that all Wi-Fi functions are offloaded from the CPU. All the BCM4709 has to do forward Ethernet packets to the 43602 where the 802.3 to 802.11 conversion happens, freeing up the 4709 CPU for faster routing and USB storage functions.

XStream's advertised 3200 Mbps link rate represents the addition of two 1300 Mbps maximum link rates for the 5 GHz radios and one 600 Mbps link rate for 2.4 GHz. So 1300 Mbps is the highest link rate you will see from any device connected to an AC3200 router. XStream is still fundamentally a 3x3 architecture.

But the dual 5 GHz radio architecture is more than just two radios stuffed into one box. XStream's "secret sauce" starts with one radio dedicated to the UNII-1 Low band (Channels 36 - 48) and the other to UNII-3 Upper (Channels 149 - 165), with one SSID broadcast. Band-limit filtering and careful RF and antenna design are added to prevent each radio from overloading the other. The diagram above doesn't show it, but the six antennas we've seen on the NETGEAR R8000 and ASUS RT-AC3200 are three dual-bands for the 2.4 GHz and 5 GHz low-band radio and three single-band antennas for the 5 GHz high-band radio.

The last element is software to assign 5 GHz clients to a radio based on the client's speed. The assignment algorithm takes not only client class into account, but actual link rate, which depends on signal strength and other factors. So XStream is smart enough, for example, to take a 2x2 AC867 class client with a low signal level and move it to the same radio as a 1x1 N150 device sitting close to the router. The Broadcom graphic below is goofy but illustrates the point simply.

Broadcom XStream Dual Highway

Broadcom XStream Dual Highway
Image credit: Broadcom

Clients can be moved from radio to radio dynamically as link rates change. But router manufacturers will be able tune this feature as needed since changing radios requires devices to reassociate, i.e. break connection, which tends to make users unhappy.

The upshot of all this is that bandwidth use will be increased because fast clients won't have to wait for slow clients. And since MU-MIMO (or even 802.11ac) support isn't required, XStream can help improve Wi-Fi performance of any mix of 5 GHz clients.

2.4 / 5 GHz band steering is also part of XStream. But since its benefits are not as straightforward to calculate and require radios to drop and re-establish connection, implementation will be up to the end-product manufacturer. Finally, if you think you can do a better job of client assignment, product vendors may also elect to provide separate SSIDs for each 5 GHz radio and let you have at it.

The Choice

There is one more factor that will either muddy or clarify the upcoming choice between AC2350 and AC3200 routers—the (un)availability of MU-MIMO clients. MU-MIMO requires support on both router and client and requires new chipsets to support it. It's not simply a matter of releasing new firmware and drivers.

I have been told that "802.11ac Wave 2" MU-MIMO capable chipsets have been quietly shipped in some number of 802.11ac equipped smartphones in the wild, but that none currently have the feature enabled. I have not heard of any consumer router manufacturers planning to release MU-MIMO capable clients when they announce AC2350 routers. Perhaps this is why NETGEAR's leaked R7500 Nighthawk X4's slides showed "MU-MIMO Ready".

Further, the first Wi-Fi "plugfest" to check for MU-MIMO capability isn't scheduled until the end of this year and will possibly move into early 2015. And MU-MIMO Wi-Fi Certification isn't scheduled until mid-2015 and could possibly slip, too. So if product manufacturers choose to push ahead and ship MU-MIMO clients and if you choose to buy them, you'll be once again playing guinea pig and paying for the privilege.

So the choice you'll soon be faced with making is to buy the biggest number on the box with AC3200 that uses mostly proven technology focusing on making better use of the same bandwidth you have today with your AC1900 router and that works with any Wi-Fi client.

Or buy an AC2350 router supporting four streams that could provide a 1733 Mbps link rate, if you had a 4x4 client (there will be none except another AC2350 router as a client bridge) and support improved bandwidth utilization via MU-MIMO if you could get multiple MU-MIMO enabled clients to use with it.

Or simply choose door number three and let the dust settle. Don't forget that 900 MHz Wi-Fi is on the way, too.

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