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Author Topic: Estimating distance to cabinet for FTTC  (Read 3076 times)

GaryW

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Estimating distance to cabinet for FTTC
« on: April 26, 2018, 10:56:34 AM »

Hi all,

This is more of an intellectual exercise than an issue...and I'd welcome thoughts from the (many) more knowledgeable folk on the forum!

So, it's possible to estimate distance to the cabinet based on line attenuation.  However, depending on which modem I use I get different DS line attenuation figures:

Billion 8900AX-2400:    27.6 dB  (DS sync at the banded limit of 15000K)
Zyxel VMG8924-B10A:  28.2 dB  (DS sync at the banded limit of 15000K)
BT HH5A:                    32.7 dB   (DS sync at around 11000K.....)

These figures were all taken within the last week or so.  To make it even more confusing, the Billion and Zyxel report signal attenuation figures of about 28.5dB and 29dB respectively, whereas the HH5A reports a signal attenuation of 25.4dB.

Dslchecker gives an upper clean limit for my line of 18.5M (it used to be 17.0M, went up to 17.5M earlier this year then up to 18.5M at the beginning of April) - I'm on an ECI cabinet.

Has anybody else seen such a wide variation between Broadcom and Lantiq based modems?  Which would be the sensible figure to use in order to estimate line length, and what would the estimate be?

(One other piece of info that may or may not help: my US sync is just over 500K with Billion/Zyxel and just over 600K with HH5A)
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re0

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Re: Estimating distance to cabinet for FTTC
« Reply #1 on: April 26, 2018, 01:38:48 PM »

The most variance I have witnessed on this current line is 0.2 dB (9.2-9.4 dB) for downstream attenuation between two different modems - 8800AXL R2 and 8800NL. Though I cannot say anything in regards to Lantiq since I won't let it touch my line as in Broadcom I trust. Don't even talk about MediaTek or I'll scream.

When comparing different chipsets and software, the displayed figure for attenuation may be interpreted differently. But not only that, slight inconsistencies in the modem at the time of manufacturing may lead to some additional (but very minor) attenuation.

I know that on the 8800NL I have, the band D3 seems to be slightly more attenuated by around 2 dB compared to the 8800AXL R2. Though, overall, the difference is not significant enough for it to have any noticeable impact on sync, attainable and the line's overall performance.

I found a nice short but explanatory reply in a different topic:
Line attenuation can only change if the physical charateristics of the line changes, e.g change of gauge of copper wire and it is only meaningful when expressed as a figure at a given frequency. However, you are not actually measuring attenuation, you are extrapolating it from power loss at various frequencies hence upstream and downstream attenuation figures are different. Different routers calculate this in different ways, and a change in tones used or a change of router can be perceived as a change in attenuation.
So, essentially, it is saying that changes you see in the attenuation are down to different modems and down to which tones are allocated. As an example, if your modem suddenly is able to allocate tones at higher frequencies due to a drop in SNR or less interference, the attenuation figure may actually see an increase by as much as a few dB because it is calculated across the tones in use. But if you get an increase in SNR or interference increases by the next time you sync, you may see a lower attenuation value as the allocation of times may occur only at lower frequencies. I believe this is true, but if someone knows I'm wrong, please point this out and correct me.

Which would be the sensible figure? There is no way of really knowing. I would be personally more inclined (biased) to believe the Broadcom figures more than the Lantiq. But that is just me.

I do not want to claim that I know how to estimate the line length on VDSL based on the provided attenuation figures. Though, I would be wary since you will only find out the attenuated distance rather than actual distance due to factors such as different gauges of cable used, and some parts could be copper with some aluminium. But if I wanted to take a guess based on how the line is performing and the approx. attenuation, I'd probably say it's close to 1.8 KM.

