Soureal wrote

Thanks, now i understand. Can the option in the MAXtream Section "Latency First" helps also for this Issue?

Yes, setting latency first can help somewhat.

Regarding channels: you could also use 2, 7 and 12 (distance 5 channels) or 1, 6 and 11 with 802.11b/g or even 1, 5, 9 and 13 or 3, 7 and 11 (distance 4 channels) with 802.11n. The channel's mid-frequency is not the concern, but the distance to other channels heavily in use is. In 802.11b/g mode, distance should be 5 channels, in 802.11n mode distance can be as low as 4, which gives you 4 non-overlapping channels in Germany.

The reason why channel 9 might be worse is because customer gear and microwave ovens use frequencies near channel 9. You won't see such devices in a WiFi survey, though. If channel 9 shows good SNRs at your location, then there is no reason to avoid it.

Soureal wrote

On the downside, you lost some bandwidth over air.

Sure, but bandwidth in the 2.4 GHz band is limited anyway in dense populated areas, since in a shared medium such as wireless technology "available bandwidth" is determined by the AirTime available to your AP in a given channel and not only by technical capabilities of the AP itself. That's the reason why the 5 GHz band is much more suitable for directional links.

Soureal wrote

Thanks for your response again! The AP is behind a wall, the client not (no view connection and a distance about 20 meter). Do you think a 5 GHz link is a better choice for that scenario?

No, in this case a 2.4 GHz link is better. 5 GHz is more sensitive to obstacles such as walls.

Snotmann wrote

Deactivate the maxstream feature will give you much better ping results

Only in pure PtP scenarios, not in PtMP, especially not if using repeater mode. And it's called MAXtream, not MAXstream.