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AndreKR
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Joined: Sat May 14, 2016 11:58 pm

Prioritize VoIP traffic, which speed to enter

Sun Sep 12, 2021 4:41 am

I have a network with a few PCs and a few VoIP phones. They share one internet connection. The download speed of the internet connection can be 20 Mbps on good days or 5 Mbps on bad days. Upload is between 300 kbps and 800 kbps.

I'd like to prioritize the VoIP phones.

All the tutorials I found assume that there is a fixed, constant speed that I'm supposed to enter in the queue configuration. What exactly do I enter there? If I enter the maximum, will the prioritization still work when it's slower? If I enter the minimum, will it still use the full speed when it is available?
 
sindy
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Joined: Mon Dec 04, 2017 9:19 pm

Re: Prioritize VoIP traffic, which speed to enter  [SOLVED]

Sun Sep 12, 2021 10:52 am

Priority means priority. A highest priority packet will always overtake those waiting in any other queue, provided it fits into the limit of its own queue. So if you mark the VoIP packets to the highest priority queue and set unreasonably high limit-at and max-limit values for that queue, and set very low limit-at values and unreasonably high max-limit to all the other queues, the VoIP packets will always be sent first regardless the currently available throughput.

However, this only works this neat and simple in the upload direction, where it's your router itself what chooses the next packet to be sent.

In the download direction, if the ISP sends a packet down your uplink, it will occupy the bandwidth on it, no matter what you do with that packet once it arrives to your router. So the download rate can only be controlled indirectly, if the sender in the internet adjusts the sending rate to some feedback provided by the recipient connected to your router; for this type of connections, it has an effect to prioritize the delivery of the traffic to your LAN clients. This is possible not only for TCP but also for some application protocols running atop UDP, the problem is that the latter ones are hard to be identified reliably. So the only working way in download direction would to set the max-limit of the parent queue to even less than the minimum download bandwidth, to provide a margin for the traffic that cannot be throttled the indirect way.

Even worse, the above may still not be enough, because if the download is so flaky, it is well possible that your VoIP packets don't fit already into your ISP's download, or into the total bandwidth of the access network serving you, during the peak times. So the only effect of throttling your own download traffic will be that other customers in the same segment of the access network (like a wireless sector) will get more bandwidth. My previous ISP had exactly this problem. So it might be worth it to talk to the ISP about prioritization of the VoIP traffic from the VoIP server(s) your phones use. If it is an issue with your ISP's own uplink, you're basically out of luck.

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