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Standard User shelluk
(newbie) Wed 16-Nov-11 12:13:43
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What do you think? 3dB SNR margin problem

[link to this post]
Hi All

I hope you are sitting comfortably...

2 phone lines, only used for ADSL (though a phone is sometimes plugged in to one). Different ISPs.

Line 1:
Over the past 6 months roughly we have a reoccurring issue where the noise margin gets set at the Exchange to 3dB on the downstream. ADSL starts dropping lots and being next door to useless. BT reset it to 6dB, fine for a month or 2 until it happens again.

This reoccurred on Friday 4th November (it's always a Friday morning about 11am it happens!). Typically they think it's a phone fault for some reason (even though I test and find it isn't) and send a phone engineer who unsurprisingly says there is nothing wrong with the phone line. By Monday they've realised what the issue is normally and sorted it with out another visit. Not this time, nearly 2 weeks now and we're still at 3dB.

So Line 2:
Been fine until yesterday, now this line has now done the same thing for the 1st time. I know for a fact this line is normally at about a 10/12dB SNR margin. Now it's around 2.5/3dB and re-syncing several times an hour at least.

Annoyingly the ISP on this line was changed 2 weeks ago (the day before the issue on the other line started) so they only have data that far back, but they have said it has always had a target SNR margin of 3dB in that time.

Now the ISP of line 1 are sure it's an Exchange fault and are fighting with BT as to why they keep sending a blimin phone engineer to our office. ISP of line 2 thinks we have interference somewhere causing the problem and that the lines should run fine at 3dB anyway so you have to work out what the noise is. I've always been told 6dB is the minimum?! I have switched off every single other thing except the router (switches, monitors, phones, etc) in the same room room and it made no difference.

Here are stats off the Cisco 877 router on line 2. Unfortunately we don't have such a nice router on line 1.

rtr01#sh dsl interface atm 0
Alcatel 20190 chipset information
Modem Status: Showtime (DMTDSL_SHOWTIME)
DSL Mode: ITU G.992.5 (ADSL2+) Annex A
ITU STD NUM: 0x03 0x2
Chip Vendor ID: 'STMI' 'IFTN'
Chip Vendor Specific: 0x0000 0x71BE
Chip Vendor Country: 0x0F 0xB5
Modem Vendor ID: 'CSCO' ' '
Modem Vendor Specific: 0x0000 0x0000
Modem Vendor Country: 0xB5 0x00
Serial Number Near: FCZ134290TN
Serial Number Far:
Modem VerChip ID: C196 (0)
DFE BOM: DFE3.0 Annex A (1)
Capacity Used: 99% 100%
Noise Margin: 2.5 dB 6.5 dB
Output Power: 20.0 dBm 12.5 dBm
Attenuation: 24.0 dB 10.0 dB
FEC ES Errors: 0 0
ES Errors: 531 41
SES Errors: 429 5
LOSES Errors: 29 0
UES Errors: 353 50
Defect Status: None None
Last Fail Code: None
Watchdog Counter: 0x59
Watchdog Resets: 0
Selftest Result: 0x00
Subfunction: 0x00
Interrupts: 8403 (0 spurious)
PHY Access Err: 0
Activations: 5
LED Status: ON
LED On Time: 100
LED Off Time: 100
Init FW: init_AMR_4.0.018.bin
Operation FW: AMR-E-4.0.018.bin
FW Source: external
FW Version: 4.0.18

DS Channel1 DS Channel0 US Channel1 US Channel0
Speed (kbps): 0 19455 0 1173
Cells: 0 348270 0 3170174
Reed-Solomon EC: 0 0 0 0
CRC Errors: 0 10 0 296
Header Errors: 0 7 0 99
Total BER: 0E-0 6760E-11
Leakage Average BER: 0E-0 6760E-11
Interleave Delay: 0 0 0 10
Bitswap: enabled enabled
Bitswap success: 0 0
Bitswap failure: 0 0

LOM Monitoring : Disabled

I have caught it just before it drops once (and resets the counters when it resyncs!) and the downstream errors went through the roof.

Can I have people's opinions please? smile

Standard User shelluk
(newbie) Wed 16-Nov-11 15:29:15
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Re: What do you think? 3dB SNR margin problem

[re: shelluk] [link to this post]
So the ISP on line 2 has got BT to reset it at 6dB and we're now at about that (sometimes 5.5dB...) and a more stable line, but...

- I'm not sure why the ISP on line 1 is having an issue getting BT to do it? Normally they do it with in a day or so as well. Not nearly 2 weeks.

- Still not right as line 2 has about a 10 or 12 dB margin when operating normally, not the 5.5dB/6dB I'm not seeing as forced by BT. Haven't seen that on line 1 in a long time though.
Standard User b4dger
(knowledge is power) Wed 16-Nov-11 18:31:09
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Re: What do you think? 3dB SNR margin problem

[re: shelluk] [link to this post]
Sounds like you need to read up a bit more on 'Target' SNRM smile

Getting most ISPs to reset TSNRM is often pretty hard - just search TBB for posts...

Target SNRM is just that - if line conditions change then it will vary. So if you line is capable of more then your margin will increase and vice-versa.

FYI - As far as I'm aware.
6db is the normal TSNRM (ADSL) but 3db is the 'newish' Target for ADSL2+ lines that can cope.

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