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Very low SN margin

Posts: 1
Registered: 08-10-2015

Very low SN margin

I have a very low download SN margin.
When the broadband modem is connected it starts around 4db and slowly drops over the day. The same is true of today.
The other day it started at 4db (speed of around 6.9) and dropped to about 2 and it had been stable with no disconnects since 7am but around 8pm it went down to 0.5db and the connection dropped and resynced at around 6.6. This was with the internal wiring. I was also hearing some hiss on the line and random clicks and noise.
I have connected the modem into the master socket with the front face plate off. There is still some minor hissing but no other noise, so I know I need to replace our extension wiring.
I am still seeing a low SN margin.
I have a TG585 v8 with plusnet firmware:
Here are my stats:
Link Information

Uptime: 0 days, 2:49:10
DSL Type: G.992.5 annex A
Maximum Bandwidth (Up/Down) [kbps/kbps]: 852 / 7,316
Bandwidth (Up/Down) [kbps/kbps]: 444 / 7,224
Data Transferred (Sent/Received) [MB/GB]: 55.36 / 1.03
Output Power (Up/Down) [dBm]: 12.5 / 20.0
Line Attenuation (Up/Down) [dB]: 24.0 / 43.5
SN Margin (Up/Down) [dB]: 20.5 / 2.0
Vendor ID (Local/Remote): TMMB / IFTN
Loss of Framing (Local/Remote): 0 / 0
Loss of Signal (Local/Remote): 0 / 0
Loss of Power (Local/Remote): 0 / 0
Loss of Link (Remote): 0
Error Seconds (Local/Remote): 0 / 0
FEC Errors (Up/Down): 32 / 1,634,823
CRC Errors (Up/Down): 1,284 / 62
HEC Errors (Up/Down): 5 / 274
I am actually getting better speed than was expected by plusnet. Any ideas what is causing the low SN margin? Do I need to get it reset by plusnet?
Is the modem reporting an accurate down SN margin?
Posts: 11,097
Thanks: 2,471
Fixes: 22
Registered: 22-08-2007

Re: Very low SN margin

A warm welcome to the forums.
I think it would be useful to explain how this stuff works in simple terms.  A line has a TARGET SNRM for which the router and exchange will negotiate a synch speed to achieve that target when making a new connection.  The 'normal' target is 6dB, but it can be set at anything from 3dB upwards in 3dB steps.  If a line has a history of instability, then the TARGET SNRM will be higher than 6dB (9, 12, 15 etc.).
After establishing synch the SNRM will vary as electrical noise comes and goes.  During the evening continental AM radio interference can cause the SNRM to fall by as much as -2dB.  Faulty (or going faulty) electrical equipment can also give rise to depressing SNRM leading to disconnection.
Poor quality internal phone extension wiring can act as an aerial picking up electrical interference and thereby degrades ADSL operation.  Interference can arise from switched mode power supplies not working within specification, central heating controls, plasma TVs, white goods, CFLs.... indeed nearly anything.  The only way of locating them is to undertake detailed monitoring (see the router stats links below) and / or going around turning things off (unplug them, putting them into standby does not help).
PlusNet can only raise your SNRM to improve stability, but that will be at the expense of synch speed - it is far better to rectify the extension wiring (use good quality twisted pair cable connect only terminals 2&5), look for a source of interference and eliminate it.
Happy hunting!