[Olsr-dev] OLSRd 0.5.4 and 0.5.5 with ETT available

[Henning Rogge]

Am Friday 14 November 2008 13:15:13 schrieb Erik Tromp:
> The problem with the reported rate is that in reality the throughput is
> usually a lot lower.
It's ALWAYS lower, thats because of training sequence/praeamble, encoding, 
wlan header... and of course the random delays between package transmission. 
It's all described in the WLAN standard.

> As an example: my WLAN connection currently reports a rate of 130 Mbit/sec
> . However, the highest transfer rate I have seen is approx. 6 MByte/sec (48
> Mbit/sec). Somehow there is some overhead. Is that a fixed factor (130/48 =
> 2,7 ?) Or is that factor dependent on other things?
130 MBit/s is the translated symbol rate... 48 MBit/s real data traffic sounds 
not bad (~60-70 MBit might be the theoretical maximum).

> This is what I meant by "it is difficult to determine the "quality" based
> on only these [L1/L2]metrics."
You cannot predict the "real throughput" on a shared access medium, because it 
depends on random waiting times and the behaviour of all communication 
stations in jamming range (which is higher than communication range).

> But, even if the reported rate is not the actual rate, at least it is
> better than nothing. That is, provided the WLAN interface has the ability
> to choose for itself its bit rate. If a node owner chooses to fix the rate
> to say 54Mbit/s, then the OLSR network will think there is a very good
> quality link there, while in fact there may be not.
Thats why ETT includes ETX... ETT measures airtime, not bandwith. If you set a 
"too high" fixed bandwith you will have packet loss, which will reduce the ETX 
part of the ETT metric.

Henning

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