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[burakkucat]

So they have two PCPs for one street?

Suppose that road in Kitz' photograph runs due east - west, for example, and suppose Kitz was facing due south when taking the photograph, then it is perfectly feasible that the PCP, which is not visible, the one on the north pavement, along with its Huawei equipped cabinet serves an area to the north of that road and the other PCP, which is visible, the one on the south pavement, along with its ECI equipped cabinet serve an area to the south of that road.

Every area served by a PCP & Fibre cabinet pair has to have a boundary and in this particular case the PCPs & Fibre cabinets for two adjacent areas are situated on the road which delineates the two areas.

Thinking about it further, it makes even more sense that the second Fibre cabinet to be installed (the Huawei equipped, under the BDUK scheme) was placed near to the first Fibre cabinet that was first installed (the ECI equipped, under BTs own commercial roll-out scheme) for easy access to the fibre-feed to/from the Fibre head-end exchange.

[Bald_Eagle1]

Yes i have if my crosstalk disturber is off my SNRM of 6dB will increase to 10-11dB and if i resync at that stage the sync rate will increase by 5Mbps.

Ah. I understand now.

My increases in crosstalk have been in measurable, but smaller steps with no single disturber appearing to be a main culprit.

[Bald_Eagle1]

As Newtronstar says, attainable rate is just a load of BT nonsense really. It doesn't say anything about your performance.

When a connection is able to sync at the maximum rate for a service, attainable rate doesn't matter so much, other than to flag up if conditions deteriorate.

However, when a connection syncs way below the service maximum, yet attainable rate suggests a much higher sync speed is theoretically possible, it can act as evidence that there is or has been a fault that caused DLM to take aggressive action or an indicator that crosstalk has increased.


e.g. Attainable rate & sync speed on my connection used to be 30Mbps+.

Now they are much closer to 20Mbps (due to increases in crosstalk).

However, when I saw big & sudden swings in attainable rates (accompanied by SNR/SNRM swings), usually shortly followed by lower sync speeds, it was part of the proof needed to pursue a line fault that was finally repaired back in 2012 after almost 11 months of intermittent disconnections.

Maybe G.INP, had it been active back then,  would have masked some of the evidence of the fault, but I'm not sure it would have been able to deal with the sudden resyncs which were actually due to a corroded joint that performed better in wet, cold weather than it did in warm, dry weather.

[Chrysalis]

my attainable was 110mbit when I first unlocked modem.

When did the attainable rate become more important than the actual sync rate think we discussed that the attainable rate is an unreliable target

given we are talking about line deterioration from crosstalk its pretty important. Newt as an active member here I am surprised at the comment.

We are talking about line detoriation not "service" deterioration which are two different things.

So obviously since I initially synced at the full 80 I have to use the attainable to calculate the affect of crosstalk on my line.

Also the attainable can remain important at other times, e.g. someone may take a crosstalk hit but manage to stay synced at a very low margin e.g. 3db, they may think ahh no effect as I still am at the same speed, but the attainable will be lower than the sync speed.

[kitz]

To further add to this, which is where I think confusion may occur.   The important fact to remember is that the max attainable rate goes skewy on some routers when interleaving and error correction has been applied.   

The max attainable on my VMG8324 has always fallen in line with any drop in SNRm that has been lost through crosstalk.  So for me at least when using the Zyxel I take it as a fairly reliable measurement of speed lost from crosstalk.

Those who are able to sync in the high 70s and certainly those whose lines are capable of over 80Mbps are unlikely to have Interleaving & Error Correction applied.  The parameter which is possibly confusing the max attainable is the INP value.   

Although the INP value when set by DLM  is a static figure (ie 3, 3.5, 4, etc) you cant say yeah that INP = 3 means 'x' Mbps, because that value is applied to the sync bit-rate to calculate the level of redundancy required.   It changes each time a new sync speed is attained and afaik no router ever displays the bitrate value of those overheads.  Although there will be a formula to calculate those overheads, all these values are always hidden.

I suspect that if we knew what that INP value meant in terms of bit rate overheads for that line.... and was able to subtract that bit rate value from what some routers display as the max attainable, then we would have a closer figure to what the true max attainable would be if Interleaving and Error correction wasn't applied.   

I made a post the other day about this when talking about the HG612.

As others have said the max attainable figure goes skewy if Interleaving (or more correctly Error Correction) has been applied. 
We are uncertain why this is, but my guess would be that its something to do with the overheads required for FEC.

