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Guest View: Will 802.11ac stab you in the back(haul)?

Hans Pang, Director of Systems Engineering, Ruckus Wireless | Feb. 13, 2014
A deeper look into wireless network traffic characteristics show 802.11ac will work just fine for your network’s backbone

Most of today's networks can't deliver it anyway.

How many networks out there can provide more than 1 Gbps WAN links-and web-based services/applications that can deliver that kind of speed? Most client-based applications now use the cloud.

Local LAN applications/servers are more likely to be able to handle 1 Gbps sustained. Cases of applications requiring more than 1 Gbps in a specific direction and operate in a silo are close to zero.

Cost is always king.

Getting business-minded for a minute, it's hard to believe that anyone will want to pay for 10 GbE at the edge for all Aps. No one wants to pay for higher-grade Cat6/7 cabling either. And of course, running multiple copper cables for each AP with link aggregation is cost prohibitive. Just show the budgeters the real-world likelihood of saturating a single, lower-cost 1 Gbps link and the budget czar will trump that decision fast.

What about 802.11ac Wave-2?

All signs point to Wave-2 11ac APs being either 3-stream (still) or-more likely-4x4:4-stream (at 1733 Mbps on 5 GHz). These boxes will also support 160 MHz channels with higher data rates. So the reasoning for the sufficiency of gigabit backhaul for Wave-2 goes something like this:

160 MHz channels are really best suited for SOHO environments. Accommodating them in enterprise products is not practical. Even if you wanted to, most enterprise client devices are unlikely to support 160 MHz-wide Wi-Fi channels.

That 4th stream won't change real-world throughput tax.

Taking all the previous arguments regarding client mixtures, application demands, backhaul problems, and high density into consideration, an additional spatial stream on the AP will have little to no impact on backhaul links. Few clients, if any, will support four spatial streams in the first place. Aggregate throughput for each AP will still be constrained by the low and mid-performing clients. Even high-performing clients will struggle to generate nearly 1 Gbps of unidirectional TCP traffic. 

Multi-User MIMO does not increase maximum backhaul load either.

Now you might be thinking that MU-MIMO, or the ability for an AP to concurrently communicate with multiple clients, has a chance to change all this. The answer is no.  

There's no doubt that MU-MIMO should improve airtime efficiency where there are many single-stream client devices and mostly downlink traffic. But, the AP still only has four spatial streams, and MU-MIMO will not be used for every transmission. In many cases, MU-MIMO transmissions will still go to only two single-stream clients simultaneously. This will not come close to the gigabit ceiling.  

Everyone has neighbors.

Wi-Fi performance is almost always dependent on RF conditions. While it's true that maximum data transfer in a clean lab environment may get up close and personal to a gigabit ceiling more often in Wave-2, the problem is that these same high-performance networks must share airtime with neighbors.  Looking forward, it's inevitable that there will still be a lot of 802.11n networks everywhere, and we will just have to cope with the realities of backward compatibility.


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