What SNR Margin should I Expect?

Hi All,

I recently returned to a FTTC broadband connection after an 18 month tribulation with VM.
Prior to that I was with Sky on FTTC using the Sky provided router.

I'll ask my question first and provide my setup details at the end of the post.

Although my connection achieves 80/20 it does this with a downstream SNR margin of 6dB (sometimes slightly less)
What seems strange to me is that my Max Downstream rate is shown as ~100,000kbps which suggests significant headroom.
I get 5ms to bbc.co.uk so I guess I'm on Fastpath.

My question is:- does VDSL use the highest possible frequencies it can which have a lower SNR so that more distant connections can get good rate on lower frequencies?

When I was on Sky my downstream SNR margin was showing as 18dB.

Here's my setup...
A TD-W9970 in bridge mode connected to a TP-Link AX20.
The W9970 is connected directly to the master socket with no POTS phone services or extensions.
The W9970 is set to the required VLAN 101 so the PPPoE link to the AX20 is untagged.
I use OneStream which seems to be Vodafone for all intents.
I'm about 250m from my cabinet which is in the local exchange compound.
The cabinet is Huawei 96 and has multiple extension pods to both the DSLAM cabinet and the PCP.
These extensions have been added since I was with Sky.

It is syncing at 80/20 …. that is the max you can get . DLM doesn’t need to tweak it down, as it runs stable at 6snr. It’s doing what I would expect it to.

You're synced at the highest rate you can get, and 6dB is a very healthy margin. There's nothing more you need to try and achieve.

For comparison, here are my own FTTC line stats (Plusnet, cabinet about 700m away):
• Max DSLAM throughput: 55,000 / 15,000 kbit/s
• Attainable throughput: 47,876 / 9,058 kbit/s
• Current throughput: 47,876 / 9,058 kbit/s
• SNR margin: 6 dB down / 5 dB up
• Line attenuation: 24 dB down / 37 dB up
• Approx. line length: 750 m
• Latency: < 1 ms both ways
• Profile: 17a
• G.INP: On downstream / Off upstream
• Bitswap: On both directions

My line is G.Fast, so the attenuation looks odd. Am about 100/110m from the cab.

uptime:12 days,01 Hours27 Mins51 Secs
Data rate:49.216 Mbps / 329.045 Mbps
Maximum data rate:62.991 Mbps / 398.424 Mbps
Full Fibre (FTTP) Mode:On
Noise margin:12.7 / 7.9
Line attenuation:0.0 / 28.6
Signal attenuation:0.0 / 28.6
VLAN id:101
Upstream error control:Off
Downstream error control:Off


Loads of spare shown on the max data rate.

Your stats are totally irrelevant for FTTC/SOGEA products.
Not sure what the OP is expecting 80/20 is the best you can get and a perfectly good SNRM.

In reply to a post by Realalemadrid:

Your stats are totally irrelevant for FTTC/SOGEA products.
Not sure what the OP is expecting 80/20 is the best you can get and a perfectly good SNRM.

Eh? The stats are very relevant for FTTC/SoGEA. Just not relevant for a line which gets the bullseye with 80/20.

In reply to a post by DFScale:

In reply to a post by Realalemadrid:

Your stats are totally irrelevant for FTTC/SOGEA products.
Not sure what the OP is expecting 80/20 is the best you can get and a perfectly good SNRM.

Eh? The stats are very relevant for FTTC/SoGEA. Just not relevant for a line which gets the bullseye with 80/20.

My question wasn't about why I don't get better that 80mbs but why my margin is only 6dB given my max rate shows a lot of headroom.

Given my line is quite short (it has 4.5dB attenuation) I assume it can use a significant proportion of the higher frequencies but at a lower margin.
I'm wondering if the DSLAM will use the higher frequencies where it can in preference to the lower frequencies to reduce crosstalk on these lower frequencies thus helping longer lines.
Does the DSLAM use frequencies that have a margin around 6dB in preference to those with a higher margin?

