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Author Topic: AC Balance issue...  (Read 4896 times)

mdc

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AC Balance issue...
« on: September 15, 2016, 11:29:32 AM »

Howdy,

ADSL developed a fault about 2 weeks ago, and finally had an engineer out today to look at it.

I suspected an HR fault at first, but line checks came back okay on resistance and capacitance, AC balance was significantly down from previous values though at around 48dB. Measured outside the premises (it's a block of flats) in the trunk at 55dB, still with a large number of CRCs (about 15-20 per minute, with a similar ES value).

About 20 spare pairs were tested, all showing similar AC balance or worse (lowest was around 45dB). I've had work done on this line before, and at that time (about a year ago) my line (before pair swap) and several spare pairs tested around high 60s, low 70s on AC balance.

Just wondering if anyone had ever experienced a situation which affected the AC balance on an entire trunk, rather than just individual pairs, and if so, what should I advise the engineer to look for? He's popped to the exchange quickly to check the MDF as I did think a grounding issue might cause this sort of problem, but just wondering if anyone else had any ideas?

Cheers :)
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Black Sheep

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Re: AC Balance issue...
« Reply #1 on: September 15, 2016, 12:55:35 PM »

A totally wet joint may cause the scenario you talk about, but I would absolutely expect other conditions to be prevalent such as grounding (or earth contact fault as we call it). In other words low resistance readings, but you comment all those taken by the engineer were ok.

So, the only other scenario that I can come up with, whereas all other parameters test ok, but the AC Balance is poor, especially on all pairs ....... is that the whole cable is on split pairs. IE: we call a pair A and B legs, if the A leg of one pair is used in conjunction with the B leg of another pair to provide broadband, this is a 'split pair'.

Hope this helps.  :)
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mdc

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Re: AC Balance issue...
« Reply #2 on: September 15, 2016, 06:39:06 PM »

Cheers for the info :)

No split pair situation here I'm afraid, 2nd engineer arrived shortly after your post, believes it's just a particularly "hot" trunk.

Attempted fix was to splice from a different trunk running adjacent to the EO trunk which serves my building; upshot is I'm now connected to a cab about 300m from the premises; D-side back to the exchange had an AC Balance of around 70, but still only syncing at 10mbps, with a ridiculous amount of errors.

SF:                   192183          183353
SFErr(CRCErr):  124188          0

Line is also rather unstable; SNRM drops to virtually nothing intermittently, leading to a large number of tones being dropped. Throughput drops to 1-2mbps during these periods, although it does eventually recover. Forcing a resync at this time will cause the line to sync at around 5mbps, although SNRM eventually climbs back up to around 12, so it looks like whatever the issue is, it's causing around a 6dB drop in SNR when it occurs.

Upshot of it all is that the engineer seems to think there's a significant issue with the internal wiring. Building management / landlord aren't likely to make the effort (or spend the money, rather) to resolve it, which means it's probably going to be down to me to inspect, repair and/or replace the length from the DP up to my flat :-/ Unless there's anything else at the exchange that you think might be worth checking?

Also, any advice RE: test equipment to check for internal wiring faults? Obviously I can visually inspect for things like physical damage to cables, loose terminations etc, and can check for continuity, but I'm thinking more along the lines of AC balance, resistance and capacitance.
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mdc

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Re: AC Balance issue...
« Reply #3 on: September 15, 2016, 06:56:18 PM »

Oh, and...
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Black Sheep

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Re: AC Balance issue...
« Reply #4 on: September 15, 2016, 07:00:15 PM »

"A hot trunk" ............. never heard that terminology before in 30+yrs in the game. 'Hot' (whatever that means) or not, it will have no impact on the AC Balance of the pair of wires.

If by 'Hot' the engineer meant there are lots of broadband circuits going down the cable, then it will be the WB Noise (Wideband noise) integers that will be affected. The aim is to get this figure to as near as -60dB as is realistically possible. If your particular circuit is surrounded by other broadband circuits (known as disturbers, or crosstalk) within the cable, then this WB figure will decrease towards '0' dependant on the amount of said disturbers.

