[MLB-WIRELESS] Speed degradation over multiple hops

[Donovan Baarda]

G'day,

disclaimer: all my comments here come from possibly incorrect theory. I have
no practical experience with this stuff (yet),

From: "Ben Short" <bshort at shortboy.net>
[...]
> The best way to explain the problem is to use an example. Say we have 6
> wireless links, each link has its own
> RF Channel, and SSID, using the same WEP. All these links are

So 7 nodes, six hops, with each node (except the end nodes) having two
links, one in each direction, yeah?

> Point-to-Point, with no Multi-point (which rules out
> halving the bandwidth available, by using the same interface for

Not sure what you mean by "rules out" here... full speed bi-directional
traffic will, in a perfect world mean each direction only gets half the
available bandwidth. In the real world, and I'm not sure how wireless avoids
or handles collisions, you get less than that.

> send/recieving between two nodes). Each of these links
> runs at 11mbps, with a signal of > 82dB and SNR of over 20dB.

I'm wondering if the SNR is only that good when there is no traffic on the
other link. Is it possible the two links are interfering with each other?
What happens when you ping out in both directions from a node in the middle?

> Now, each link from hop to hop you can send or recieve at about 400kb/s,
> which is to be expected. However if I download
> direction from hop 6 to hop 1, I am usually lucky to get 10kb/s. There
> is no other traffic on the network at the time, and
> hidden nodes are not an issue. I am assuming interference is not an
> issue, because each link can do 400kb/s.

but can a node in the middle do 400kb/s sec out both links at once? Or
recieve at 400kb/s on one link while transmitting at 400kb/s on the other?

> tracerouting and pinging gibe favourable results, with no indication of
> why the above happens.

Pings are small packets... sometimes this can mask problems that only occur
with larger packets. Most linux pings you can use "-l" to specify a larger
size. Another interesting one is "-f" to flood ping... sends packets as fast
as it can.

> Can someone please tell me why this speed degradation occurs, and how I
> may be able to rectify it.

Good luck.

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Donovan Baarda                http://minkirri.apana.org.au/~abo/
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