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Author Topic: FEC issues  (Read 754 times)

nsmhd

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FEC issues
« on: July 08, 2017, 11:07:35 AM »

Hi all,
First i would like to say what a fantastic site this is full of great people and resources.
I have an issue with my FTTC connection and thanks to DSL Stats and using the unlocked OR HG612 modem i can pin point when this latest issue started.
I have been using DSL Stats since 3/6/17 and my FEC errors for downstream have been around 0. My speed had been 60308 Kbps down 14087 Kbps up.
On 20/6/17 during the snapshot taken at 16:51 SNR down had dropped to 1.3 , upstream was still ok at 5.3.
Then on the next snapshot at 18:51 there had been a resync SNR returned to 6.1 and speed dropped to 58336 Kbps Retrain Reason:4. then at 20.52 snapshot it had done another Resync to 59196 Kbps SNR still ok at 6.1 Retrain Reason:1.  Line Attn had increased form 19-19.2.
Then on 21/6/17 at 7:33 there was a further re-sync this time the Downstream had decreased to  54997 Kbps SNR went from 6.3 to 14.2 and line attn decreased back to 19. Retrain reason was 1 it has stayed this speed ever since .
This is when the FEC errors started and since then have been increasing. In the past week i have recorded how many spikes over 900 i have had :

Today 8/7/17:
9:18
01:00
***************************
7/7/17:
23:50
20:55
20:45
13:30
05:36-05:56 7*
03:53
00:56

************************
6/7/17:
21:27
07:13
06:58
06:54
02:18

***********************
5/7/17:
20:15

**********************
4/7/17:
17:58-18:12 *4

**************************
3/7/17:
21:11
00:55-00:58 *2
************************
2/7/17:
14:38
02:22

***********************
1/7/17
18:58
01:56

I called my ISP (Sky) last weekend and explained about the high SNR and the banded connection but they said they cant do anything or raise a fault unless the speed drops below my minimum speed of 53 mbps.  Im now wondering what to do next i had a similar fault a couple of months ago and OR came out but because i had to get them out as a line fault they could not do much with the Broadband and when i logged it as a broadband fault it was fixed without a house visit but it looks like the fault is back. I am on an ECI Cabinet which is 482 M away. any advice would be greatly appreciated.
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j0hn

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Re: FEC issues
« Reply #1 on: July 08, 2017, 12:25:19 PM »

It's perfectly normal.
One of those resyncs enabled interleaving+FEC.
It's just a type of error correction. FEC's are errors that were prevented from happening.
The drop in sync is also expected. Interleaving takes roughly 10% of your sync away.
The only thing you posted that I would be concerned about is the 54997 figure. If it remains this after multiple resyncs then the line may have been banded. DLM "sometimes" forgets to remove banding, but in most cases it should remove itself.

Sky is right, there's nothing they or any ISP can do. FEC's are errors that didn't happen.

Your post explains perfectly why ISP's lock down modems to give little or no stats. Sometimes a little info is a bad thing.
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kitz

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Re: FEC issues
« Reply #2 on: July 08, 2017, 01:07:10 PM »

An SNRM of 1.3 doesn't give the line much operating headroom and the vast majority of lines would highly likely experience an increase in CRCs and Err Secs when running at such a low Noise Margin.

Why your SNRM dropped to such a low figure, I dont know...  but the usual culprit is a new FTTC connection which causes crosstalk on your line.   Nothing much can be done about crosstalk atm, weve all experienced it when neighbouring lines take up FTTC.  :/

Quote
at 18:51 there had been a resync SNR returned to 6.1

At this point your line will have been struggling badly, the Errored Secs will have turned into SES -> LOS and at some point caused the line to retrain bringing it back to a more sensible headroom of ~6dB.

Quote
Then on 21/6/17 at 7:33 there was a further re-sync

This will be DLM.   It will have spotted all the previous day's errors and is trying to stabilise your line.  It has various parameters and usually the first step is the addition of INP [Interleaving and RS encoding].

It is the RS encoding which will be causing the FECs - See here for more info.
Unfortunately it also causes a reduction of sync speed to allow for overheads.

900 FEC isnt too concerning..  I quite often will get circa 1500pm when my line is interleaved, and I have had higher than that.


----
Sometimes the problem with DLM is that it takes action the day after regardless if the line has already taken its own action to stabilise :/
The problem with yours is that from the sync speed you 'may' have been banded.   Full line stats would be useful.