Could you upload some graphs from something like HG612 Modem Stats? You should be able to use it with your 8900AX. Just setup the program and use the "current stats" executable to graph all the stats for uploading.
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GaryW

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Re: Estimating distance to cabinet for FTTC
« Reply #2 on: April 27, 2018, 07:36:28 AM »

Here's my bitloading, currently using the VMG8924-B10A (bought on fleabay as an experiment to see how it compares to the Billion).  As you can see, for some reason the highest tone used is 777 even though DS1 on the ECI could go up to 857 and the SNR looks like it would be OK (and HLOG is fine).  It's also a little inconsistent as the highest tone used seems to vary fairly randomly between 777 and 789 at each resync.  Maybe it's due to the line being banded, but more tones would probably mean higher SNRM....
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re0

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Re: Estimating distance to cabinet for FTTC
« Reply #3 on: April 27, 2018, 05:18:15 PM »

Could you please provide full stats for your connection from something like HG612 Modem Stats?

As many tones as possible will be allocated as the SNR margin allows. If you're banded, then it will allocate as many tones as it requires to hit that speed - there would be no reason for more tones to be allocated otherwise it would be synchronising at a higher speed. The highest tone allocated will vary depending on line conditions and whether or not it is believed that it is needed to be allocated to sync at the banded speed.
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GaryW

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Re: Estimating distance to cabinet for FTTC
« Reply #4 on: April 27, 2018, 05:34:06 PM »

Could you please provide full stats for your connection from something like HG612 Modem Stats?

As many tones as possible will be allocated as the SNR margin allows. If you're banded, then it will allocate as many tones as it requires to hit that speed - there would be no reason for more tones to be allocated otherwise it would be synchronising at a higher speed. The highest tone allocated will vary depending on line conditions and whether or not it is believed that it is needed to be allocated to sync at the banded speed.

It would seem logical for the number of tones to be based on what you need to sync at a particular speed, but that's not quite the way it seems to work.  For example, if I resync during the day it'll use up to tone 777ish with an SNRM of 7+dB and a sync speed of 15000; if I resync late at night when SNRM is at its lowest it'll still only use up to tone 777ish with an SNRM of 6.3db and a sync speed a little under 15000 (e.g. the other night it was a little over 14500).  I live in hope of enough "green" days to get the banding raised again so I can see what happens  :)

Full stats:
Quote
xdslctl info --stats
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:   0
Max:    Upstream rate = 517 Kbps, Downstream rate = 19282 Kbps
Bearer: 0, Upstream rate = 519 Kbps, Downstream rate = 14563 Kbps

Link Power State:       L0
Mode:                   VDSL2 Annex B
VDSL2 Profile:          Profile 17a
TPS-TC:                 PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        6.9             6.0
Attn(dB):        28.0            0.0
Pwr(dBm):        5.2             5.2

                        VDSL2 framing
                        Bearer 0
MSGc:           26              33
B:              47              15
M:              1               1
T:              55              23
R:              16              0
S:              0.1047          0.9481
L:              4888            135
D:              619             1
I:              64              16
N:              64              16

                        Counters
                        Bearer 0
OHF:            26250130                52831
OHFErr:         155             16
RS:             1480024020              591219
RSCorr:         26401309                0
RSUnCorr:       7746            0

                        Bearer 0
HEC:            1942            0
OCD:            1               0
LCD:            1               0
Total Cells:    4251711005              0
Data Cells:     203853126               0
Drop Cells:     0
Bit Errors:     0               0

ES:             29              17
SES:            0               1
UAS:            22              22
AS:             151860

                        Bearer 0
INP:            8.00            0.00
INPRein:        0.00            0.00
delay:          16              0
PER:            5.78            16.41
OR:             44.26           19.00
AgR:            14606.94        537.89

Bitswap:        64652/64654             0/0

Total time = 1 days 18 hours 11 min 22 sec
FEC:            26401309                0
CRC:            155             16
ES:             29              17
SES:            0               1
UAS:            22              22
LOS:            0               1
LOF:            0               1
LOM:            0               0
Latest 15 minutes time = 11 min 22 sec
FEC:            4904            0
CRC:            0               0
ES:             0               0
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Previous 15 minutes time = 15 min 0 sec
FEC:            7825            0
CRC:            0               0
ES:             0               0
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Latest 1 day time = 18 hours 11 min 22 sec
FEC:            9535259         0
CRC:            64              8
ES:             12              8
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Previous 1 day time = 24 hours 0 sec
FEC:            16866050                0
CRC:            91              8
ES:             17              9
SES:            0               1
UAS:            22              22
LOS:            0               1
LOF:            0               1
LOM:            0               0
Since Link time = 1 days 18 hours 10 min 59 sec
FEC:            26401309                0
CRC:            155             16
ES:             29              17
SES:            0               1
UAS:            0               0
LOS:            0               1
LOF:            0               1
LOM:            0               0
NTR: mipsCntAtNtr=0 ncoCntAtNtr=0


