Problem with cable modem

[deathtrap3000]

This morning my broadband was working fine. I come home this evening and its not working. Phoned vm and they couldnt fix it and are now sending a technician out. I have narrowed down the problem to the splitter. The tv is working on either of the splitters outputs but the modem will only work if its plugged directly into the connection on the wall.

Can I buy a new splitter (99p) from ebay and just use that? As it will probably arrive before vm come to fix it.

[deathtrap3000]

Does nobody know if those cheap splitters on ebay will work??

[Lord Nikon]

they will, but the problem is more likely to be with the cable feeding your home, or the amp in the street cabinet. The relevant point you make is that without the splitter it works. The splitter reduces the signal strength to the modem, without the splitter you have a more powerful signal and this seems to be the cure.

Are there any additional filters screwed into the modem? they will possibly say 3.5dB Forward Path Attenuator, or be colour coded. If so, try removing one and see if it helps, if not then you are better waiting for the technician who will check and adjust signal levels as necessary.

[calmpitbull]

If the splitter outputs both work for the TV then the problem won't be the splitter.

I think you have a signal issue.

[deathtrap3000]

No other filters on the modem. Ive just screwed it all back in and now its all working again. This is very annoying as its the second time in a week that ive had a technician booked to fix a problem thats fixed itself.

[Lord Nikon]

keep the technician visit, the chances are your signal strength is very low and at the threshold of the power required for the modem. let him check it out regardless.

[calmpitbull]

I agree, don't cancel the visit.

[deathtrap3000]

Thanks for your advice guys. Just hope I dont get charged for wasting the technicians time.

[moaningmags]

The only way you'll get charged is if you're not home.

[Peter_]

Thanks for your advice guys. Just hope I dont get charged for wasting the technicians time.
As you are having issues with your connection the technician will have a fault to sort out, so no time wasted.:)

[deathtrap3000]

Well the tech came and swapped my ambit 255 modem with a new ambit 256. Said the signal to or from the cab was a bit low and called another tech who came a few mins later to sort the cab. I think everything is ok now assuming that something was wrong in the first place.

[Mick Fisher]

Why not post your signal levels here. There are many who are qualified to give an opinion and advice. :)

[deathtrap3000]

Hmm that sounds like a good idea. Maybe I should have done that in the first place.

Signal levels before modem moved upstairs.

Downstream
Downstream Lock : Locked
Downstream Channel Id : 27
Downstream Frequency : 298750000 Hz
Downstream Modulation : QAM256
Downstream Symbol Rate : 6952 Ksym/sec
Downstream Interleave Depth : taps12Increment17
Downstream Receive Power Level : -3.1 dBmV
Downstream SNR : 35.1 dB

Upstream
Upstream Lock : Locked
Upstream Channel ID : 4
Upstream Frequency : 45800000 Hz
Upstream Modulation : QPSK
Upstream Symbol Rate : 5120 Ksym/sec
Upstream transmit Power Level : 48.2 dBmV
Upstream Mini-Slot Size : 1


Signal levels after modem change and tech visit.

Downstream
Downstream Lock : Locked
Downstream Channel Id : 0
Downstream Frequency : 586750000 Hz
Downstream Modulation : QAM64
Downstream Symbol Rate : 6952 Ksym/sec
Downstream Interleave Depth : taps12Increment17
Downstream Receive Power Level : -11.1 dBmV
Downstream SNR : 32.0 dB

Upstream
Upstream Lock : Locked
Upstream Channel ID : 2
Upstream Frequency : 25800000 Hz
Upstream Modulation : QAM16
Upstream Symbol Rate : 2560 Ksym/sec
Upstream transmit Power Level : 46.5 dBmV
Upstream Mini-Slot Size : 2

[Welshchris]

hes made it worse LOL!

Downstream power levels majorly to low.

U need to get tjem back

[Ignitionnet]

Any technical type people spotted the oddity in his modem stats yet?

[pabscars]

Qam ???

---------- Post added at 18:28 ---------- Previous post was at 18:18 ----------

Upstream Frequency ......??

As he wildly stabs in the dark

[Ignitionnet]

Nope.

Come on Seph I know you looked at this thread an hour ago and nearly responded :p:

[Sephiroth]

Upstream Modulation : QPSK
Upstream Symbol Rate : 5120 Ksym/sec

---------- Post added at 19:52 ---------- Previous post was at 19:50 ----------



Come on Seph I know you looked at this thread an hour ago and nearly responded :p:

Got called to dinner.

[Ignitionnet]

Correct - 6.4MHz wide DOCSIS 2 upstream, presumably running on QPSK because the network in the area couldn't handle 16QAM at the time.

