[Olsr-users] Question about hidden node behavior

[ZioPRoTo (Saverio Proto)]

> Isn't it possible simply to do a QoS where you say: if there is a OLSR
> packet in the queue send it first. No need to evaluate bandwidth?

the key idea with QoS is that you have to drop packets because the
rate of packets to deliver is higher of the rate of packets you can
deliver. Then you queue packets in a way that you drop packets with
less prio, in other words you try to decide what to drop.

so, unless you have a 802.11e card, usually the buffer of the card is
1 or 2 packets. What you do with the Linux kernel is doing the queing
and the ordering before passing the packets to the 802.11 device.
The bandwitdth you set up in the tc script is the rate that your
kernel will send packets to the device, assuming that device will
always be able to deliver those packets without losses.

Now if you don't know what rate your wireless medium can handle, it is
just useless to queue and prioritize in the kernel, because if you
then lose the packets after passing them to the device. So again is
not you deciding what to drop, because you are losing packet after the
QoS: your QoS is not working.

Yes, you can queue first OLSR packets, and send then to the device,
but if there is no bandwidth the prioritized packets will die in the
air, so what is the sense of this ?

So, to do QoS, you MUST know what is the available bandwitdh.

I hope this time I explained my self.

Saverio