Now that the world is starting to talk seriously about 802.11ac, I’m answering many more questions about it. One of the most common questions I get when I speak about 802.11ac (as I did at our Boston user group, and will be at our upcoming New York user group) is why it makes sense to upgrade to 802.11ac when the client population is still largely 802.11n.
The answer lies in the basis of packet-switched networking, and how networks behave in response to congestion. Congestion is unfortunately a fact of life in networking, and it’s something that network technology designers have to deal with. Packet-switched networks cope with congestion well, up to a point, and then they suffer from a sudden collapse.
One of my favorite analogies for talking about the behavior of network technologies and congestion is to compare your computer network to the transportation network. Let’s say that you set out to drive somewhere early in the morning, say around 2 am. The road will be empty, so that as you merge on to the freeway you’ll be able to go at the speed limit. There’s no congestion, so you can fly on to your destination. An unloaded network behaves the same. Packets line up for transmission, and head out to their destination immediately.
Add a few cars to the road, and not much happens. I try to drive to work at Aerohive before traffic starts, so I get on the road and there’s a reasonable load on the road. Provided I hit the road early enough, though, there’s enough space, and the throughput of the road (the number of cars) is high, but traffic is not yet high enough to degrade my latency (otherwise known as the time I spend in traffic).
Eventually, though, too many cars get on the road, and – boom! – traffic suddenly starts to crawl. What happened is that there’s just too much demand for the road, and adding more cars still increases throughput, but the cost of adding a car is slowing everybody else on the road down to a near-stop. What does a congested network look like? Think mid-day traffic in any major world city. If you’ve been in a cab in New York, London, Tokyo, or Paris, you know what I mean.
So, if the network is so congested that latency is high, what can be done? Remove the congestion. (Put more formally, the average latency of a packet on the network is inversely related to available airtime, so free airtime is a general good for the network.) One of the reasons that major cities have focused on increasing trips taken by bicycle is that bicycles take up much less road capacity. For the “price” of a car’s worth of roadway, you can have a half dozen bicycles.
So, what’s the “bicycle fix” for Wi-Fi? We have a fixed and costly resource – the radio medium – and need to wring more out of it. By reducing the transmission time for frames, 802.11ac enables network administrators to push back against congestion collapse. The lesson in here is that if you have a network running close to the traffic volume at which congestion takes hold, you should move to 802.11ac. As you add 802.11ac clients, they will reduce the amount of airtime required to transmit data, and increasing free airtime will improve the quality of user experience.
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