What causes low upstream signal levels - Canadian TV, Computing and Home Theatre Forums
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post #1 of 25 (permalink) Old 2019-11-17, 03:02 PM Thread Starter
 
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Question What causes low upstream signal levels

Have started to experience intermittent outages with our cable internet service.

After talking to tech support, it was determined our upstream signal levels are too low resulting in T3 errors. A ticket has been created for the supplier of broadband services to our TPIA to investigate.

My question is, what causes low upstream signal levels? My understanding is, this is entirely controlled by the service provider by "telling" our modem how much return signal level to generate?

It will be interesting if the tech coming out on Wednesday finds anything amiss with our physical connectivity.

I'll let you know how it goes.

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post #2 of 25 (permalink) Old 2019-11-17, 03:18 PM
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if ur talkin a Cable ISP, you may be able to login to your cable modem and check the downlink strength and SNR on each qam channel, along with the Uplink Signal level
it is sending back in the Uplink. That would at least point you to the specific channels they are using for the Uplink.
on my motorola sb6141 the ip address is 192.168.100.1 to login to the diagnostics pages.

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post #3 of 25 (permalink) Old 2019-11-17, 03:29 PM
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Quote:
My question is, what causes low upstream signal levels?
There are a variety of things that can cause this. One being a problem with the terminal. However, it can also be caused by defective cables. In addition, the cable company has to maintain appropriate signal levels through their network There are many things between you and them that can affect signal levels. These include amplifiers, splitters, filters, attenuator pads and more. Periodically, they'll send out a tech to check the levels at various points in the network.

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post #4 of 25 (permalink) Old 2019-11-17, 04:30 PM
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Things that can be checked in the home:
1. Loose RG6 connections either not screwed on tightly or bad connector fitting on the cable. This can usually be remedied by simply unscrewing all connectors, checking the connector condition and screwing back on. Connectors should be the newer compression type. Screw-on connectors and connectors with a thin metal ring should be replaced.
2. Corroded connections. This can happen in outdoor and other damp locations or on the inside of wall plates due to condensation. It can be checked by looking for oxidation on the connectors (white powder) and center conductor (green discoloration.) Remove the wall plate for inspection if necessary. Replace all components, such as connectors and splitters, where oxidation is found.
3. Poor quality cables. They should be marked RG6. RG59 cable should be replaced.
4. Damaged, tightly bent or crushed cables.
5. Too many splitters or low quality and corroded splitters.
6. Overly long cables. Keep cables runs as short as possible.
7. Too many connections. Try to avoid the use of extra splitters and barrel connectors. Splitters should not have any unused outputs.

Anything outside the home is the cable company's responsibility. Wiring inside the home past the demarcation point is supposed to be the homeowner's responsibility. Some companies, such as Rogers, do some of that work as part of the service. Other companies, such as Bell, charge a fee for work done inside the home.

The company tech will check signal levels in the home and probably check connections and cable condition as well. If the signal level is low entering the home, they will check outside connections and cables. If that doesn't help, they will put in a ticket for a network tech to check the network. That usually takes several more days.
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post #5 of 25 (permalink) Old 2019-11-17, 04:52 PM Thread Starter
 
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Hey thanks fellas,

majortom I have the same modem as you and my upstream power levels are half of yours. My highest is 27dbmv. So something is not right. Most times it is around 25 dbmv which accounts for the T3 errors.

ExDilbert My setup in the home is the same as it was when Rogers initially installed it in 2004. They brought in the RG6 from the demarc point outside, terminated it with a compression fitting and there are no splitters in the home as our cable is only used exclusively for broadband. I believe though, there might still be a filter on the tap feeding our home to block analog cable, back in the day.

A Rogers tech is scheduled for Wednesday. I'll report back on what he/she finds. My guess is this will get resolved (at the CMTS) without an on site visit.

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Last edited by Gentleman; 2019-11-17 at 05:25 PM.
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post #6 of 25 (permalink) Old 2019-11-17, 05:36 PM
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The analog filter could be an issue since those frequencies are now used by other services. The signal loss would probably be greater than 10-12dBmV though.

I've seen signal levels drop at low frequencies with time even on good installations. Barely visible oxidation can occur on the copper center conductor and device center connection. Simply unscrewing the connections, pulling them out and reseating them can resolve the issue. Simply tighten them finger tight. This seems to be an issue below 60MHz which were used by channel 2 and for other purposes by some cable companies.

This seems to be worse if the center prongs in the device connector have become oxidized or damaged. It can be an issue with things like modems and receivers which are often recycled between customers. Another issue are devices like splitters hidden in boxes or walls and not being removed or replaced with new installations.
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post #7 of 25 (permalink) Old 2019-11-17, 06:43 PM
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Considering we are talking about logarithmic measurements, not linear...
Yours is way more than "half" lower than mine.

