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Author Topic: G.Fast connection  (Read 2176 times)

adslmax

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Re: G.Fast connection
« Reply #30 on: August 04, 2018, 11:39:18 AM »

Blimey. Line Attenuation 39.1dB (very long distance from the cabinet) but a good Sync Rate 241 Meg down with 33 Meg up.

Enjoy your new G. Fast Kitz.

My goodness me copper line length 3.55km to the exchange
« Last Edit: August 04, 2018, 11:42:19 AM by adslmax »
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kitz

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Re: G.Fast connection
« Reply #31 on: August 04, 2018, 12:57:45 PM »

Quote
Line Attenuation 39.1dB (very long distance from the cabinet)

No its not.  :'(  Its 120m from door to cab not sure about actual line length - perhaps 200m?   It could get 80Mbps on FTTC.  So FTTC attenuation would have been something like 8-9dB.

You have to remember how attenuation changes depending on the technology in use.  Even swapping between adsl1 and adsl2+ increases the attenuation despite the copper still being exactly the same length.  The higher the frequencies in use, then the more the line attenuates.     

g.fast uses higher frequencies, less power, different tone spacing and TDD..  all of which can have an effect on the attenuation measurement.

Quote
copper line length 3.55km to the exchange

But its the copper between the cab and home that matters.
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adslmax

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Re: G.Fast connection
« Reply #32 on: August 04, 2018, 01:00:52 PM »

Ok but why is my FTTC line attenuation changed from 11.3dB to 11.9dB? It's been staying the same Line Attenuation for a very long time even the cabinet didn't move itself lol as increase Line Attenuation caused sync rate to drop.
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kitz

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Re: G.Fast connection
« Reply #33 on: August 04, 2018, 01:24:45 PM »

Read this - What is Attenuation?

There's different attenuation measurements, the ones provided by your router are not always accurate and can change depending upon certain things - heat, weather, power, gauge of cable, number of tones in use etc.

Quote
True line attenuation - or Insertion Loss - can be measured at the DSLAM at the exchange via diagnostic tests and this figure should remain fairly static.

Only the Openreach engineer can get that figure.. and I just noticed its in the 2nd image - 8dB
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kitz

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Re: G.Fast connection
« Reply #34 on: August 04, 2018, 01:34:36 PM »

PS also bear in mind that the line is faulting and there's apparently a length of cable that needs replacing which is why I showed the 2nd image.   
Blacksheep may be able to explain some of those figures.   Im not sure what the A-B, A-E, B-E points are, although there is probably a really simple explanation.
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Ixel

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Re: G.Fast connection
« Reply #35 on: August 04, 2018, 02:28:22 PM »

As I understand it, very basically, 'E' is ground/earth, 'B' is ring and 'A' is tip. I don't know however why one is called tip and the other is called ring, perhaps it's something to do with positive and negative? Beyond that I don't know.

EDIT - Wikipedia may clarify: https://en.wikipedia.org/wiki/Tip_and_ring

EDIT 2 - Looks like I might be right perhaps? http://www.tech-faq.com/tip-ring.html

"Tip is the nickname for the electrically positive wire and Ring is the nickname for the electrically negative wire. Tip is the ground side, which is positively charged, and Ring is the battery side, which is negatively charged, of a telephone circuit."
« Last Edit: August 04, 2018, 02:32:01 PM by Ixel »
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burakkucat

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Re: G.Fast connection
« Reply #36 on: August 04, 2018, 04:53:58 PM »

PS also bear in mind that the line is faulting and there's apparently a length of cable that needs replacing which is why I showed the 2nd image.   
Blacksheep may be able to explain some of those figures.   Im not sure what the A-B, A-E, B-E points are, although there is probably a really simple explanation.

"A-B" is A-wire to B-wire.
"A-E" is A-wire to Earth.
"B-E" is B-wire to Earth.

Looking at the second image, we can clearly see that the pair (in the section of cable being examined) is faulty. The HHT even helpfully highlights, in red, the out of specification AC balance, which is shown at 44.9 dB. (I believe that an active AC balance > 60 dB is what would be expected to be seen.)

The insulation resistance, both "A-E" and "B-E", also strongly hints at a sickly pair.

[Edited to correct a typo.]
« Last Edit: August 04, 2018, 10:13:54 PM by burakkucat »
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Bowdon

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Re: G.Fast connection
« Reply #37 on: August 04, 2018, 06:32:49 PM »

Would I be right in thinking that using technology at a higher frequency like G.fast might expose any potential cable faults on the line?

The reason I'm asking is if they are having to replace a cable now for G.fast it seems very coincidental that the line could run well at full FTTC speed but nearly immediately things start faulting on G.fast. Or is this purely coincidence?
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burakkucat

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Re: G.Fast connection
« Reply #38 on: August 04, 2018, 07:48:53 PM »

A more demanding service can highlight defects in the cable pair.