and pbParams:
Quote
xdslctl info --pbParams
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:   0
Max:    Upstream rate = 531 Kbps, Downstream rate = 19328 Kbps
Bearer: 0, Upstream rate = 519 Kbps, Downstream rate = 14563 Kbps

Discovery Phase (Initial) Band Plan
US: (6,31) (882,1193) (1984,2770)
DS: (33,857) (1218,1959) (2795,4083)
Medley Phase (Final) Band Plan
US: (6,31)
DS: (67,777)
                  VDSL Port Details               Upstream                Downstream
Attainable Net Data Rate:             531 kbps              19328 kbps
Actual Aggregate Tx Power:             5.2 dBm                5.2 dBm
====================================================================================
        VDSL Band Status                U0              U1              U2              U3              U4              D1              D2              D3
  Line Attenuation(dB):  9.1     73.5    63.2     N/A     N/A    28.0     N/A     N/A   
Signal Attenuation(dB):  9.0      N/A     N/A     N/A     N/A    28.9     N/A     N/A   
                SNR Margin(dB):  6.3      N/A     N/A     N/A     N/A    6.9      N/A     N/A   
                 TX Power(dBm):  5.2      N/A     N/A     N/A     N/A    7.3      N/A     N/A   
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re0

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Re: Estimating distance to cabinet for FTTC
« Reply #5 on: April 27, 2018, 06:13:04 PM »

... ECI could go up to 857 ...
Correct for ECI. Huawei DSLAM band plan is up to 859 for DS1, just to note. I forgot to mention in my last post.

For example, if I resync during the day it'll use up to tone 777ish with an SNRM of 7+dB and a sync speed of 15000; if I resync late at night when SNRM is at its lowest it'll still only use up to tone 777ish with an SNRM of 6.3db and a sync speed a little under 15000 (e.g. the other night it was a little over 14500).
It looks like, from your stats, that your line is only allocating tones in DS1. 6.3 dB is quite close to 6 (which is probably your target), but it is only an average so at certain frequencies the margin may not be great enough for it to be determined that certain tones can be allocated. Perhaps it cannot allocate beyond 777 because there is no spare margin? I could be wrong in this case and maybe it is a product of banding (I cannot confirm because I've never been banded that I can recall).

Perhaps you could provide full stats in the form of graphs (including HLOG, QLN and full bitloading graph with SNR plotted)? It could help a lot as opposed to making me speculate. ;)
« Last Edit: April 27, 2018, 06:21:58 PM by re0 »
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GaryW

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Re: Estimating distance to cabinet for FTTC
« Reply #6 on: April 28, 2018, 10:32:49 AM »

The bitloading graph with SNR is in the third post above  :)

Here are QLN and HLOG which certainly look like higher tones in DS1 should be available, although the HLOG plot doesn't contain anything for higher ranges.



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re0

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Re: Estimating distance to cabinet for FTTC
« Reply #7 on: April 28, 2018, 12:28:06 PM »

In that case, I really think it must be a situation where the DSLAM doesn't think it needs to use it. I could be wrong about this, but there is no other logical reason I can think of so maybe someone else could shed some light onto this. :hmm:

The reason to why the HLog is not plotting any higher downstream bands is because they are too attenuated. I can see that there are upstream bands that have attenuation values but are not plotted, so perhaps they're only plotted if they have a distinguishable signal? Again, I am not sure.

I have attached an old bitloading graph back when the other line was on 40/2. You can see that on U1 it hardly allocates any tones and while there is sufficient SNR in other (but not all) nothing is allocated there and there is quite a sudden drop off. Of course, the line was able to sustain 2 Mbps upstream with a very large overhead (attainable approx. 30 Mbps at the time). This is the only example I have to back my educated guess.

