Enter the maze

Enough is enough!

by Klajd Karaj, Undergraduate at Queen Mary University of London

Grid of city roads: copyright istock.com 1766805

You are watching a Zombie movie online, when things go wrong. No not the Zombies winning... Big squares appear on the screen, or maybe the screen freezes completely. Perhaps you need to wait for the video to start, or its quality is very low. Perhaps the video stops in the middle and the dreaded loading sign appears. You are in for a wait! You check the network connection but everything seems to be ok. You want to complain, but who do you shout at? Whose fault is it?

It turns out we are all to blame. The ever-increasing number of devices that we are connecting to the Internet plays an important part. The number of tablets, phones and other devices that will be connected to the Internet by 2015 is estimated to be more than twice the global population. We also stream ever more videos on Netflix, Xbox Live and YouTube. Everything we do over the Internet whether emailing, tweeting or streaming a video involves sending packets of data. Streaming a video involves sending a massive number of those packets as a video is made of lots of data. Each packet joining the network is like another car joining a motorway. Streaming several videos at once is like a whole town trying to leave at once. We are all generating vastly more traffic on our network and all are competing for the limited space. Achieving a high level of quality of experience, avoiding all those problems that frustrate us, is challenging to say the least.

The overall performance of a computer network is traditionally measured in terms of the quality of service. It is about measuring things about the network itself like bandwidth (the amount of data that can be transmitted in a given time), transmission delay (the amount of time it takes to get data into the network to start with), packet loss (how much data is lost along the way) and so on. Quality of Experience is a wider view of the problem. It takes into account the acceptability of the service to the people using it. If we send a birthday present through the post and the package arrives, it may have met its quality of service guarantee However, if the vase inside arrives shattered or the day after the birthday, then we have a Quality of Experience problem. Less than a quarter of video streams in the UK are provided with really good Quality of Experience. Over the next few years this could cause losses of over #1bn for UK online video distributors. It only takes a delay of two to three seconds in loading a video for users to give up. It's a problem that needs to be solved.

There are lots of ways to try and improve the Quality of Experience. You could improve the quality of service - increase the bandwith of the network (pay more for better broadband!). If that isn't practical, another way is to give priority to some services - if video streaming is most important because problems in a video are immediately noticed and lead to a really bad experience, then video traffic gets to go first. A more complicated way is called adaptive bitrate streaming. The idea here is that several versions of the video are created with varying quality. The different versions need varying network capacity to get through satisfactorily. The system then identifies second by second how much data the network and the user's computer can deal with and streams the next segment of the version that fits best. That means sending just the amount of data that the network and end computer can cope with well. The picture is a little worse but in a controlled way that is barely noticable. The quality of service is just as bad but the quality of experience has improved.

So why is it you have all those problems watching that Zombie movie? Well as we have seen it is just like what is going on in the movie itself when everyone tries to evacuate the town at once when the Zombies arrive: too much traffic. The highway gets clogged up and everyone has a bad experience. To avoid it those in charge have to plan ahead just in case their town does one day become infested by Zombies. They can do that by building bigger roads, or they can police the traffic, letting full coaches through first for example. Adaptive bitrate streaming is a bit like having a supply of coaches and motorbikes ready. When the roads are moving freely you let coaches leave, but if it gridlocks you send people on motorbikes instead till things free up again.

It takes the right sort of network planning, if you want to have a good quality of experience watching the movie in peace, and not ending up a Zombie.