Above explanation is dumbed down version.  More info about DLM and types of Errors are available on the main site.
ETA.  I started this post before I saw j0hn's reply.  I seldom post these days as it takes me so long to type anything out...  and usually by the time I have, someone else has responded  :D
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ejs

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Re: FEC issues
« Reply #3 on: July 08, 2017, 01:58:29 PM »

900 FEC over what time interval? If it's one minute or even 10 seconds, it's nothing.
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nsmhd

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Re: FEC issues
« Reply #4 on: July 08, 2017, 02:22:15 PM »

Many thanks for all your reply's and the clarification on FEC errors that makes sense now. Sorry i forgot to post my full connection stats:
Stats recorded 08 Jul 2017 14:14:38

DSLAM/MSAN type:           IFTN:0xb206 / v0xb206
Modem/router firmware:     AnnexA version - A2pv6C038m.d24j
DSL mode:                  VDSL2 Profile 17a
Status:                    Showtime
Uptime:                    2 hours 38 min 51 sec
Resyncs:                   4 (since 23 Jun 2017 08:32:30)
         
            Downstream   Upstream
Line attenuation (dB):     19.1      0.0
Signal attenuation (dB):   Not monitored      
Connection speed (kbps):   54997      13676
SNR margin (dB):           14.3      6.0
Power (dBm):               6.9      6.9
Interleave depth:          1      1
INP:                       0      0
G.INP:                     Not enabled      Not enabled
Vectoring status:          5 (VECT_UNCONFIGURED)      

RSCorr/RS (%):             0.0010      0.0007
RSUnCorr/RS (%):           0.0001      0.0000
ES/hour:                   28.0      3.16

Telnet Data:
 xdslcmd info --stats
xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason:   1
Last initialization procedure status:   0
Max:   Upstream rate = 13603 Kbps, Downstream rate = 59600 Kbps
Bearer:   0, Upstream rate = 13676 Kbps, Downstream rate = 54997 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):    14.3       6.0
Attn(dB):    19.1       0.0
Pwr(dBm):    6.9       6.9

I suspect cross-talk form new connections is properly to blame OR are always at the cab's! When i upgraded to 80/20 product back in March i was on 75mbps but has been dropping ever since. If the DLM banding has got stuck what would be the best course of action?
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Dray

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Re: FEC issues
« Reply #5 on: July 08, 2017, 02:40:53 PM »

Looks like your connection has been banded due to excessive disconnections
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j0hn

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Re: FEC issues
« Reply #6 on: July 08, 2017, 03:12:10 PM »

Your line is back to fastpath so the FEC's should stop now.
The SNRM looks strangely high when the banding is only a little bit under the max attainable.
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ejs

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Re: FEC issues
« Reply #7 on: July 08, 2017, 05:31:02 PM »

It is technically possible to have some FEC with an interleaving depth of 1, which the full stats with all the framing parameters would show.
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j0hn

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Re: FEC issues
« Reply #8 on: July 08, 2017, 07:10:06 PM »

It is technically possible to have some FEC with an interleaving depth of 1, which the full stats with all the framing parameters would show.
I've seen that with ECI cabinets upstream but never saw it on the downstream. No idea how OpenReach configure things.
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BT FTTC 55/10 ECI Huawei Cab - Zyxel VMG8924 bridge mode + Asus RT-AC68U running Asuswrt-Merlin - minted on MDWS via DslStats

kitz

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Re: FEC issues
« Reply #9 on: July 08, 2017, 10:08:34 PM »

Yes it is theoretically possible because RS encoding and Interleaving are 2 separate technologies. 
By default, Openreach use a small amount of RS redundancy on the upstream without any interleaving on the [ECI] upstream.   

Ive not seen them do this for downstream though as they use INP.  INP being a combination of RS redundancy and Interleave to provide a minimum level of protection against noise.  eg INP=3 is the most common.
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kitz

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Re: FEC issues
« Reply #10 on: July 08, 2017, 10:25:56 PM »

Looks like your connection has been banded due to excessive disconnections

I'd tend to agree.  54997 looks like one of the banded params.   The excess SNR at 14.3 seems awfully high.    DLM must really have not liked something.  I would normally expect it to be around 9/10 dB.    I suppose that if the line only had 1.3 SNRM then it could well have been erroring like crazy.  :(  We dont know the normal [daily] SNRM variance.

Quote
banding is only a little bit under the max attainable.

Can sometimes happen.  The modem is supposed to take the Target SNRM into account when it calculates the max attainable. 
I'm not sure what effect banding has on the Target SNRm, but it certainly wouldnt be the first time Ive seen what appears to be some sort of odd correlation between the Target SNRm and banding.   



Unfortunately banding appears to be rather hard to get rid of.  Openreach engineer full DLM reset is the quickest way.   
What speed does the BTwholesale checker say for your line?
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MartinGoose

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Re: FEC issues
« Reply #11 on: July 09, 2017, 08:44:24 AM »

900 FEC over what time interval? If it's one minute or even 10 seconds, it's nothing.

I would be interested to know how many would be regarded as a problem over what time interval?
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Black Sheep

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Re: FEC issues
« Reply #12 on: July 09, 2017, 09:27:57 AM »

With certain REIN interference ............ you can see them incrementing in their hundreds of thousands per second, and the hand-held tester will still retain its synch.

