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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

Now that 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), suddenly the industry is focused on the potential bottleneck of AP backhaul links. In other words, 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 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, this will not happen under real world conditions.

1.75 Gbps is a data rate. Real TCP throughput, however, (what the client experiences), has historically been somewhere near 50% of this data rate. With 11n/ac frame aggregation and other enhancements, 65% is becoming more realistic in best-case scenarios (usually for single-client tests only).

Client mixtures do not support the maximum capabilities.

In reality, you'll have some single-stream client devices like mobile phones and tablets, some two-stream client devices, like tablets and many laptops, 11a/g/n devices that don't support 11ac maximums and clients in the service area that aren't 3 meters from the AP-and thus subject to lower data rates.

So if your network has any of these client types (and it does!), then you don't have to worry about gigabit saturation. Every lower-capability client on your network will reduce the average airtime efficiency, making gig-stressing conditions impossible.

Don't Forget: Ethernet is full duplex.

When comparing Wi-Fi speeds to Ethernet speeds, we must remember that Wi-Fi is half-duplex. All airtime is shared for uplink and downlink. So 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. So to really stress that gigabit link, you need to push either ALL uplink or ALL downlink traffic from Wi-Fi clients. In reality, this just won't happen.

Application requirements will not stress 1 Gbps backhaul links.

In combination with the limitations of client capabilities, there are very few client applications and services that can generate even bursty-let alone consistent-load 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.

At first glance, high-density networks seem cause for gig stress, and thus more likely to tax network maximums. However, if anything, high-density scenarios are MORE likely to have single-stream mobile devices that don't support protocol maximums-as well as airtime challenges that increase retries and non-data overhead-thus bringing aggregate network potential down.

 

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