packet loss unacceptable?

Answered Question
Jun 15th, 2010

I have a test setup of a cisco 2106 wlc and a 1231ag access point in lightweight mode.  ive been having issues with dropped connectivity trying a lot of different wpa2 setups.  i've finally peared it down to a pretty plain config of a wpa2 psk non-broadcast ssid with most of the options turned off under advanced in the wlan config.  i have a linux laptop and a windows laptop testing, both located about 10ft from the ap with perfect signal strength.

i can connect fine, and i do a steady ping from the laptops to another (wired ip) for testing.  i'll let it go, and then look at the ping statistics.  i always have a couple of pings lost.  it's usually 0% because its like 3 pings out of 800.  but is this acceptable? i thought wifi was supposed to be collision avoidance.  is there an easy way to debug this besides putting wireshark inbetween the AP and the WLC, then the WLC and the core switch, etc...

I have this problem too.
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Correct Answer by Aaron Leonard about 6 years 7 months ago

I consider 3 pings lost out of 800 to be entirely normal/acceptable behavior in 802.11.  If you look at our design guidelines for VoWLAN (voice being pretty much the "canary in the coal mine"), you will see that we specify no more than 1% packet loss for a voice call, so 0.4% would be considered well under that threshold.

An unlicensed wireless channel is nothing like a Cat5E point to point cable.  It's a half duplex channel with very high BER - many many orders of magnitude worse than 1000BASE-T, typically with all kinds of 802.11 and non-802.11 interferers encroaching.  To be sure, the 802.11 CSMA/CD works hard to avoid collisions and to keep retransmitting to get its packets through at layer 2 - but when radar or a microwave oven starts stepping all over the physical layer, no upper layer magic is going to get the bits through.

If you don't like 0.4% packet loss and want to troubleshoot it ... then you would typically need something like wireless packet capture on your channel, plus spectrum analysis (like Spectrum Expert) to catch any non-802.11 interference.  Maybe you will find that there are 5GHz channels available that are cleaner than the 2.4GHz that you're trying to use now.  (Assuming that you are using 2.4GHz, you don't say.)

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Correct Answer
Aaron Leonard Thu, 06/17/2010 - 10:36

I consider 3 pings lost out of 800 to be entirely normal/acceptable behavior in 802.11.  If you look at our design guidelines for VoWLAN (voice being pretty much the "canary in the coal mine"), you will see that we specify no more than 1% packet loss for a voice call, so 0.4% would be considered well under that threshold.

An unlicensed wireless channel is nothing like a Cat5E point to point cable.  It's a half duplex channel with very high BER - many many orders of magnitude worse than 1000BASE-T, typically with all kinds of 802.11 and non-802.11 interferers encroaching.  To be sure, the 802.11 CSMA/CD works hard to avoid collisions and to keep retransmitting to get its packets through at layer 2 - but when radar or a microwave oven starts stepping all over the physical layer, no upper layer magic is going to get the bits through.

If you don't like 0.4% packet loss and want to troubleshoot it ... then you would typically need something like wireless packet capture on your channel, plus spectrum analysis (like Spectrum Expert) to catch any non-802.11 interference.  Maybe you will find that there are 5GHz channels available that are cleaner than the 2.4GHz that you're trying to use now.  (Assuming that you are using 2.4GHz, you don't say.)

bdial Thu, 06/17/2010 - 10:48

thank you for the answer! i find getting everything working a lot easier than actually understanding how it works,

a common situation for people setting up wifi i think.  things are working good now i just wanted to make sure i was starting with a good setup.  we have a 5508 and some 3500 APs on order so hopefully with the clean air and maybe 5ghz N we'll see even better performance.

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