Why does latency increase when downloading or uploading?

Question

I recently discovered that latency increases when a download or upload is in progress on my DSL line, sometimes quite drastically. Initially I thought there was a fault on the line but discovered the same thing happening on several other connections. On some connections the latency would increase from 10ms to an average of 100ms while downloading and 800ms while uploading, whereas other lines increased by a smaller margin.

Why does consistent traffic seem to increase latency?

NOTE: I realise that an increase from 10ms to 800ms is an indication of there being something wrong with the line but I'm wondering why I noticed similar behavior on other lines, even when the variance wasn't so high.

Solution 1

This increase in latency is likely due to bufferbloat. Wikipedia describes it as:

... a phenomenon in packet-switched networks, in which excess buffering of packets causes high latency and jitter, as well as reducing the overall network throughput.

You can use the ICSI Netalyzr to check if your network is suffering from bufferbloat.

Solution 2

Why does consistent traffic seem to increase latency?

Why does it take longer to drive across town when the traffic is heavy? Same reason. For a packet to cross a connection, it has to wait its turn.

Comments

[Ramhound]

It doesn't or it shouldn't. What is happening is the data your sending is being send on a saturated upstream/downstream which means the data is simply slower. Latency measure how long a packet took to be reach. Smaller packets wouldn't have latency in your example

[mcryan]

Fair enough, but to what extent is increased latency "acceptable"? What is the average? Because even on the better lines latency would increase by 7 times the usual.

[David Schwartz]

@mcryan That depends on your particular application. If the latency increase is unacceptable for the application, then you may need to purchase more bandwidth, throttle high-throughput applications when latency-sensitive applications need to run, and so on.