How can Upload SNR be much bigger than Download SNR?

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May 12^th, 2015 at 10:25am

Upload SNR = 11.4 dB, Download SNR = 8.65 dB
How is this possible?

Reply #1 - May 15^th, 2015 at 8:04am

Hi,

Download and upload SNR levels are independent from each other. Download SNR is related to your receive signal while upload SNR is related to your transmit signal. If you are sure about your antenna well pointed, your location may be close to beam edge or there is a rain fade at gateway station.

Reply #2 - Jun 21^st, 2015 at 10:01am

The frequencies of both upload and download are also far apart, and the beam width is not the same for either of them. Slight off-pointing of the dish and obstacles like twigs (with leaves), can make the figures change.

The two figures are never the same on my link.
Also, I believe that the uplink figure depends on the transmission power level that the satellite hub asks the terminal (your sat modem) to apply in order to maintain the link.
Even the downlink figure changed e few months after the start of the Ka-Sat service, down from 14.5 dB to 11.5 or 12, in my area. This signal level is also remotely commanded separately by the hub for every spot beam. - correct me if I'm wrong.

Reply #3 - Jun 21^st, 2015 at 11:37am

Thank you that is helpful. But I am still puzzled by the measurements I'm getting. Here (attached) is a typical plot. These are the numbers for yesterday, which was good weather all day.

I would have expected the 'Rx SNR' (reported by 192.168.100.1/?page=modemStatus) to be similar to the 'Download SNR' (reported by checkportal.skylogicnet.com)

Actually the Rx SNR is consistently similar to the Upload SNR and different from the Download SNR. This doesn't seem to make sense.

I'm also wondering why the Download SNR is so variable, while the Upload SNR is relatively stable.

Reply #4 - Jun 21^st, 2015 at 12:37pm

you have a point there - I will have to monitor simultaneaously what the modem (and Tria) pages say, and what the checkportal says. But then again, some values in the modem are "commanded" "to be shown" or "calculated" values (just look at what is supposed to be cable attenuation when the system is powred up and scanning - it shows total nonsense for cable attenuation until it finds it's spot frequency (because it tries all spot channels regardless what your set-up was).
The upload SNR should actually reflect the signal thet the sat receives from you and reports (probably not often, and with a delay), to the hub, while your Tria shows how much power in dBm it is sending to the sat on request of the hub. So this figure can vary depending on the needs of the link. Good pointing, as I said earlier, also comes into play. The transmit beam, if I'm not wrong, is on the higher band end, and much narrower than the receive beam, so you could be "a tad off" and get weird results on the figures. Even with pointing as per manual, I ind the last test has to be slight bending of the dish (still a bit too flexy, if you ask me), to the left, right, up, and down, to see if you're really spot-on the beam centre. Cloudy weather doesn't help in such cases, because the pointing beep reflects the quite significant signal variations created by the clouds. On a Astra2connect site, I have once even reverted to the old "point and shout" method where a helper loudly reads out the exact rx decibels to me while I fine point. Very stupid impractical 60's method, but works. I'm not yet ready to go up on roofs with the laptop end sat modem, lol, but it could happen one day.

Reply #5 - Jun 21^st, 2015 at 1:27pm

Another thing: I start seeing Tooway speed tests going weird - as if they crippled every client creating a fast download burst, *including* the one that the official Tooway Ookla speed test creates. See attached screen shot

Reply #6 - Jun 22^nd, 2015 at 2:36pm

Is it possible the fields at https://checkportal.skylogicnet.com/ are mis-labelled, i.e. they field they have labelled 'Upload SNR' is actually the Download SNR, and the field they have labelled 'Download SNR' is actually the Upload SNR?

Reply #7 - Jun 22^nd, 2015 at 2:51pm

This is good question actually.

The DSSS (Direct Sequence Spread Spectrum) technology uses very wide spectrum at very low power. It is done by transmitting same information several times, this is why the bandwidth is so wide, but on the other hand, if the receiving party manage to get at least one of the identical transmits, it manages to get the data right. One of the main advantages of this technology is that it resists interference and noise very well. But as far as I know, eventually other methods took the lead, for example in cellular communication the CDMA standard which uses spread spectrum is not as popular as it used to be, while the GSM is now the leading option.