Blog | Will 802.11ac stab you in the back(haul)?

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By Don Herana

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Wave-1 of 802.11ac is here. With vendors promising 1.3 Gbps in 5 GHz and 1.75 Gbps aggregate per access point (AP), the industry is suddenly focused on the potential bottleneck of AP backhaul links.

Is a single Gigabit Ethernet uplink enough for each AP? The answer is ?yes,? and applies not only to Wave-1, but also to Wave-2 11ac. Here are some of the reasons why:

Theoretical maximums do not happen in real-world conditions. Even though 11ac Wave-1 promises a combined 1.75 Gbps theoretical rate, it does not happen in reality. The client only experiences close to 50 percent of this data rate historically.

Client mixtures do not support the maximum capabilities. Single-stream client devices like mobile phones and tablets, and two-stream client devices like tablets and laptops, are 11a/g/n devices that do not support 11ac maximums. Your network usually has these clients, which are sometimes beyond 3 meters from the AP, and thus subject to lower data rates. Every lower-capability client will reduce the average airtime efficiency, making gig-stressing conditions impossible.

Wi-Fi is half-duplex. In Wi-Fi, all airtime is shared for both uplink and downlink. When you start with a theoretical maximum channel capacity, you have to divide it between uplink and downlink. Conversely, Ethernet is full duplex with a 1 Gbps uplink and 1 Gbps downlink simultaneously. To really stress that gigabit link, you need to push either all uplink or all downlink traffic from Wi-Fi clients, which won?t happen in reality.

Application requirements will not stress 1 Gbps backhaul links. Few client applications and services can generate consistent or even bursty loads above 700 Mbps. But again, the issue isn?t the potential of a single client device, but the potential of all combined client devices passing traffic and sharing airtime.

High density does not stress 1 Gbps. High-density scenarios are more likely to have single-stream mobile devices that don?t support protocol maximums and experience airtime challenges, bringing aggregate network potential down.

Most of today?s networks can?t deliver it anyway. Most client-based applications now use the cloud. Local LAN applications/servers are more likely to be able to handle 1 Gbps sustained and cases of applications requiring more than 1 Gbps are unlikely.

Cost is always king. Hardly anyone will pay for 10 GbE at the edge for all APs or a higher-grade Cat6/7 cabling, and running multiple copper cables for each AP with link aggregation is cost prohibitive. Show 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 or 4×4:4-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. Most enterprise client devices are unlikely to support 160 MHz-wide Wi-Fi channels.

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That 4th stream won?t change real-world throughput tax. Considering the previous arguments about client mixtures, application demands, backhaul problems, and high density, an additional spatial stream on the AP will have little to no impact on backhaul links.

Few clients 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. MU-MIMO improves airtime efficiency where there are many single-stream client devices and mostly downlink traffic. However, 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, hardly coming close to the gigabit ceiling.

Everyone has neighbors. Maximum data transfer in a clean lab environment may get close to a gigabit ceiling more often in Wave-2, but the problem is that these same high-performance networks must share airtime with neighbors. In future, there will still be a lot of 802.11n networks everywhere, and we will just have to cope with the realities of backward compatibility.

Stop gig stressing. There?s just no way that real-world client mixtures, RF environments, application requirements, and network infrastructures are going to saturate the full capacity of a high-performing full-duplex gigabit link. Don?t be fooled by vendors wanting you to upgrade your wired networks. Just say no.

Last word. ?If you?re planning an 11ac upgrade, there are several deployment considerations. Certainly upgrade switching infrastructure to gigabit speeds, if it isn?t already, and be sure to use Cat5e or better cabling (Cat6 is recommended for new deployments).

Many 11ac APs require support for 802.3at PoE for full functionality, but some can support all features on 802.3af power budgets. Double check your PoE switches or injectors to make sure you will have compatible products without more spend.

Also, IT folks may be quick to upgrade to 11ac because of marketing claims from vendors, but some enterprises may save money without sacrificing performance by using localized deployments of 11ac with continued support for 11n in some areas.

And finally, 11ac does benefit all clients (even those that are not 11ac), but since there are a few features that bring the most value (e.g. better modulation and wider channels), make sure that your AP solution can maximize those features with smarter technologies that improve signal quality and intelligently select channels.”

The author is the country manager for the Philippines at Ruckus Wireless

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