Given that many of us have been working from home now for some months since the arrival of COVID, I thought it might be timely to take a look at the performance of asymmetric network links which are typical of many domestic Internet connections (at least this is certainly the case in Australia, and from what I can make out also common in many other parts of the world).

So what exactly is an ‘asymmetric’ network connection? Well, put simply it is where the upload capacity is different to the download capacity. For example, download speeds may be 50Mbps whereas upload is often considerably less at say, 10Mbps. This was very common in with older style aDSL connections, the ‘a’ even standing for ‘asymmetric’.

In most domestic environments asymmetric connections work fine. Most households download far more than they upload, typically watching streaming video such as NetFlix which is pretty much a one way download stream. In fact talking to a local carrier in pre-COVID times I was told that up to as much as 70% of their evening traffic was made up of NetFlix streams. So in this environment it makes perfect sense to tune the link to favour download rather than upload.

All good so far, until we all started working from home (WFH) on our asymmetric network connections. Rather than downloading streaming video, the WFH application of choice is more business related … enter video conferencing via applications such as Zoom.

Zoom quote on their website that for group video calling at 720p HD you need 1.5Mbps/1.5Mbps up/down (you have probably seen the Zoom ‘Gallery’ view). That is, we need as much upload as we do download.  Makes sense right? I mean we are transmitting video as well as receiving.

But we should be cool, our download speed is 10Mbps, well above the specified Zoom requirements, and yes, in most situations, we should be just fine. But even so, I do occasionally have issues even when the stated bandwidth is clearly enough and even verified by running various speed tests that clearly show upload capacity well above 1.5Mbps.

So why do I still experience occasional issues?

Turns out that asymmetric links exhibit quite unusual behaviour when it comes to latency and jitter. Sure the throughput might be high but the variance of inter packet arrival time can vary greatly, especially when compared to the download traffic. Turns out that real time streaming traffic such as video and voice are very sensitive to variations in inter packet arrival time. That is video and voice are very sensitive to jitter.

Let’s drill down on this in more detail, at Byte25 we believe one of the best measures to gauge the performance of applications is what we call the Jitter Index or Inter Packet Arrival Time Variance Coefficient. This is a measure of relative variability calculated as the ratio of the standard deviation to the mean of inter packet arrival time. Put simply, the higher the coefficient of variation, the greater the level of variation of inter packet arrival time and hence the greater the impact on performance on apps like video and voice.

Don’t worry if this seems confusing, let’s look at some picture of real life traffic that will hopefully make this clear. Below is a Jitter Index graph from a network with a symmetrical Internet connection.

There are two things of note here, firstly the upload and download (from/to) Jitter Index values are consistently alike throughout the monitored period and secondly the ‘variance’ of the values are also reasonably consistent, that is, they all come in at around an index value of 1.0 (note what we are looking for here is not an absolute value, but rather the ‘shape’ of the values over time). Because there is little variation between the Jitter Index values, real time streaming applications like voice and video perform well.

When we contrast this with the same dashboard for an asymmetric link we see quite a different picture.

The shape of the Jitter Index is quite different. The Jitter Index From values are consistently higher than the Jitter Index To, that is the upload Jitter Index is higher than the download Jitter Index. Worse still, the Jitter Index from is not consistent but range from around 1.0 through to -4.0, that is, there is significant variance. This is not great for real time streaming protocols and it is likely that we could experience performance issues for apps such as Zoom.

In an ordinary troubleshooting scenario using a tool like Byte25, we would drill one and examine the Jitter Index per application, in this case Zoom, to quick diagnose potential issues relating to the connection but even with this high level example we can see significant differences between symmetric and asymmetric Internet connections.

Traditionally bandwidth and throughput have been the default go to metrics for network performance, but to really analyse and diagnose application performance it is often necessary to dig a little deeper in to latency and jitter measurements. Especially as we increasingly work from home and are reliant on video streaming apps such as Zoom, it is necessary to measure beyond the traditional metrics to ensure performant connections.

And just in case  you are interested, we live and breathe this stuff at Byte25 so feel free to reach out if you want to discuss or need more information.