Hi Eric,
Definitely a good call out for the reasonings behind the additional settings modifications. I will try my best:
1. Transmit power at 0db – currently the distance between my access point and the 902AC is 60cm and I use a 10db gain external antenna on the the 902AC and while trying all the options I found out that even this 10db gain is almost too much power when the distance is so small. This means that I do not need any additional amplification from the 902AC. While negotiating interference in my place I found out that the antenna power setup on both, the access point and the 902AC, plays a significant role when it comes to retries. If too strong, or too weak, the retries will increase. If anybody is interested how a rough setup guide, continue reading… I found a way to quickly find this relatively „optimal“ strength. Basically, keep increasing the power on the access point (Tx) until you reach a plato value where the receiving unit (902AC) does not register extra signal (Rx) in db no matter how much you increase the transmit power. Set the the transmit power at the lowest setting that gives this max db reading in the 902AC (or even 1dbm less). Repeat the same exercise, but this time increasing the transmit signal (Tx) of the 902AC and monitoring the signal receiving strength (Rx) on the access point. Note that the amplification of both units should be in the similar range or the unit with a much stronger signal will choke the transmission of the other unit, again having a significant impact on retries. For example, currently antenna power settings I run are 12dbm on the access point and 10dbm on the 902AC (the strength of the external antenna alone). Finally, I found out that access point to client distance should not be too short. I had significantly lower retries at 1,5m distance than at 50cm. Yesterday I increased to 60cm and this helped much more then I hoped for, but due to my living room limitations this is the max distance I can have right now. Definitely, keep the distance 60cm or more. One final note, once the signal is stronger than -30db to -28db, it is too powerful and retries start increasing again.
2. NTP settings – this is something I had an experience many years ago in a completely non audio networking situation where I had encountered background processes when NTP settings were not synchronised. I can’t say I experienced or noticed any issues in the current setup, but since the 902AC with openwrt is still a router communicating between two IP subnets, I like it to have the least variables to try and sort out. Still not sure how to best implement this. Maybe the most efficient is to have it set to use the DHCP communicated values. This way it is automatic and will have the same setting as the main router and that is ideal.
3. Long vs short preamble – it is an error redundancy check and only recommended to use the long preamble in higher interference, low signal environments as it is more robust, but it is slower and makes the client work more as it relies on a larger and a slower header overhead. Ideally, both the access point and the client should have the same preamble enabled. In other words, if there is no interreference it is better to keep the short preamble.
4. Rx LDPC – it is a decoding „algorithm“ which is used to maximise the wifi network at full bandwidth usage by detecting and correcting errors in data transmission. In layman’s terms, it is created so that it reduces retries by fixing the received data and sending it downstream. It sounds good, but it involves many iterations to „fix“ the data and in turn requires significantly higher compute power to perform. If this option is disabled then is is a simple yes/no situation where good data gets send downstream and the corrupted data is requested again. Other than the processing power it also impacts latency due to time it takes to fix these errors. Bottom line, it is meant to enable high quality throughput in full bandwidth usage wifi scenarios. Streaming uncompressed audio is only using 1% bandwidth for 16/44 files and approximately 10% for 24/192 files on a 5ghz channel with 20mhz width. This is not even close to full bandwidth scenarios. I suggest to try disabling this one, it was some time ago I did it, but the impact was noticeable.
Through a bunch of tests I realised that the speed of wifi has nothing to do with audio streaming quality. The stability of the signal has everything to do with quality of audio streaming as impacts retries which impact the workload on the client (or even stream drops). Most of these tweaks are all geared towards this wifi stability and/or reduction of any kinds of processes in the 902 AC.
Hope I didn’t over complicate it:)
Cheers,
B.
PS. This is potentially a Pandora’s box, but if you are running two 902ACs, one as AP and one as client, I would love to se what happens if WDS or 802.11s protocols are used. Basically, they are supposed to create a completely transparent connection making the client act like a wireless switch where all the IPs are in the same IP subnet. I tried enabling it between the Aruba and the 902AC, but no luck (Even changed my main router’s (Fritzbox) IP to the openwrt range)