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I want to setup QoS on my DD-WRT router. I found this wiki page, but what puzzles me is that it needs to know my WAN bandwidth.

"Set your download and upload speeds. You can use a speed test like Speedtest.net to check your actual connection speed. Some ISPs also provide their own bandwidth testing service, which may be more reliable than the links provided. Enter no higher than 95% of the values you measured into the proper fields."

I don't want to provide my bandwidth because my bandwidth may change depending on my ISP, which is not always fixed.

What I want to do is to give my VoIP phone the highest priority. That means every packet to and from the IP (or its interface) is allowed to jump to the head of the queue.

Why does the router need to know my bandwidth?

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    QoS provides bandwidth, not priority. It needs to know your bandwidth so it can divide it up appropriately. May 31, 2016 at 23:59

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Say there's VoIP traffic and 4Mbps of HTTP traffic. What should the router do? Should it drop or delay some of the HTTP traffic or not? If you have an 8Mbps connection, it should do nothing. If you have a 4Mbps connection, it should start dropping/delaying the HTTP traffic to preserve bandwidth for the VoIP traffic.

Without knowing your bandwidth, your router has no way to know when it needs to give the VoIP priority. If it gave the VoIP priority all the time, all your other services would suck because you give a service priority on inbound traffic by dropping (or delaying) packets for other services.

Remember, your router cannot do anything about an inbound packet until it receives it from your ISP. At that point, it has already consumed your bandwidth.

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    What I have in mind is the router should just process the VoIP traffic first (if any) and then process other traffic. Basically having 2 queues, one high priority and the other normal. I don't mind the other services suffer because they are not high priority and I know VoIP uses very little bandwidth. As far as in-bound packet goes, it does not matter as LAN bandwidth is plenty. Throttling should happen naturally as the out-bound traffic of the same flow is being slowed down. I have no control over how ISP route me traffic anyway.
    – some user
    Jun 1, 2016 at 6:30
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    @someuser The order in which traffic is processed won't affect anything. (Why would you think it would?) Having two queues will help if the outbound bandwidth is congested, but what if the outbound bandwidth is perfectly fine and it's the inbound bandwidth that's congested? How can you ration something if you don't know how much of it you have? Jun 1, 2016 at 17:13
  • The main issue you will have is inbound bulk traffic (due to downloads or watching videos) clobbering the inbound VoIP packets. You handle that by dropping some of the inbound non-VoIP packets, which triggers backoff. You can't do that unless you know when your inbound bandwidth is the limiting factor. Jun 1, 2016 at 17:14

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