I'm also sure I read something about cabinets that are in an exchange compound (i.e. exchange only lines) not using the ADSL frequencies, or at least at a lower power.

In reply to a post by Eeeps:

My question wasn't about why I don't get better that 80mbs but why my margin is only 6dB given my max rate shows a lot of headroom.

You're at max sync, so the DLM doesn't need to make any adjustments. Should you get a lot of disconnections your margin could be increased, but as you say you have a 4.5dB attenuation, its likely you will have no issues.

I'm wondering if the DSLAM will use the higher frequencies where it can in preference to the lower frequencies to reduce crosstalk on these lower frequencies thus helping longer lines.

At max sync I don't think the DLM will do anything to your line.

Don't understand your comment. Please tell me in what way Zarjaz's G.Fast stats are relevant for FTTC/SOGEA. I don't believe they are.

My stats show a short line, hitting the profile cap for the product in use, as does the OP’s opening post.

I cannot post VDSL stats, as I don’t have that service on my line. There are enough similarities between the ways VDSL and G.Fast work to make comparisons.

The DSLAM in the cabinet backed off transmit power to save electricity and increase longevity most likely. No point in wasting energy and driving components harder pushing the SNR any higher when the signal is already reaching you at quadruple the bare minimum.

In reply to a post by XGS_Is_On:

The DSLAM in the cabinet backed off transmit power to save electricity and increase longevity most likely. No point in wasting energy and driving components harder pushing the SNR any higher when the signal is already reaching you at quadruple the bare minimum.

That makes sense.
Dropping the TX power from the DSLAM would also help reduce crosstalk where there are many lines bundled together.
I note that my upstream stats do show a higher margin which I assume is because the power can be left higher at the modem end.

Current Rate (kbps) 19999/79999
Max Rate (kbps) 31033/100775
SNR Margin (dB) 13.2/6
Line Attenuation (dB) 6.7/4.5
Errors (pkts) 64/0

Just got some more stats from the telnet interface...

dataPath=Fast
interleaveDepth=0
lineNumber=0
upstreamCurrRate=19999
downstreamCurrRate=79999
upstreamMaxRate=31040
downstreamMaxRate=100575
upstreamNoiseMargin=132
downstreamNoiseMargin=60
upstreamAttenuation=67
downstreamAttenuation=45
upstreamPower=-52
downstreamPower=117

I assume the stats in dB have a decimal place so upstream power is -5.2dB and downstream 11.7dB

Close: the unit is decibel-milliwatts for power, dB is a relative measure only while power is absolute 😊

Just to be clear, OP must be right about the decimal point.

117dBm would be over 500GW, nearly half the capacity of the EU. 11.7dBm is a more reasonable 15mW.

I think you mean half of 1GW
30dBm =1W
and each additional 30 is a factor of 1000
so 60, 90, 120 would correspond to k, M, G respectively?

(117dBm is 3dBm below 1GW)

For things like attenuation, it can be in dB without needing to refer to any absolute value,
since 0dB would be a reference for the full strength of the signal where it was transmitted,
and -20dB would be an attenuation of 20dB or equivalently that it was received at 1% of the transmitted level.

Edited by prlzx (Sat 11-Oct-25 13:03:22)

In reply to a post by Eeeps:

Just got some more stats from the telnet interface...

dataPath=Fast
interleaveDepth=0
lineNumber=0
upstreamCurrRate=19999
downstreamCurrRate=79999
upstreamMaxRate=31040
downstreamMaxRate=100575
upstreamNoiseMargin=132
downstreamNoiseMargin=60
upstreamAttenuation=67
downstreamAttenuation=45
upstreamPower=-52
downstreamPower=117

I assume the stats in dB have a decimal place so upstream power is -5.2dB and downstream 11.7dB

No, they are probably in centiBels - cB

You are right. 500MW

G.fast is faster than 80/20 or supposed to be.

