DSL Noise margin

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jseko

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Jul 29, 2010
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I ran 80ft of Cat5e between the NID to my office and used the tool-less key stones from Monoprice for Cat5e on both ends. One of these.

I wired both ends using the 568B standard. I noticed that when the modem is plugged in at the NID the noise margin is about 4 to 6 db higher than when plugged in at the office. I only have 6-7db noise margin so it's not much to work with at the office.

From the NID to the jack, I have about 10 ft of Cat3 with bare copper connected to the splitter and terminated on the other end with a RJ11 plug. This goes into the tool-less jacks for the Cat5e. On the other end of the Cat5e, I am using the 6 ft flat phone cord that came with the modem.

The attenuation stays the same whether at the NID or office. Why's there such a big difference on the noise margin? I thought the major factor here would be the run from the CO to my premise - 7000 ft.
 
I wouldn't worry about the 4-6dB fluctuation of the SNR, quite honestly. You obviously have something in the house that's creating noise on the DSL side of things.
6-7dB SNR is terribly low. What's the attenuation at?

I would verify all house phones are filtered, and filter is plugged in correctly.
 
attenuation shouldn't change much based on the location in your house you plug the modem in. Attenuation is basically related to distance from the CO/DSLAM. A couple hundred feet shouldn't affect attenuation much, if at all.

So, for clarification, SNR is around 6-7dB when plugged in to the CAT5 you ran between the NID and your office. But when plugged in to the NID, SNR increases to around 10-13dB?

Correct?

ehh... 48dB is a relatively high attenuation. Once attenuation starts getting up there, SNR starts to drop and not much you can do about.
This is all based on my years as a DSL field tech; although we only dealt with older DSL techs (GDMT, ADSL/2+, some SDSL flavors)
 
Yes, the noise margin you stated is correct.

I am on ADSL2+. Will the noise margin make much difference in speed? My modem is currently capped below the tier of service to maintain stability.

I'm about 8000 ft from the CO, but the copper could be longer. There was an AT&T tech out here a while back and he said it looks like the wires were probably strung in the 70s or 80s.
 
SNR won't necessarily affect the speed at which the modem negotiates with the DSLAM, but it can cause data corruption.

to explain SNR... SNR = signal to noise ratio.
so, with an SNR of 20dB, you have 20 signal to 1 noise.
As the SNR drops, it gets closer to a 1:1 ratio.
Obviously when the ratio gets closer to 1, the noise causes interruptions to the DSL signal. 'Bout the best way I can explain it...

I honestly wouldn't worry about it unless you start getting a lot of dropped packets.

Although, it is strange for the SNR to increase that much.

Did you run the CAT5 yourself, or was it already there? Are you positive it's a "home-run?" Meaning, straight from the NID to the office, with no splicing, daisy-chaining, etc?
 
I did the home-run myself with cat 5e. I had to do it because originally the ISP served up ADSL2+ on separate pairs than my POTS with the telco. They rent the copper between the NID and CO/DSLAM from the telco. But now the ISP serves up POTS on the same pair so I had to get that Wilcom splitter. With or without the splitter the noise margin is the same near the NID.

This is an old house so the topology is daisy chained quad cable with some cat 3 in a renovated part of the building.

I have a bunch of microfilters too, but I was hesitant to use them. I didn't want anyone to be able to plug in anything that wasn't filtered and I've been told the whole-house splitters are much better quality than the microfilters by a few telco techs when they were working on our DSL lines at work.
 
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What is the SNR on the download and upload side (usually different numbers)? When I was a DSL install contractor, I found that if the customer had anything less than 8dB SNR on the download and roughly the same on the upload, the DSL would probably flap frequently and the speeds would be low. Also remember that SNR is calculated in dB, which is a logarithmic unit. For every 3dB of SNR increase, you are doubling your effective signal.

If I saw low SNR & low sync speeds at the loop's test port in the NID with the customer premise wiring temporarily removed, I called in a trouble ticket to have the loop checked out. Further into my job I got a Tempo Sidekick meter & a CO RFID badge which allowed me to check the loop myself and verify whether it was good or bad.

Sounds to me like the loop you have is marginal at best. Calling in a trouble ticket is most likely the only way to get the issue fixed since you can reproduce the issue at the NID test port ( you did this already didn't you? )
 
for up, the typical noise margin is about 5 db higher than download. Attenuation is 29db.
 
If you are seeing higher SNR levels on the upstream than the downstream, that makes me think the loop is not balanced correctly or has some other issue on it. Call your provider and have them test the loop at the CO and/or NID.
 
An ISP tech has been out here three times and the telco has been here once. None of them could find anything wrong with the line. Or at least they said the line was clean at the NID. Although for 8000 ft I think that the attenuation is quite high. But without SLA, I don't think I have much leverage...
 
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