1 Mar
2011
1 Mar
'11
8:51 a.m.
On Mon, Feb 28, 2011 at 05:00:23PM +0100, Daniele Furlan wrote:
Hmm, I guess that depends
on the interference pattern a lot.
Afaik minstrel for instance is one of the rate selection
algorithms which can chose higher enconding rates to reduce
interference in case of bursty loss. So when your rate selection
algorithm selects 54MBit/s, that does not necessarily mean that
you have a higher net throughput / that the link is "better"
compared to another link where the rate selection algorithm has chosen
for instance 36MBit/s. So to make the selected rate information
somehow useful, you'd have to take the loss rates at that
enconding rate into account too.
2011/2/28 Andrew Lunn <andrew(a)lunn.ch>ch>:
In my opinion the use of broadcast is somehow useful. If many OGM
packet collide it means that the link is congested and batman will use
another.
And what if the other link is not really useful for e.g. TCP?
batman would switch to that link then anyway. At least that's what
I had been experiencing in some indoor scenarios here. The hidden
node can lead to degrading the measured LQ values from ~90% down to
< 10% packet delivery ratio.
I'm
not a kernel guru but i find out that the new core mac80211 and
cfg80211 (http://linuxwireless.org/) offers the possibility to obtain
a per-station bit-rate information that should be driver independent.
O.K. I will take a look at this.
I have not mentioned the hidden node problem. But
I think this problem
is very difficult to remove, the only best practice to reduce the
problem is to force the RTS/CTS mechanism to be active.
RTS/CTS helps, true. But it has a few problems. e.g. most meshing
protocols use broadcast packets for there management. e.g. BATMANs
OGMs are broadcast. These cannot be protected with RTS/CTS. So the
OGMs can collide with RTS packets from a hidden node, or OGMs from a
hidden node.
It would be wrong to protect OGM from collision or
hidden node.
So I'd not quite agree with that.
But I'm actually also a little confused why you, Andrew, came up
with the hidden node problem :) (which these papers did not try to
solve, if I'm not missing something). Or do you mean, that this
idea could be extended maybe be able to solve that hidden node
problem for OGMs, using the bitrate measuremens to detect / avoid
switching to bad links?
Another way to help reduce the hidden node
problem, or interference
with other nodes, is to use a low coding rate and transmit power when
possible. So for example when we don't have much traffic to send to
node X, it could send the packets with the lowest coding rate and low
power. This keeps the interference with other nodes to a minimum. As
the amount of data for X increases, the coding rate and transmit power
can be increased, so increasing the available bandwidth to X, but at
the same time increasing the amount of interference the traffic
produces.
Yes, I think that it is always better to use low transmission power
and favor multi-hop so short link, especially in dense network. The
schema I propose effectively favor short link offering the possibility
of reducing transmission power.
Anyhow a transmission power management system is out of the scope of
my work, but it is a very interesting area. It is certainly possible
to add transmission power level to OGM in order to use also this
information in routing decision. More information we now of link and
more accurate will be the routing decisions.
Such a scheme to minimize interference does not
play too well with
your idea of putting the coding rate into the OGMs. We would have to
ensure that the coding rate is the highest possible coding rate usable
between two peers, not the currently used coding rate, which could be
low in order to avoid interference.
The current rate is decided by the driver basing on link quality and
collision so indirectly tells us how much interference there are in
the link. Why it is a bad idea to use this information?
>> > Will you be at the WBMv4 next Month?
>> >
>> I'm afraid but I cannot be at WMBv4, I hope I can come to the next
>> WBM, maybe with a working implementation :)
Hmm, what a pitty. But, ok,
see you next time then :).
>
> Shame, i plan to talk more about the hidden node problem during WBMv4.
> It is also a good place to discuss new ideas like adding information
> into the OGMs.
>
> Andrew
>
Nice papers there, thanks for sharing! Just looked at them briefly
so far but seems very interesting.
Cheers, Linus
--
Daniele Furlan
PS: Just one thing I noticed so far, isn't it possible to simplify
the formula B = 1 + \sum^{N\\{0\}}_{k \in N} p^{l_{o,k}} to just:
B = \sum_{k \in N} p^{l_{o,k}}? p^{l_{o,k}} is already 1 for k = o
isn't it?