We have a few subscribers who use the service in ways which are very sensitive to jitter and packet loss, such as Skype calls where the screen is being shared, so any performance change causes immediately noticeable artifacts, and this is enough of a problem for them that they may leave our service if the problems can't be corrected.
What I'm seeing that I believe is responsible for these artifacts is that the signal level is fluctuating slightly - not more than 3dB in any direction or polarity, but that variation is causing the modulation to shift from 6x MIMO-B to 4x MIMO-B and back. Running a continuous ping to the SM shows no substantial variation in ping times when the signal is stable, but when the signal is in flux, we'll see occasional roundtrip times drastically increase to more than 120ms, sometimes as high as 500ms.
We had seen similar issues on the PMP320, and one of the tools we had on that platform was a CINR Backoff setting - this would alllow the thresholds of the particular modulations to be adjusted so that a radio which regularly jumped between QAM-64 and QAM-32 as the signal changed could be adjusted so that it would stay in QAM-32. Yes , the performance suffered slightly, but the stability was greatly improved.
I do not see any similar option on the PMP450 platform - does anyone know of a way to effect such an adjustment?
There isn't such a setting in the current software, but we have debated this internally for quite a while. I will discuss with engineering again and see if we can find a way to implement something that can help this situation.
Referring to the main example, we can't really improve the signal - he's about 11 miles out from the tower, and this is as good as it gets. Ping tests to his SM from the router where the AP is shows the jitter is bad over that RF jump, so it's not really a Skype issue so much as it is a "the problems of this connection are much more noticeable on something like Skype."
I was able to switch him to a Medusa AP on the same tower at a slightly lower RSSI, but his modulation is more stable now, and that seems to have helped somewhat.
There isn't such a setting in the current software, but we have debated this internally for quite a while. I will discuss with engineering again and see if we can find a way to implement something that can help this situation.
Matt
That would be super handy to have. I can't say we've had any customers complaining of this, but it would certainly be nice.
Otherwise I suggest making use of the High Priority channel. If you always prioritize those packets then the impact of changing modulation is greatly lessened. However you need to make sure the packets are marked properly before they hit the radios, so this may not be practical.
For what it's worth we roll a "Digital Phone" service that leverages QoS with the HP VC with fantastic results. Never had an issue with modulation changing impacting voice quality.