So I tihnk the OP is asking why his line is only running at 80/20. Why not 100/20 or 90/20 etc

The OP doesn’t have g.Fast on his line. He has FTTC. Top sync is 80/20

Opps.

Tired brain

I think this is the OP's question:

My question is:- does VDSL use the highest possible frequencies it can which have a lower SNR so that more distant connections can get good rate on lower frequencies?

and I think the answer is no. I think the line optimisation system deals with individual lines and does not try to "trade off" one line's settings with another. This sounds like "vectoring" which I'm pretty sure has not been rolled out by Openreach.

In reply to a post by Eeeps:

I get 5ms to bbc.co.uk so I guess I'm on Fastpath.

Fastpath ?, maybe, but not conclusive.
I'm on 80/20 FTTC with interleave depth = 8 (i.e. NOT fastpath), and just pinged bbc.co.uk @ 5.089ms.

In reply to a post by Eeeps:

My question is:- does VDSL use the highest possible frequencies it can which have a lower SNR so that more distant connections can get good rate on lower frequencies?

I wonder whether viewing your modem's VDSL tone bits/BIN graph would reveal anything interesting regarding the frequencies used ?.
Unfortunately the TD-W9970 appears to not include the tone graph in it's GUI, but Grok suggests that it's possible (but slightly tedious) to enable Telnet on the the TD-W9970 to allow a nearby PC running RouterStats, to plot the tone graphs.

This sounds like "vectoring" which I'm pretty sure has not been rolled out by Openreach.

A handful of Openreach cabinets do have vectoring enabled.
What criteria was used to decide that it would be deployed on a particular cab I know not. Unless you had prior knowledge, that the line was vectoring enabled would be mentioned in the notes .. but the usual sign was graffiti on the inside of the cab door . Written to stop engineers thinking they were on a faulty DSLAM port due to the greatly increased time to gain sync.

Back when I was on a VDSL2 connection, I also had a solid 80/20 connection. With a max attanable of around 100mbps.

It sucks they dont let the line run as fast as it can but they dont do that as it would cause more support calls due to unstable conections etc etc

In reply to a post by Zarjaz:

A handful of Openreach cabinets do have vectoring enabled.
What criteria was used to decide that it would be deployed on a particular cab I know not.

Possibly a response to a lot of reports of poor speeds/low SNRs due to crosstalk in the cabling to the customer premises connected to a particular cabinet, but I have no idea how comprehensive the OR data on such things would be.

It's a bit of a vague memory but I believe vectoring was used in some rural areas with long D-side lines to reach a particular download speed required by the installation contract, presumably BDUK deployments. Not sure of the figure but so called superfast speeds of 25 or 30Mbps seem likely.

Edited by Realalemadrid (Mon 27-Oct-25 14:55:41)

In reply to a post by rippedcotton:

In reply to a post by Zarjaz:

A handful of Openreach cabinets do have vectoring enabled.
What criteria was used to decide that it would be deployed on a particular cab I know not.

Possibly a response to a lot of reports of poor speeds/low SNRs due to crosstalk in the cabling to the customer premises connected to a particular cabinet, but I have no idea how comprehensive the OR data on such things would be.

I was experiencing some unexplained drops even with what seemed to be a very good line.

So I tried connecting the modem directly to the 'test socket' and was surprised to see a 2.5dB improvement in downstream margin and about 8Mb/s in the max rate.
The socket is about 3 years old and the pins on the PCB that mate with the test socket are simply tinned.

I reckon my line could easily provide a stable 100Mb/s if allowed!

I've unclipped the plastics in the NTE5C faceplate and removed the PCB.
The faceplate cover is now back in place with the wire feeding out through one of the lower cut-outs.

Stats are now ...


Upstream Downstream
Current Rate (kbps) 19999 79999
Max Rate (kbps) 34069 107919
SNR Margin (dB) 14.9 8.6
Line Attenuation (dB) 6.8 4.5