Re: [B.A.T.M.A.N.] OGM overhead and routing metric proposal

Daniele Furlan
2011-02-28T16:00:23Z

2011/2/28 Andrew Lunn <andrew@lunn.ch>:
>> 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.
>
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.
It would be wrong to protect OGM from collision or hidden node.

> 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 :)
>
> 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