Setting QoS and AQM

Note: The instructions for setting QoS below were relevant for CeroWrt prior to the Modena build (February 2013). We have preserved them here for historical reasons.

Setting QoS: Improve latency problems with a CeroWrt router

Note: These have been superseded by the Setting up AQM page.

CeroWrt has the ability to solve latency using a number of mechanisms. One is our major focus: decreasing bufferbloat. Another technique is AQM (active queue management) and bandwidth shaping: enabling AQM with sane values will make it more possible for you and your family to do more work simultaneously - doing uploads while your son is downloading and your daughter is making phone calls and your spouse is playing games, with fewer complaints from everyone.

AQM set to good values is best. Getting good values automatically is hard. We’re working on it.

Since it’s hard to determine good values automatically, CeroWrt ships with QoS (Quality of service) turned OFF by default. This will make your router and network connection seem slow until you use these steps to manually measure your network’s speed and set good values:

To set up QoS for CeroWrt:

  • Go to the Network->QOS screen on to see the CeroWrt router configuration page.
  • Turn off QoS by un-checking the box labeled Enable and clicking Save and Apply at the lower-right corner. (It may already be unchecked: if so, skip this step.)
  • Run a suitable bandwidth performance test such as http://speedtest.net to get your network’s Download and Upload Speeds. Multiply each by 1000 to get the values in kilobits/second. For example, 2.45Mbps on the Speedtest.net site would be converted to 2450 kilobits per second (kbit/sec). You can ignore the Ping times.

A HUGE problem with most network tests is that they don’t run for long enough. Over a minute is required in order to get a reasonable value, most run for less than 20 seconds, and many ISPs are now gaming that. We use
netperf to various sites supporting it to get a reasonable value.

  • Use these values to fill in the Download and Upload speed fields in the CeroWrt web GUI. It is best to use values a few percentage points lower than what the bandwidth test shows you. For example, if the value is 2450 kbit/sec, you could set it for 2300 kbit/sec.
  • Re-enable QoS by checking the Enable box in the GUI.
  • Click Save and Apply at the lower right.

AQM doesn’t seem to do any good! I can’t do a big upload/download and make a phone call!

Decrease the Upload and Download speeds somewhat in the web GUI until you can do both. You should make adjustments in 256kb increments (or do a binary search between 60% and 100% of your provisioned bandwidth).

How accurate do these speeds need to be?

The values can be approximate, in the range suggested in the previous question. AQM is “bandwidth” shaping, and attempts to limit the filling of queues in the broadband equipment. Unfortunately, this can miss out on the advantages of Comcast’s PowerBoost, and it cannot take into account other dynamic changes in availability of bandwidth caused by your neighbors or even the temperature. We note that SPEEDTEST reports misleading results on broad band service which provides temporary bandwidth boosts such as Comcast’s PowerBoost, and we are working on a better test.

It’s important to set some AQM speed settings. Although disabling AQM can (sometimes) make single stream downloads run nearly as fast as possible, it occurs at the expense of other activities. And it may not always work.

To edit this page, submit a pull request to the Github repository.
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Jim Gettys' Blog - The chairman of the Fjord
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