Understanding Speedtest
Alternative title: Why you can’t actually use your cell phone’s data plan.
There’s one great misunderstanding that most people have about speedtest: that your download speed isn’t controlled by your upload speed. 
Consider the speedtest shown in the photo. Looks great, right? 21 megabits of download speed is good enough to do just about anything, right? Um, no. You aren’t going to be able to do anything…probably not even email at these speeds. There is no YouTube in your future with this lousy, crappy service.
While the download speed is 21 megabits, the upload speed is basically zero. It’s 0.38 megabits, and worse, it has a loss rate of 99%.
When a packet of data is downloaded to you from a server, your system must acknowledge receipt and then request the next packet of data. These ACKs and handshaking are sent via the upload channel back to the server. Without good upload speeds you can’t access the download speeds.
A very broad rule of thumb is that your download speed will be limited to 5x the upload speed when the two are unequal. So in this case you are limited to 2 megabits. But because the loss rate is so high you will be bludgeoned by “Retries” which will be delayed by 62 milliseconds (32ms x 2). With this type of latency combined with low upload bandwidth you will be lucky to get 25 packets per second through your pipe, so basically you don’t have squat.
I am increasingly seeing this “game” being conducted by cellular companies. I am not sure what the motivation is but I can think of two things:
- It is a secret, almost unnoticed way of throttling customers because customers largely don’t look at upload speeds. By cutting the upload speeds, cellular companies can choke off demand for download speeds, keeping their numbers “looking good” while not actually providing the service they promised.
- Cellular companies seem to be doing this more and more on LTE. Since everybody is watching 5G speeds it seems that cellular companies are cannibalizing their LTE and letting it whither on the vine. They seem to be stealing bandwidth from the upload channels since this is the least noticed.