FEC overheads aren't part of the sync speed, yet the modem still has to transmit those overheads on top of line speed.   If we work on the basis that FEC overheads are circa 12%* and you are syncing at 40Mbps, then in all reality you are pushing 44.8 Mbps down the line. - the extra is hidden from view but theres a couple of different methods in how that overhead can be transmitted.... and how much is subtracted from sync or sent via a reserved channel or frequency (too long of an explanation for this post).  Its that overhead which seems to confuse the max calculation on some routers.

*12% is an average, but it can be more depending upon the INP value set by DLM

But basically if your line doesnt have Interleaving, Error Correction, INP and isn't one of those modems (such as the HG612 and several more where it does skew the results), then the max attainable rate is a very good indicator of line deterioration from crosstalk.   Certainly on my line I'd take it to be a fairly accurate figure.  Chrys's line doesn't have INP so it would also be reasonably accurate for him too.

[kitz]

I'm splitting off this topic - because its more moved over to discussion of Vectoring and crosstalk.

Bear with me whilst Im juggling some posts

[kitz]

Vectoring looks like it will help more on shorter lines than on longer lines.

Yep.  Vectoring should actually help those on short lines the most as those are the ones most impacted by crosstalk.
Plus its the shorter lines that have the most PCB applied to prevent their lines interfering with other longer lines.   

There's a graph here from Alcatel which shows improvements that can be made by using vectoring.



As you can see vectoring makes massive gains for lines up to 400m and some decent gains for lines up to 800-900m.
There's still some not to be sneezed at gains to be made though at  1200m.  If youre only getting 20Mbps, Im sure you'd welcome a possible increase to 30Mbps.

[GigabitEthernet]

I don't understand that graph as it seems to suggest the maximum speed at 100m is only 60Mbps...

[kitz]

Its showing the impact how far crosstalk could reduce the line to.
Maximum speed without any interference is the green line.

[GigabitEthernet]

I seem to be very lucky as my line isn't affected by crosstalk (much) at all

[Chrysalis]

fair point on INP kitz, and as you said my 110 was taken with fastpath.

All the figures I put in the post were fastpath also, I didnt include any banded or interleaved sync speeds.

[CrazyTeeka]

Can crosstalk happen in the cable from DP to NTE5 or is it normally just in the cab?

[NewtronStar]

given we are talking about line deterioration from crosstalk its pretty important. Newt as an active member here I am surprised at the comment.

I was making reference to a line which has been interleaved with error correction and most of the time that's what my line was on and the attainable rate just gave me false readings, but since G.INP has been enabled the Sync and Attainable are evenly matched.

When looking at my attainable before and after G.INP i can't see any relationship that shows up deterioration from crosstalk on this line, all i can see from past events when disturber is off the SNRM will increase and the Attainable will increase and after a reboot the sync rate will increase but never matched the Attainable, but in saying that have not been able to test that out on G.INP.

[burakkucat]

Can crosstalk happen in the cable from DP to NTE5 or is it normally just in the cab?

It can happen . . . if it is Drop Cable 10 (containing two pairs) and both pairs are carrying separate xDSL services.

For the more normal users -- those only taking one VDSL2 service -- any cross-talk would occur in the multipair cable(s) between the (fibre) cabinet and the DP.

[WWWombat]

Its showing the impact how far crosstalk could reduce the line to.
Maximum speed without any interference is the green line.

In terms of giving you an idea of how much crosstalk can really affect a line, I like the graph from the Broadband Forum's MR-257
https://www.broadband-forum.org/marketing/download/mktgdocs/MR-257.pdf

(See figure 6 on page 12, attached below).

This shows a similar picture to Kitz' picture, but adds a few interesting details
- The upper and lower triangles show the most extreme range between "no other users" to "maximum crosstalk ever possible"
- The blue crosses show the range of actual speeds for 80 users causing crosstalk
- The red circles show the range of actual speeds when vectoring is activated.

At 350m, these details correspond to:
- Absolute best: 140Mbps
- Absolute worst: 45Mbps
- Actual results with crosstalk range between 65Mbps and 90Mbps
- Actual results with vectoring range between 130Mbps and 140Mbps.

These actual results don't apply in the UK (it is for 0.4mm cable, and likely doesn't include the power masks we use to keep compatibility with exchange-based services), but it gives you a clear idea of the potential impact of crosstalk if subscriber numbers get too high: losing two-thirds of the theoretical capacity!

Incidentally, I chose the distance for the example at 350m, because one property I had service at was at this length.
BT currently gives a range A prediction of 62-80Mbps; in reality our attainable started at around 90Mbps, and gradually dropped (along with the actual speed) to 78Mbps.

The picture, in the end, agrees that crosstalk can account for losses of around one-quarter to one-third of speed. And that doesn't include the effect of DLM adding error protection.