Assymteric ping across point-to-point link

[drobbins]

I have a point-to-point 802.11b link across a distance of about 2 km. It works pretty well, giving about 3 mbps throughput.

I'm puzzled however by asymmetric ping result. In one direction ping times are great. In the other direction ping times are slower and more erratic. See the results below.

It could be a misaligned antenna or a bad cable, but -- here's my question: on which end would you first focus troubleshooting? Point A or Point B?

Pinging point A from point B:

PING 172.21.3.22 (172.21.3.22) 56(84) bytes of data.
64 bytes from 172.21.3.22: icmp_seq=0 ttl=64 time=4.31 ms
64 bytes from 172.21.3.22: icmp_seq=1 ttl=64 time=4.19 ms
64 bytes from 172.21.3.22: icmp_seq=2 ttl=64 time=4.42 ms
64 bytes from 172.21.3.22: icmp_seq=3 ttl=64 time=4.45 ms
64 bytes from 172.21.3.22: icmp_seq=4 ttl=64 time=4.16 ms
64 bytes from 172.21.3.22: icmp_seq=5 ttl=64 time=4.50 ms
64 bytes from 172.21.3.22: icmp_seq=6 ttl=64 time=4.36 ms
64 bytes from 172.21.3.22: icmp_seq=7 ttl=64 time=4.01 ms
64 bytes from 172.21.3.22: icmp_seq=8 ttl=64 time=4.40 ms
64 bytes from 172.21.3.22: icmp_seq=9 ttl=64 time=4.03 ms

--- 172.21.3.22 ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9007ms
rtt min/avg/max/mdev = 4.018/4.287/4.504/0.173 ms, pipe 2

Pinging point B from point A:

PING 172.21.3.21 (172.21.3.21) 56(84) bytes of data.
64 bytes from 172.21.3.21: icmp_seq=0 ttl=64 time=22.6 ms
64 bytes from 172.21.3.21: icmp_seq=1 ttl=64 time=64.3 ms
64 bytes from 172.21.3.21: icmp_seq=2 ttl=64 time=190 ms
64 bytes from 172.21.3.21: icmp_seq=3 ttl=64 time=40.9 ms
64 bytes from 172.21.3.21: icmp_seq=4 ttl=64 time=84.2 ms
64 bytes from 172.21.3.21: icmp_seq=5 ttl=64 time=70.3 ms
64 bytes from 172.21.3.21: icmp_seq=6 ttl=64 time=190 ms
64 bytes from 172.21.3.21: icmp_seq=7 ttl=64 time=60.4 ms
64 bytes from 172.21.3.21: icmp_seq=8 ttl=64 time=190 ms
64 bytes from 172.21.3.21: icmp_seq=9 ttl=64 time=30.9 ms

--- 172.21.3.21 ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9008ms
rtt min/avg/max/mdev = 22.605/94.571/190.668/65.239 ms, pipe 2


I can supply more detail on the setup if it helps. Both endpoint use identical wireless routers & antennas.

[Alan87i]

I'm still cleaning the fog out of my head , but if possible I'd use a laptop and a program like net stumbler to have a look at the noise on the channel your using for the link at both ends.
The end with the most noise could be having problems receiving.

[drobbins]

I've had a quick look at Netstumbler, but can't see how I might monitor the point-to-point link, only a laptop-to-access-point link.

The management software for the wireless routers doesn't show a significantly larger amount of noise on one end compared to the other (screenshot attached).

[Alan87i]

That's from the AP's in question?
100% good packets.
I'd say it's working good regardless of ping count.
Usually in my experience one end will have TX retries and the good end won't. Or not as many.
The end with more transmit retries has the problem.
Last time it was a bad antenna that had a crack and must have got moisture inside it.

[drobbins]

Guess I'll just let it be. Oddly, today we have high winds (up to 80 km/h) and the troublesome ping time is down to a consistent 30-40 ms :)

[Alan87i]

I had one link that acted up as I mentioned. High retries , sometimes it would be a nasty hard rain and the link seemed better than a sunny day.
Other days during the winter months it acted up more when the temperature was changing. Like in the evening when it dropped or when the sun came out and warmed things up.
It ended up being the sealed element on a 24 dBi grid antenna.
After I swapped it out I noticed the crack in the plastic. It was dry inside but the problem has disappeared since changing it.
The TX retries were on that antenna. RX was always good. SO I would look at the end that shows problems with TX , point B in your case.