uMPX "Limit Rate Below" Setting

Updated by Mark Manolio


This document provides an explanation of the "Limit Rate Below" setting in the MPX Node Encoder.


Q: What is the "Limit Rate Below" setting in the uMPX Setup screen of the MPX Node Encoder?


A: "Limit Rate Below" controls how fast packets can be sent through the network. 

You should normally just leave this at the default setting. All recovery packets in a block are generated at once. If the number of recovery packets is high, this can lead to many packets being sent to the network at high speed. This could in itself cause packet loss. So, without rate limiting, adding a lot of recovery packets would actually make the connection less reliable. Because of this, it's usually best to leave this setting slightly higher than the bitrate multiplied by the error correction factor, and always lower than the maximum network capacity available.

"the error correction factor" here is the number of extra packets that are sent per packet. So if you are sending 4 recovery packets per 64 normal packets, it's 1 + 4/64 = 1.0625, so if the bitrate is 320 kbit that would lead to a theoretical bit rate of 340 kbit. "Limit rate below" must be set higher than that.

Note that in certain cases, recovery packets can be a bit bigger, which can lead to a slightly higher bitrate than this, which is why the slider is by default set about 100 kbit higher. If your connection can handle the resulting bitrate (440 in this case), that's fine. If you want to send it over satellite, for example, set it to the maximum that you can actually send. When needed, recovery packets will be thrown away if they got too big in that case.

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