SatSig topic:

KA-SAT signal strength

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Jul 19^th, 2011 at 3:43pm

Hello,

As part of the investigation to resolve the unreliable SSL connections to Apple (in particular) sites I have been looking at signal strenght. After tweeking the alignment and dish position I am getting around 13-14dB SNR which Bentley Walker say is below average.

Is there anyone on here who has Tooway in the UK, ideally the Midlands who would have a look on their modem stats and tell me what SNR they are getting so I can compare.

Thanks for your help

Jul 19^th, 2011 at 4:53pm

When I was setting up my dish a few weeks ago I asked BW what was a good SNR level to aim for and I was told 14 is good. Mine is around 13.8 - 14.5 after careful tweaking. Whether the system is working at full capacity is something I'm still not sure of.
Colin

Jul 19^th, 2011 at 5:03pm

Meant to add that I'm in North Cambridgeshire.

Jul 19^th, 2011 at 8:34pm

Provided the S/N is above threshold (i.e. less than say 1E-8 bit error rate) there is no effect on download speed. In effect you are getting a perfect error-free downlink. Increasing the S/N from say 10 to 14 makes no difference, it is still error-free.

S/N is not the explanation for SSL connections.

If the S/N was too low, like 6 dB, your receive link would fail with high error rater and no web site would be accessible. These clever digital system have a fairly sharp threshold, between failed and working perfectly. The threshold depends on the modulation method (e.g. QPSK or 8PSK) and FEC coding rate ( e.g. 1/2, 3.4, 7/8 ).

If a link gets really bad due to extra heavy rainfall the Surfbeam modem and the hub would recogize the problem and would temporarily adapt the modulation and coding (ACM) of outlink DVB-S2 packets addressed to your site so as to maintain normal service.

Even this would make no difference to SSL operation.

If you S/N is very high is just means that there is more rain margin, before ACM needs to be applied.

It is important that the S/N is actually peaked up. It is not acceptable to just get above some arbitary level (as might be the cae for satellite TV receive only). The reason is that the transmit beam is narrower than the receive beam and the transmit beam needs to the centred accurately.

Regarding your SSL problem it may be relevant that satellite system use complex accelleration techniques to speed up normal TCP/IP traffic flow. Some encrypted systems may not be fully compatible. I don't know enough to help further.

Best regards, Eric.

Jul 20^th, 2011 at 10:03am

Regarding your location, North Cambridgeshire UK, see the spot beam coverage map below.

I have added a red dot in your general area. The coastal outline of the Wash is just visible !

You are in spot beam 34 which is an Orange type beam, or type 2. I don't know if you can verify this from your modem status screens. Beam 22 would work fine also but the higher level of the Orange beam would be best.

Incidentally, GW5 means your hub is in Berlin.

As side issue, I am sceptical about the type of beam aimed at Bristol. I think it is Green Type 4 as shown in the map above which I believe is correct. I don't think it Blue Type 1 as indicated in the Tooway dish pointing web page. If anyone in the Bristol beam knows the best beam in that area, please say.

Best regards, Eric.

Jul 20^th, 2011 at 10:23am

Yep, I'm on orange. I fully understand what you say about digital signals. But I sent three trace routes to BW last Friday because as you may remember, I was concerned about slow initial appearance of webpages (more so than could be explained by single instances of latency). Although my SNR is normally about 14, BW looked at it remotely when it was low at about 10, perhaps because it was raining heavily. Their judgement was therefore that I should re-point my dish. Which I haven't. Which advice seems spurious in the light of what you are saying (which I agree with). I don't want to sound cynical but I can't help wondering if this will be the catch-all advice in the way that in the old days (a couple of months ago), when one complained about ADSL internet speed the answer was always some thing like "try a new filter". In the case of satellite I imagine that dish pointing will be cited especially in the case of those who have self-installed.

Jul 20^th, 2011 at 1:57pm

In the event of problems with in-service VSAT terminals, my catch-all advice, as it always has been, is to first check the cable F connectors (water corrosion) ; then the dish mounting for rigidity and pointing. (coming loose in the wind)

For new installations there is more variety to the possible troubles. I think Viasat and Skylogic have done well to simplify the installation process so as to encourage self-installation. In new networks, like Tooway KA-SAT, there are often problem with the network itself that need diagnosing and fixing. If these are obscure and intermittent then it is not easy and the more feed back from end users the better.

Best regards, Eric.