The important point to remember is that xDSL technology uses the cable pair as a radio frequency transmission line. So thinking about the circuit in terms of low voltage DC or AC (at 50 Hz) would be quite wrong.

Let's make the assumption that a typical pair, in a typical cable, has, over the last 17 - 18 years, carried each category of xDSL service as soon as it became available. The upper frequency that the pair has been required to carry has steadily increased. In the beginning was G.992.1 (fmax 1.1 MHz), then G.992.5 (fmax 2.2 MHz), G.993.2 Profile 8a (fmax 8.5 MHz), G.993.2 Profile 17a (fmax 17.6 MHz) and finally G.9700/G.9701 (fmax 106 MHz, with up to fmax 212 MHz proposed). As the frequency increases so does the "skin effect". With ever higher frequencies being propagated along the pair, lesser and lesser of the metallic pathway is being used -- the current is being constrained to an ever decreasing annular ring, hence the descriptive phrase "skin effect". A less than perfect pair may have been adequate for telephony and, say, G.992.5 (ADSL2+) services but could be significantly disruptive to a G.9700/G.9701 (G.Fast) service.

I include the use of aluminium or aluminium-alloy conductors within my phrase "less than perfect pair". Aluminium is a highly reactive element which passivates rapidly in the presence of moisture and oxygen to form an amphoteric oxide. Aluminium oxide is far less conductive than the pure metal. Hence a cable pair made of an aluminium-alloy will have an outer layer (a "skin") of less conductive aluminium oxide. Which would be a big "no" when attempting to pass a high radio frequency current over such a conductor.
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kitz

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Re: G.Fast connection
« Reply #39 on: August 04, 2018, 09:03:29 PM »

Would I be right in thinking that using technology at a higher frequency like G.fast might expose any potential cable faults on the line?

The reason I'm asking is if they are having to replace a cable now for G.fast it seems very coincidental that the line could run well at full FTTC speed but nearly immediately things start faulting on G.fast. Or is this purely coincidence?

Yes, because each new technology pushes the line harder.  We are now entering a period where lines that behaved well on FTTC could show up any less than perfect copper when on g.fast.

Very simple example using the SNRM graph below.   
On a short FTTC connection they could have had 11dB SNRM but because the Margin is so good it can easily withstand such faults and remain in sync.   Note on this graph SNRM can fractionally dip to 0dB but the line still remains in sync.     If the line isn't monitored then no-one will know it does this.   If its only a few times a day then DLM wont care and no-one is ever any the wiser.
 
Put that same line on g.fast and it no longer has 11dB SNRM buffer so the connection is going to start dropping.

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kitz

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Re: G.Fast connection
« Reply #40 on: August 04, 2018, 09:14:20 PM »

Thanks Ixel and B*cat for the explanations.     
I was obviously having a slight senior or FM moment.  I was thinking about it when I was out this afternoon and it suddenly clicked what the A,B & E were.  d'oh  (fresh air rather than being in front of the PC must have been beneficial)  :D
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Bowdon

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Re: G.Fast connection
« Reply #41 on: August 04, 2018, 09:36:10 PM »

Thank you for the indepth explanations :)

This isn't something I thought about before. But reading about a couple of people who have G.fast and their speeds deteriorate over a short time makes me wonder what percentage of them are experiencing the problem, and is this something OR anticipated?
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ejs

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Re: G.Fast connection
« Reply #42 on: August 04, 2018, 09:45:18 PM »

I thought one of the ideas of G.fast was that you've got such a large amount of bandwidth that losing a few tens of Mb isn't supposed to matter, and also it's much faster to reconnect.
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Chrysalis

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Re: G.Fast connection
« Reply #43 on: August 04, 2018, 10:27:12 PM »


it shows higher number weaver as its measured on a higher frequency than adsl.
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burakkucat

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Re: G.Fast connection
« Reply #44 on: August 04, 2018, 10:57:33 PM »

There is theory . . . There is practice . . . And often the two are found not to agree.

When the Profile 8a G.993.2 service was first made available every end-user had to have an Openreach supplied modem. By implementing that rule, live performance data was obtained for every circuit. (Hence the existence of the VLAN tagged 301.) One advantage of the mass data collection was that, after suitable analysis, it became clear the average U.K. metallic pathway was capable of supporting a Profile 17a service. Hence my surprise to know that mandatory usage of an Openreach provided modem and mass performance data monitoring has not been implemented for the G.9700/G.9701 (G.Fast) service.

As to the secrets within the Openreach branded Huawei MT992 modem, I know not. No rumours have reached me; no written note has been pushed under the door of The Cattery during the early hours of the morning.  :-\
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