Monday, 22 June 2015

Wifi: not as easy as you might think

Why is managing wifi on a large scale so tricky? Matthew Seymour explains.



Netflix would need more than 23 days to buffer
an episode of Game of Thrones (1)
For those who remember a time before wifi, when internet access was via a slow modem and viewing videos an impossible dream, it often seems amazing we can move so much data, so quickly, without wires.

Wifi is now everywhere, almost... It's such an ubiquitous technology that we just expect it to be there, we rely on it and make the mistake of thinking it's an easy thing to implement.

The impression of simplicity is understandable because on a small scale, wifi is pretty easy. Scaling it up to a large building and campus is a different story.

There’s always a place to escape from wifi (2)
To understand why wifi gets tricky for big networks it's helpful to know a little bit about how it works and one crucial way in which it's very different to a modern wired network. So here's a tiny bit of history.

Most computer networks are built with Ethernet. It was designed in the early 70s and has seen a lot of development over the years. In its early days all computers on an Ethernet segment shared the wire with everything transmitted on the network visible to all machines.

Because it was a shared wire, only one computer could talk at once and part of what made Ethernet so successful was the simplicity of the system for handling this. If two computers talked at the same time, both detected the resulting collision on the wire almost immediately. They'd both stop, then wait a random amount of time before trying again and hoping for the best. This is really effective, up to a point.

Network switches - loved by network technicians and kittens alike (3)
As you might imagine, as you increase the amount of data traffic you can reach a point where collisions become so frequent that all the machines spend most of their time waiting to try again. A previously fast Ethernet network could be brought to its knees by excessive collisions caused by too many computers, too much traffic or even a faulty computer belching out rubbish onto the wire.

Fast forward a few years and we no longer use a shared wire, each computer connects to a network switch capable of two-way communication and collisions are (or at least should be) a thing of the past.

But what has this got to do with wifi?


If I tell you that wifi uses a shared medium (the air) and that only one station can talk at a time… does that sound familiar? Because we haven't yet found a way to provide a separate universe for each wifi user to occupy, this time there's no option but to share the air.

How different wifi devices use the network affects this shared air time. Older devices that connect at a slower speed therefore take up more air time. They talk more slowly, and while they’re doing so nobody else can get a word in. But even the latest, super fast Macbook with AC wireless will drop to a slow connection speed if the signal level isn't so good, or there's a lot of interference. What people do is even more significant: a single access point might support 150 clients happily if they're just doing email and a bit of web browsing; bursty traffic works well on a shared medium. The same setup might only support two clients streaming Netflix, which requires a reliable high speed connection.

Wifi design for Ainsty Court, Halifax College
Looking at the building in 3D shows that we don't have
enough channels to avoid overlap
Just as you begin to see how complex the picture is, there's a whole other challenge presented by the shared medium: wifi works in three dimensions.

Imagine our wifi network is a group of people sitting around a table. We know only one can speak at a time without words colliding into garbled nonsense that nobody understands, but it works because everyone's polite.

Now another meeting has started up next door. Because both groups can hear each other the number of people who can cause a collision by talking at the same time is increased.

Then another meeting kicks off in a room above. Because all three groups can hear each other, collisions become even more likely but worse still some of the people upstairs can't hear some of the people downstairs and vice-versa so collisions become almost guaranteed.

Wifi segments the network by using different channels so our three meetings could all take place on separate, non-interfering channels. But it's rare to have enough channels to go round, so for wifi to cover a whole building there's going to be some channel re-use going on across different access points between floors and they'll be in range of each other. This is known as channel contention.

Not only is wifi design challenging, it's also an iterative process because nothing stays the same. Not that long ago most people had one wifi device (laptop) now it's common to have a smartphone, laptop, tablet, console… and maybe throw a desktop into the mix too.

The way people use the network has changed dramatically too. More of us stream audio and video to our devices, placing a much greater demand on that shared air time.

As a result we're seeing some areas where a network design that worked really well for a few years starts to struggle as more is demanded from it. Because of the channel contention issue, just installing more wireless access points doesn't work, and often makes the problem worse.

IT Services are working hard and investing a lot of money in wifi but essentially all this boils down to: getting wifi right is hard and even when you get it right, it won't be right for long.


Image credits:


  1. By Ralf K├╝hne - used under a Creative Commons license
  2. By Dushan Hanuska - used under a Creative Commons license
  3. By Michael Himbeault - used under a Creative Commons license


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