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ADSL instability fault occurred in MA5600 due to noise on copper line network

Publication Date:  2016-09-13 Views:  69 Downloads:  3
Issue Description

After cutover (migration to MSAN network), many users started malfunctioning. After service ports are configured and modem      is connected, some users do not have stable internet because their modems lose ADSL synchronization after a random while and this may happen many times a day and last for many minutes. For some other users, we just observe the modem
trying to synchronize and fail over and over again.



Alarm Information
None
Handling Process

W  When connecting the modem at Customer house.

     We activated the port by running the command: activate portid profile-index
profile-index_number.
The modem tries and fails to activate and the port state remains at activating.

We check port configurations and status. Run display parameter portid to see the ADSL profile in which line has
been activated.

We can see the target SNR margin is too small, it must be between 6dB and 9dB.  A small SNR means a
smaller activation time but also lower capability to activate. With a target SNR set to 1dB the line may fail to activate.

Channel mode is fast, this channel mode doesn’t perform FEC error correction and thus is more
vulnerable to error that can affect line and prevent it to be activated. It is better to choose interleaved mode. 

Run display statistics performance portid current-15minutes and check port functioning and line quality within the current 15 minutes:


The command output shows us CRC anomalies on the line, CRC (Cyclic Redundancy Check) is a transmission error detection tool which adds redundancy to the errored transmitted information. CRC usually occur when data is not genuinely transmitted due to noise. According to the number of CRC generated on a line during an hour we can deduce the amount of noise affecting the line

0 CRC in 1hour : perfect line

1-5 CRC in 1hour : slightly noisy line

6-15 CRC in 1hour : noisy line

30 CRC in 1hour: very noisy line

More than 30 CRC in 1hour: abnormal line

-       Given the information above and the number of CRC on our
line in the example (1635), we can conclude there is noise in line cable.

Once the port finally synchronizes we can run display line operation 24 and see the line conditions are poor. Downstream SNR margin is 20.0dB and upstream SNR is 4.6dB while Upstream signal attenuation is 14.1dB and Downstream signal
attenuation is 26.2dB. Given that Downstream SNR margin is supposed to be at 35dB or higher for a good line quality and Downstream signal attenuation should be lower than 20dB we can see that those values are way beyond the required
values for a stable line.

Ch Check whether line activation and parameter from Main distribution frame (MDF)  
First connect the modem at MDF side, make sure network side is disconnected
Run display parameter portid and check the ADSL line-profile configured and make sure parameters such as target
downstream/ SNR margin, channel mode, minimum and maximum downstream/upstream rate are well configured.

      Run display port state portid, we see that port state is activated

No We check the line condition and observe for 10 minutes. Run display line operation portid

No We see that modem did not lose synchronization during the last 15 minutes, to confirm it we run display statistics performance portid current-15minutes


We  also see that the number of FEC and CRC anomalies has decreased significantly; it means the noise is mostly located on network side or distribution side.












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Root Cause
Noise in cable line
Solution


To fight against CRC anomalies, we can configure INP (Impulse Noise Protection) parameter which is a parameter configurable on the DSLAM toprotect lines against Impulse noise. It applies a protection on DMT (Discrete Multi-Tone) symbols, INP represent the number of DMT symbols that will be protected. The DMT symbol rate is an influence factor for conversion between INP parameter values and pulse noise duration. The DMT symbol rate is defined as 8000 DMT symbols per second in the 30a profile and as 4000 DMT symbols per second in other spectrum profiles. "INP=16" means that the system can
correct the bit errors produced in the noise duration of 16 x 1/8000 = 2 ms in the 30a profile, and 16 x 1/4000 = 4 ms in other spectrum profiles. The smaller the INP value is the better it is for line activation rate, so we usually
choose INP=0.5 or 1. Here below the extline-profile configuration:



MA5600T(config-if-adsl-0/1)#adsl extline-profile modify 20

Start modifying profile 20. New setting will take effect
automatically after


the modification succeed

Notes: If pilot tone is included in the forbidden tones, the link
may not be


activated

When port activated with annexA in mode G.992.1, 64tone is pilot

When port activated with annexB in mode G.992.1, 96tone is pilot

Press 'Q' to quit the current configuration and new configuration
will be


neglected

> Will you configure the disabled tone? (y/n) [n]:

> Will you configure the minimum INP in downstream? (y/n) [n]:y


> Minimum INP on downstream direction:

> 0--auto

> 1--noProtection(0)

> 2--halfSymbol(0.5)

> 3--singleSymbol(1)

> 4--twoSymbols(2)

> 5--threeSymbols(3)

> 6--fourSymbols(4)

> 7--fiveSymbols(5)

> 8--sixSymbols(6)

> 9--sevenSymbols(7)

> 10--eightSymbols(8)

> 11--nineSymbols(9)

> 12--tenSymbols(10)

> 13--elevenSymbols(11)

> 14--twelveSymbols(12)

> 15--thirteenSymbols(13)

> 16--fourteenSymbols(14)

> 17--fifteenSymbols(15)

> 18--sixteenSymbols(16)

> Please select (0~18) [2]:2

> Will you configure the minimum INP in upstream? (y/n) [n]:y

> Minimum INP on upstream direction:

> 0--auto

> 1--noProtection(0)

> 2--halfSymbol(0.5)

> 3--singleSymbol(1)

> 4--twoSymbols(2)

> 5--threeSymbols(3)

> 6--fourSymbols(4)

> 7--fiveSymbols(5)

> 8--sixSymbols(6)

> 9--sevenSymbols(7)

> 10--eightSymbols(8)

> 11--nineSymbols(9)

> 12--tenSymbols(10)

> 13--elevenSymbols(11)

> 14--twelveSymbols(12)

> 15--thirteenSymbols(13)

> 16--fourteenSymbols(14)

> 17--fifteenSymbols(15)

> 18--sixteenSymbols(16)

> Please select (0~18) [2]:2

> Will you configure the maximum nominal transmit PSD? (y/n)
[n]:


> Will you configure the maximum nominal aggregate transmit power? (y/n) [n]:

> Warning: The configuration of transmission mode in
extline-profile will


replace the one configured in line-profile when the transmission
mode is


enabled!

> Will you enable the transmission mode? (y/n) [n]:

> Will you set PSD mask value parameter downstream? (y/n) [n]:

> Will you set Spectrum transfer mode? (y/n) [n]:

> Will you set power management parameters? (y/n) [n]:

> Will you set DPBO parameters? (y/n) [n]:

> Will you set RFI notch configuration parameter? (y/n) [n]:

> Will you set INM downstream parameter? (y/n) [n]:

> Will you set channel initialization policy selection? (y/n)
[n]:


> Will you set network timing reference? (y/n) [n]:

> Will you set force framer setting for inp? (y/n) [n]:

> Will you set PHY-R function? (y/n) [n]:

> Will you set the G.998.4 retransmission function? (y/n) [n]:

Now apply the ext-line profile to the port having issue, run extline- config portid profile-index
extline_profile-index_number.
Run display port state portid again and see the output.

To see extline-profile and adsl line profile configurations, you can run: display adsl extline-profile profile_number and display adsl line-profile profile_number.

You can see that the number of CRC has highly decreased and modem doesn't re-initialize anymore.





Suggestions

While setting up an MDF, we should make sure grounding and cable connection are OK. We should also avoid to build MDF room next to an interference source such as base station. Also perform maintenance of copper network and make sure all lines are can carry signals properly.

END