You'd have some fun on a 10Mbps upstream on that when it fell back to QPSK or was at it - that upstream delivers 9. :)

[deathtrap3000]

So the guy actually made things worse then?

Will wait and see if I have any more problems with it before getting someone out again though.

[deathtrap3000]

Ive been told today that they are going to be putting a bigger cable from the cab into my property. What will they actually do? Will they need to dig my garden up again?

[deathtrap3000]

Ive been told today that they are going to be putting a bigger cable from the cab into my property. What will they actually do? Will they need to dig my garden up again?
Can anyone answer my question?

[jb66]

Yes

[Sephiroth]

Can anyone answer my question?

Sounds like a load of rubbish to me. A "bigger cable"? They would need to ensure that the impedance between your wall box (or indeed your cable modem) and the street box is 75 ohms. So that might require a re-pull of the cable to your wall box thriugh the green pipe you mentioned. They'll use the old cable to pull through the new.

HTH.

[deathtrap3000]

Thanks for confirming. I did wonder what they meant by bigger cable. They were saying that as I had 2 v+ boxes and a modem what I had now wasn't enough. But everything is working... most of the time.

[Neil22]

Thanks for confirming. I did wonder what they meant by bigger cable. They were saying that as I had 2 v+ boxes and a modem what I had now wasn't enough. But everything is working... most of the time.


Get the Tech back and shoot him with some rubber slugs, don't allow him to leave until he's sorted the problem out.

[deathtrap3000]

Internet just went down and when it came back I checked the signal levels again and they are now perfect.

Downstream Receive Power Level : 0.8 dBmV
Downstream SNR : 35.3 dB

Wonder what they did, and why it was at this hour.

[jb66]

Could have been a network fault that affected a large amount of people and by doing it at this hour it would not disturb as many people. Im just guessung though

[deathtrap3000]

Whatever they did didnt last long. My downstream power level is back down to almost -20 dBmV.

Also, what does "downstream pulse failed" mean? It was in the event log.

[jb66]

Whatever they did didnt last long. My downstream power level is back down to almost -20 dBmV.

Also, what does "downstream pulse failed" mean? It was in the event log.

Im sure it has something to do with star trek

[Ignitionnet]

Modem lost downstream signal, as you would expect given the extremely low power level.

---------- Post added at 10:29 ---------- Previous post was at 10:18 ----------

Sounds like a load of rubbish to me. A "bigger cable"? They would need to ensure that the impedance between your wall box (or indeed your cable modem) and the street box is 75 ohms. So that might require a re-pull of the cable to your wall box thriugh the green pipe you mentioned. They'll use the old cable to pull through the new.

HTH.

Thicker cable = less attenuation / signal loss so higher power level received on the other end of the cable. This isn't related to impedance at all, all cable VM use is of the correct impedance. Where cable were of an incorrect impedance it'd indicate a fault with the cable which would ideally need replacing. The reported power level is voltage relative to 1mV RMS and impedance doesn't enter into the calculation.

Problem seems to be nothing more interesting than a damaged cable or loose connector, maybe on the outside of probabilities a totally buggered amplifier, hence the >20dBmV variations in power levels.

[Sephiroth]

Of course it's related to impedance. Of course the cable is 75 ohm. But in the wider picture, there are connectors between modem and street box and the end to end impedance has to remain 75 ohms. A damaged cable, oxidisation, a loose connector all affect impedance.

The relationship between dBmv and power (watts) is impedance dependant. Relative values to 1 mV RMS are meaningful only in the context of pwer delivered (energy per symbol or bit or whatever). That's why 75 ohms has to be assured.

[Ignitionnet]

By the time a cable is degraded enough for impedance mismatch to cause noticeable issues with regards to power levels reflected signals are going to cause serious issues with signal quality. SNR would be seriously degraded way before power were taken out of spec.

My comment regarding impedance calculation is perfectly accurate, it's assumed to be 75ohm in the calculation due to it being such a non-issue.

As far as I know impedance isn't even measured when installs take place and it's certainly not a reason for repulling with a higher grade coax, that is simply to reduce attenuation across the drop by using a cable with a thicker core.

[Sephiroth]

Look - I'm not the one suggesting a re-pull. I'm merely putting the electrical considerations that can affect the signal. One customer, after a repull, still found water in their tap box, corroded metals and although clearly 75 ohms wasn't there any more, you're right - impedance wasn't checked.

And you know what impedance mismatch does to signal.

As for "assuming that it's a potential issue" - it is a potential issue although I'm not assuming that it is the issue.

[Ignitionnet]

I reckon we just test the drops by getting the installed to pop his tongue onto the cable. Once he's done a few he should get some idea what feels right and what doesn't.

Sound good?