A multi-meter will perform some tests that you require for the internal wiring, but AC balance is measured in decibels, so somebody else on here may be able to guide you towards the equipment you need ??  :)
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mdc

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Re: AC Balance issue...
« Reply #5 on: September 15, 2016, 07:54:25 PM »

If by 'Hot' the engineer meant there are lots of broadband circuits going down the cable, then it will be the WB Noise (Wideband noise) integers that will be affected. The aim is to get this figure to as near as -60dB as is realistically possible. If your particular circuit is surrounded by other broadband circuits (known as disturbers, or crosstalk) within the cable, then this WB figure will decrease towards '0' dependant on the amount of said disturbers.

Yep, that's exactly what he meant :)

They did remark that the trunk was kicking out a ton of noise, and the second chap put it down to approaching saturation in terms of the amount of broadband provisioning in the area. Apparently the "ideal" solution would've been to eliminate the internal DP entirely and reprovision from a different cab about 400m away, which apparently has plenty of spare capacity (at least 100 lines). However, with a "working" setup - from a voice point of view, anyway - it would be hard to justify the work to OR management. And unfortunately, the majority of the residents of the block are "older folks", who are less likely to ring up their providers and complain  :P If it was student housing, I daresay it'd be shifted over within a week due to the number of complaints.

AC Balance is one of those things that I know "is a thing", but not quite sure what affects it; is it something which can drop in the presence of crosstalk, or is it a physical measurement of the line itself (ie, pair twist consistency / frequency, number of joints, etc). Apparently it's copper all the way to the Joint Box, and then aluminium to the DP; not ideal, or so I've heard.
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Black Sheep

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Re: AC Balance issue...
« Reply #6 on: September 15, 2016, 08:08:39 PM »

AC Balance

The AC balance is the pairs ability to reject external noise ( very important on digital lines ) such as crosstalk. If the pair is balanced electrically in terms of resistance & capacitance, it should have good AC balance.
Any line faults such as battery , earths & HR's will significantly effect the pairs AC balance .
The higher the reading in dB the better the pair, the test will fail under 45dB , but 45-55 is poor for digital services. Readings 60dB + are good ,the higher the better


Taken from our hand-held tester 'layman's guide' to testing ^^^^^.

Absolutely aluminium cable is incredibly poor with regard to AC Bal ........ but it is what it is ..... and it won't alter due to increased X-talk, only the WB Noise value will do so.

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mdc

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Re: AC Balance issue...
« Reply #7 on: September 15, 2016, 08:17:50 PM »

Hmmm, I can't help but wonder whether the internal wiring might not be at fault after all then. The sheer number of CRC errors I'm getting (errors now at 82% of total frames) don't seem to match up with a run of cable (between the JB and the NTE5) that was getting an AC Bal of 58, and with good resistance/capacitance values.

I can't help but wonder if the actual issue might be E-side :-/ apparently it tested fine at the MDF (back to the property), so possibly something on the MSAN?
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Black Sheep

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Re: AC Balance issue...
« Reply #8 on: September 15, 2016, 08:27:52 PM »

Incredibly difficult, if not impossible to give a definitive answer mdc  :'(.

Aluminium cable really is the sperm of the devil with regard to DSL signals. Without being on-site with a JDSU tester and a lot of time to spend, I'm just clutching at straws really ??.

My own thoughts on CRC's and being on Fast Path, is to hopefully allow the DLM put you on Interleaved and have FEC's instead. Yes, a slight lag to ping-times and a slight decrease in speed, but hardly noticeable unless you are a gamer or someone who sleeps with their connection stats under their pillow.  ;) :)

Stability over speed any day of the week.
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mdc

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Re: AC Balance issue...
« Reply #9 on: September 15, 2016, 08:37:20 PM »

Haha, not a hardcore gamer or anything, but I am a game developer; to be honest, my main need is throughput, not latency. Hadn't even noticed interleaving was disabled actually, if I remember rightly I had it switched to Fastpath to bypass the nasty 888kbps limit on upstream (one of the reasons I'm dying for fibre, as uploading assets that are in the GBs is a nightmare). Might be worth a call to PlusNet to change the line profile for the time being.

I suppose at this point it can't hurt to request a lift and shift as well, just in case the issue IS at the E-side. Gave a local ex-BT engineer (who now does domestic installs etc) a call RE: redoing the internal wiring and was quoted no more than around £150, which seemed pretty damn reasonable to me; always a last-resort option, trick will be convincing the landlords to foot the bill... don't want to set a precedent for self-funding repairs :P
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