[OLSR-users] WLAN and Broadcasts (e.g. OLSR)

[Roar Bjørgum Rotvik]

Joerg Pommnitz wrote:
> Hello all, I have stumbled accross a problem with olsrd. It seems that HELLO-Messages
> are only visible over a very short distance. Other protocols (ICMP-Echo, ssh, iperf)
> work fine, however. The difference is, that these protocols are unicast while olsr
> HELLO-messages are sent to the broadcast address of the subnet. I have verified that a
> broadcast ping has the same problems, so its not really OLSR-specific.

I don't know if it is the problem you are observing, but WLAN unicast performs on-the-air 
ACK and retransmit when ACK is missing within a timeout value.
Broadcast (and also multicast) over WLAN (802.11) does not use ACK (since it is 
one-to-many transmission and the sender does not know which nodes to expect ACK from).

This means that when broadcast packets collides with other traffic, some or all of the 
receivers may not receive the broadcast packet at all. More nodes means that the chances 
of an on-air-collision is more likely and broadcast will suffer a lot.

For unicast traffic the WLAN will perform retransmit if an collision occurs, so this will 
not be noticed on the application level.

If it is possible to see statistics from the WLAN card you are using, check RX/TX counters 
and retransmit counters and see if RX counter increases when sending broadcast traffic 
that is not received. If sender TX increases, but receiver RX does not increase for 
broadcast (but RX increases for unicast) it could indicate collision on air (if the nodes 
are within hearing range).

> This got me thinking. My understanding is, that 802.11 modifies the over-the-air coding
> of the data packets according to the error rate when talking to the peer. It trades
> bandwidth for reliability when the error rate goes up. However, this is a
> point-to-point concept. In AdHoc-Mode where you talk to all the nodes in a cell, this
> is not applicable. My suspiction is, that the 802.11 card sends the broadcast packets
> in whatever coding is the default for the current mode which might not be optimal for
> nodes that are further away. Has anybody seen something like this? Am I on the right
> track? Is there a solution? Something I could try?

I think 802.11 will try to reduce the rate to try to increase the range when the error 
rate increases, if auto rate is enabled. But it is possible to force the WLAN card to 
stick to one rate if you want to (most card I have used supports this).

As far as I know 802.11b will send broadcast using something called Basic Rate Set (or 
similar), this means a set of low rate to increase the possibility of success and longest 
range. For 802.11b I thought broadcast was sent with 1Mbit/s, but I may be mistaken.

-- 
Roar Bjørgum Rotvik