Curious behaviour

[#1]

For some time my connection has experienced curious behaviour - reflected in jumps in the downstream SNRM that sometime are accompanied by disconnects and re-connects. Together with two helpful people from the Plusnet side we have been trying for some time to understand what's going on with only marginal success.

This morning the downstream SNRM suddenly jumped from around 7dB to around 9.3 dB. [snapshot attached] with synch speed 64.773Mbps. Out of curiosity I decided to reboot my router and lo and behold the connection was re-established at 6.6dB and synch speed was 76.901Mbps.

I'm wondering what to make of this - anybody got an idea?

[#2]

Hi @BobR1

I also run dslstats and these S to N jumps are due to the removal of "a crosstalker".
One of xDSLs issues is interference between the pairs of cables in the Openreach large cable bundles.
This is referred to as crosstalk.
In essence another houses broadband reduces your broadband signal to noise margin.
When the other household turns their modem off your noise margin increases.
Unless the others householder has completely ceased broadband, they will at some point turn the modem back on. Then the crosstalk will return, the noise margin decrease and the error rate rise.
I suggest you do not resynch to chase these changes in margin.
Regards
Richard

[#3]

@RichardB 

Thanks for those thoughts. Vectoring is active on my downstream isn't that supposed to help with crosstalk?

[#4]

@RichardB

A further thought Richard - even if the other user's arrival was what triggered the SNRM increase, that doesn't explain why the re-boot of my router reduced the SNRM. Clearly it is highly unlikely that the crosstalk, if indeed the problem is crosstalk,  ceased at the same time as my router was re-booting.

[#5]

Vectoring mitìgates crosstalk but does not eliminate crosstalk.
In addition, vectoring only works when all the modems support it properly.
Openreach have standards for the modems but not all modem/routers on sale in the UK properly support it.
It maybe your interferer used a modem whuhc could not support vectoring.
Hardly any Openreach FTTC deployments have vectoring.
It requires additional hardware in the cabinet and that costs Openreach money for no more income.
It is mostly BDUK funded cabinets, where it can lift a sufficent number of customers into the superfast category, that have vectoring deployed.

[#6]

You misunderstand.
Assumming no banding, when switched on a modem will connect at the fastest speed it can to maintain the target noise margin (typically between 3dB and 6dB depending on cabinet and line).
The speed is then fixed (unless DLM takes action).
The noise margin will vary as interferers come and go.
Lets use the example of a neighbour turning their router off due to going away.
Your noise margin increases and the speed does not change.
You resync your modem.
The speed will increase and the margin reduces to the target margin (6dB).