I don't really know how it works, but I am guessing that the return power is controlled by them to achieve a certain SNR at the next node up the line, which in turn would optimize the modulation coding scheme, without disturbing your neighbors???

eg - Comparing your numbers to mine, I would think something is seriously wrong for yours to be 22 dB down from mine. It's possible that they see a problem somewhere that gets better for everyone else in your hood, when yours is powered way down?

27 dBmV -> -21.75 dBm
49 dBmV - > 0 dBm


That is >100 times less signal strength coming out of your modem than mine. That is interesting, keep us posted what happens.
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post #8 of 25 (permalink) Old 2019-11-17, 07:14 PM Thread Starter
 
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Levels are up slightly.


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post #9 of 25 (permalink) Old 2019-11-17, 07:47 PM
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For the record, my Downlink signal strength is + 11 dBmV (-38 dBm), and the SNR is 39 dB on each of the 8 channels.

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post #10 of 25 (permalink) Old 2019-11-17, 07:56 PM Thread Starter
 
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Downstream


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post #11 of 25 (permalink) Old 2019-11-17, 09:29 PM
 
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opposite

LOW levels on upstream indicates the opposite of what folks here are saying.
Yes, the level from the modem is controlled by the far-end equipment. There is an ideal point where modems would work best, that level is +45dBm.

The return path (upstream) has many variables not the least of which is cable loss and splitter losses. Since the return must include signals coming from ALL modems in a given area(node), and they're all transmitting in-turn on the same frequency(ies), the levels arriving at the far end must be controlled. The levels all need to arrive both in-sequence AND at very similar levels so that they all can be understood without having to resort to instantaneous AGC or other impractical wackiness.

Modems are only able to generate signals within a certain range, due to fundamental RF electronics reasons. In order to be certified as whatever DOCSIS compliant, they need to be able to output within a certain range. This range ends up being anywhere from +38 to about +54, which can also depend on how many upstream channels and which modulation are used.

If a modem is connected to the coax network with a very low customer-premesis loss(single device), and happens to be at the far end of a very long outdoor system(long cable and low value splitter), you can end up with a scenario that the downstream signal is kind of strong, while the upstream level is beyond minimum(less than +38). The outside system is configured for consistent downstream signal levels, but they don't take upstream levels into account, so since cable loss is a function of frequency(higher frequency = much higher loss), and upstream frequencies are very low compared to downstream signals, you end up with very low loss in the upstream, and controlled loss in the downstream. the result is that your upstream signal cannot be understood by the far end because it's too loud, so packets are lost.

in this relatively rare scenario, the service person has three options:
1: install an attenuator to get the downstream signal as weak as possible while still being good enough for no errors(about -10db), while hopefully introducing enough loss to get the modem transmit levels out of the dump and into the +38 or better.
2:install an "equalizer" that attenuates low frequencies more than it does higher frequencies, when option#1 fails to achieve the goal. The equalizers might be unobtainium depending on the company.
3: ask the network design/maintenance team to alter the network and reduce the amount of amplification of the upstream side of the inline amplifiers in order to make it possible to get this equipment working correctly, and without causing customers closer to the active equipment from having the opposite problem.

In my days at a tech, I encountered a LOT of resistance to option #3, as it was always slow, and usually they disagreed with my assessment... as we're delving into territory that many techs in those systems fail to completely understand. I usually had problems with high transmit levels in high--rises because of lack of amplification in the upstream, as the designers failed to anticipate that there would never be any low-value taps on that leg of the system, and all customers in a highrise would usually be fed by -26 to -29dB of taps before ever getting to the customers, where normal outdoor suburb taps values are -23 coming right out of the amp.

I hope this helps to clear up some of the non-intuitive-ness that comes with such high tech...
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post #12 of 25 (permalink) Old 2019-11-17, 09:49 PM
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thanks sparky, but you DO realize that +45 dBm is over 30 watts??
No way a cable Modem is putting out 30 watts of rf power. You must have meant +45 dBmV?


I would just think the -22 dBm he has would be an indication that it doesn't need to be as high as mine in order to be heard... But...he said his issue is that they are seeing errors in his return path too... in which case you'd think they would increase it until it IS heard error free. That is unless something else is wrong in the loop.

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post #13 of 25 (permalink) Old 2019-11-20, 08:24 PM Thread Starter
 
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Ticket closed Here are the upstream levels

As expected, no tech appeared onsite today to check physical connectivity.

Levels have changed slightly since the ticket was created. Here are our upstream levels now.


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post #14 of 25 (permalink) Old 2019-11-25, 09:00 PM Thread Starter
 
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New ticket same issue

Just got off the phone with TSI support. Upstream levels are very low.

Issue has now been escalated with Rogers. Hopefully this will get resolved.

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post #15 of 25 (permalink) Old 2019-11-29, 10:02 PM Thread Starter
 
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Problem solved

Rogers tech showed up onsite today. Turns out, those analog cable TV filters, the ones installed to prevent broadband subscribers from receiving analog cable television are to blame for low upstream signal levels.

Once the tech removed the filter, our upstream levels returned to the normal range. See below.


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