|Netflix would need more than 23 days to buffer |
an episode of Game of Thrones (1)
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)|
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)|
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
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.
- By Ralf Kühne - used under a Creative Commons license
- By Dushan Hanuska - used under a Creative Commons license
- By Michael Himbeault - used under a Creative Commons license