What sort of setup do you have in regards to cabling and filtering?
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Chrysalis

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Re: Estimating distance to cabinet for FTTC
« Reply #8 on: April 28, 2018, 12:40:18 PM »

I agree re0, I believe it stops either when out of range for the DSL spec in use or if too much loop loss.
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re0

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Re: Estimating distance to cabinet for FTTC
« Reply #9 on: April 28, 2018, 12:56:53 PM »

In terms of HLog, the maximum attenuation reported on VDSL2 is 96 dB, though I would imagine it would not be possible to allocate anything at this value as this would signify the loop loss is too significant. The highest attenuation figure where tones are still allocated is around 70-80 dB in general anyway.
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GaryW

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Re: Estimating distance to cabinet for FTTC
« Reply #10 on: April 28, 2018, 09:15:44 PM »

Quote from: re0
What sort of setup do you have in regards to cabling and filtering?

Cabling is a 0.5m Tandy STP RJ11-RJ11.  Filtering is a Mk4 filtered faceplate on an NTE5C.  To be honest, I don't expect it to use anything other than US0 and DS1 given that I am a long way from the cabinet, it just seems weird that it doesn't use more of the tones in DS1 given that HLOG and QLN look OK beyond the point where it stops.  True, it doesn't need to given that the line is banded, but it would improve the SNRM and therefore would hopefullly make the line less susceptible to noise.  The latest sync uses up to tone 777, lowest it ever stops is 775...highest is 789...and it seems to be quite random.
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re0

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Re: Estimating distance to cabinet for FTTC
« Reply #11 on: April 30, 2018, 02:25:02 AM »

Do you have any microfilters? I was going to suggest using one of those in the test socket and see if it made any difference to your stats (QLN, HLog, Bitloading, etc.) I am not saying your MK4 filter has gone bad, but rather I am curious to see if the use of a decent microfilter could allow you to attain more (even so, the difference would most likely be very marginal).
« Last Edit: April 30, 2018, 02:27:37 AM by re0 »
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GaryW

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Re: Estimating distance to cabinet for FTTC
« Reply #12 on: April 30, 2018, 10:15:23 AM »

Do you have any microfilters? I was going to suggest using one of those in the test socket and see if it made any difference to your stats (QLN, HLog, Bitloading, etc.) I am not saying your MK4 filter has gone bad, but rather I am curious to see if the use of a decent microfilter could allow you to attain more (even so, the difference would most likely be very marginal).

I've tried a microfilter in the test socket recently and the results are identical (within experimental error...) to using the mk4.  I think the line is just too long to use anything beyond DS1, but all of the plots indicate that at least some of tones at the top-end of DS1 should be perfectly useable (i.e. beyond the point where allocation is currently stopping).

One thing I have done is call time on my Zyxel VMG8924-B10A experiment.  I thought it was worth a try given that a lot of people swear by them and make specific mention of the common mode noise filter, but it seems to give slightly worse attenuation and DS SNRM than the Billion (with the same tones in use i.e. 67-777) and seems more sensitive to REIN/SHINE overnight when SNRM is at its lowest.  Of course, its never truly an apples-to-apples comparison....but it would be a big coincidence if my line conditions deteriorated so much within a hour of switching to the Zyxel (and if they get better now I've reverted to the Billion it'll be verging on the miraculous  :) )
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re0

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Re: Estimating distance to cabinet for FTTC
« Reply #13 on: April 30, 2018, 07:26:06 PM »

While I can agree that the graphs would show that the top-end D1 should be usable, I think it is the case where the DSLAM has ultimately decided it doesn't need to/cannot allocate tones in those higher D1 frequencies. Whether that is because it can (typically) achieve the banded speed without them or some other reason I honestly don't know. I am not sure how the DLM works in this respect, but perhaps someone can confirm whether problematic tones can be disabled by it?

I can agree that my post was more of a speculative post rather than fact, so it may be best to take a pinch of salt unless someone can provide good facts that would back it up (because I can't :().

Sticking with the Billion 8900AX-2400 is probably a good choice anyway. Seems like a good modem-router combo, and I usually swear by Billion as they usually have good features and full SSH/Telnet access.
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anything