Tuesday, 15 January 2013

Steady data transmission over WIFI - Part 4 - Other weirdnesses

So far, 1024 bytes every 20 ms represents a meagre 51KB/s. It's more than enough for transmitting RC commands, but with WIFI 802.11 g it should be possible to peak at about 1MB/s in perfect conditions (which might be incompatible with my need for low latency/steady transmission but anyway...).

Just out of curisosity, I naively tried to increase the amount of data sent per second (x4). Unfortunately it didn't go too well. Here is an example of catastrophic result :)

Weird increase of latency on the reception side
And after few seconds the latency reduces
It's not always broken! Here's a pretty acceptable transmission
Not sure what happens here, but the uglyness seems to happen only in one direction: from the Windows-7 netbook to the Raspberry Pi. Things are relatively normal in the other direction.

Amongst the things that might be worth looking into:
- The MTU
- The way I create the 2-machines WIFI network (I've been using Connectify so far). Maybe I could try an adhoc network
- Nagle's algorithm. It's supposed to be TCP only, but this weird increase/decrease latency phenomenon looks a bit like an algorithm is trying to do something clever.
- Network settings/parameters at the OS level on Windows and Linux
- Or to simplify things a lot: using identical hardware and OS on both sides

I'll investigate that later and try to cover it in another article. If anyone's got advice on the matter, please let me know!

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