If the REIN is more advanced, you will see the same FEC increments but with the addition of incrementing CRC's ..... this will see the HHT lose connection within seconds.
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nsmhd

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Re: FEC issues
« Reply #13 on: July 09, 2017, 09:35:14 AM »

Many thanks again for your replys  :)
Here is the BT DSL Checker results the top speed has been getting lower used to be 71 a month ago that will be down to crosstalk.:
Featured Products

Downstream Line Rate(Mbps)

Upstream Line Rate(Mbps)

Downstream Handback Threshold(Mbps)

WBC FTTC Availibility Date

WBC SOGEA Availibility Date

WBC FTTC 18x2 Provide Availability

WBC FTTC 18x2 Sim Availability

High   Low   High   Low         
VDSL Range A (Clean)   65.1   48.5   19.6   13.4   41.2   Available   --   --   Yes
VDSL Range B (Impacted)   54   32.4   16.2   7.7   26.5   Available   --   --   Yes
Featured Products

Downstream Line Rate(Mbps)

Upstream Line Rate(Mbps)

Downstream Range(Mbps)

WBC FTTP Availability Date




FTTP on Demand   330   30   --
Normally the downstream and upstream SNMR are between 5.4 and 6.4. The last time I had an issue the SNMR for downstream went as low as 0.9! Back then I was on 69mbps when I rebooted the router the next day the speed plummeted to 56mbps to bring the SNMR to 6 again BT OR confirmed there was a fault with the Broadband and it was fixed at the cabinet (I think).  I believe I have been on fastpath since OR came last and did a full DLM reset he was a great engineer and showed me the results on his machine.
I have all the graphs and stats saved from 3rd June to present if you wanted to see any more info?

I have my Hg612 modem connected into the test socket using a 0.5m shielded twisted pair patch cable. When OR were here last I got upgraded to the latest 5C master socket but I found when I was connected though the filtered faceplate I was getting lower speeds and worse SNMR even after disconnecting the extension wireing hence I have left it connected to the test socket.

Edit just seen Black sheep's reply the last time OR were out he tuned his radio to 612Khz AM on the drive and the further away we got from the house it got quieter and he said it is REIN looks like it is back :(.  We have overhead cables to the pole and then a thick cable goes to the next pole down the street though trees which in the past has also caused issues.
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nsmhd

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Re: FEC issues
« Reply #14 on: July 09, 2017, 09:45:16 AM »

Here are my full stats for this morning and also attached Bitloading and QLN:
xdslcmd info --stats
xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason:   1
Last initialization procedure status:   0
Max:   Upstream rate = 13812 Kbps, Downstream rate = 60028 Kbps
Bearer:   0, Upstream rate = 13676 Kbps, Downstream rate = 54997 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):    14.4       6.2
Attn(dB):    19.1       0.0
Pwr(dBm):    6.9       6.9
         VDSL2 framing
         Bearer 0
MSGc:      20      19
B:      247      238
M:      1      1
T:      64      13
R:      6      16
S:      0.1435      0.5562
L:      14160      3668
D:      1      1
I:      254      255
N:      254      255
         Counters
         Bearer 0
OHF:      34512152      1377113
OHFErr:      691      31
RS:      2208750025      902299
RSCorr:      6550      152
RSUnCorr:   4041      0

         Bearer 0
HEC:      5191      0
OCD:      199      0
LCD:      199      0
Total Cells:   4117900655      0
Data Cells:   29841372      0
Drop Cells:   0
Bit Errors:   0      0

ES:      33465      719
SES:      1136      2
UAS:      1265      388
AS:      79552

         Bearer 0
INP:      0.00      0.00
INPRein:   0.00      0.00
delay:      0      0
PER:      2.30      9.07
OR:      90.23      22.04
AgR:      55086.86   13697.89

Bitswap:   45301/45403      263/269

Total time = 1 days 46 min 13 sec
FEC:      320011      11973
CRC:      559880      801
ES:      33465      719
SES:      1136      2
UAS:      1265      388
LOS:      9      2
LOF:      56      2
LOM:      474      0
Latest 15 minutes time = 1 min 13 sec
FEC:      0      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:      121      0
CRC:      3      0
ES:      2      0
SES:      0      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
Latest 1 day time = 46 min 13 sec
FEC:      210      0
CRC:      22      0
ES:      19      0
SES:      0      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
Previous 1 day time = 24 hours 0 sec
FEC:      8359      226
CRC:      873      52
ES:      658      50
SES:      0      0
UAS:      37      37
LOS:      0      0
LOF:      0      0
LOM:      0      0
Since Link time = 22 hours 5 min 51 sec
FEC:      6550      152
CRC:      691      31
ES:      550      29
SES:      0      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
#
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