Unexpected jump in FTTC sync speed and SNR margin change

Hi all,

I’ve noticed something unusual with my FTTC connection and was wondering if anyone else has seen similar or if there have been any recent changes on the BT Wholesale side.

My ISP is BT Broadband. For a long time, my line was syncing in the ~7,000 Kbps range and was fairly stable. However, after a resync last night, it has suddenly jumped to around 79,999 Kbps.

What’s also changed is the downstream target SNR margin, which now appears to be around 1.6 dB, whereas it previously sat at the usual 4 dB target.

Has there been any recent change in BT Wholesale or Openreach DLM behaviour that could explain this? For example, a profile change, removal of banding, or a shift in how target SNR margins are applied?

It feels like the line has either been reset or moved onto a different DLM profile, but I haven’t made any changes on my end.

Any insight would be appreciated, especially if anyone has seen target margins drop this low recently or had similar full-rate syncs appear after a resync.

Thanks in advance.

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FTTC Stats:

xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason: 2
Last initialization procedure status: 0
Max: Upstream rate = 20647 Kbps, Downstream rate = 77948 Kbps
Bearer: 0, Upstream rate = 20000 Kbps, Downstream rate = 79999 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 Kbps
Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode(0x0)
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 1.6 4.4
Attn(dB): 17.1 0.0
Pwr(dBm): 13.9 7.2
VDSL2 framing
Bearer 0
MSGc: -6 20
B: 130 239
M: 1 1
T: 0 64
R: 8 0
S: 0.0518 0.3819
L: 21468 5028
D: 16 1
I: 139 120
N: 139 240
Q: 16 0
V: 14 0
RxQueue: 60 0
TxQueue: 15 0
G.INP Framing: 18 0
G.INP lookback: 15 0
RRC bits: 0 24
Bearer 1
MSGc: 186 -6
B: 0 0
M: 2 0
T: 2 0
R: 16 0
S: 5.3333 0.0000
L: 48 0
D: 3 0
I: 32 0
N: 32 0
Q: 0 0
V: 0 0
RxQueue: 0 0
TxQueue: 0 0
G.INP Framing: 0 0
G.INP lookback: 0 0
RRC bits: 0 0
Counters
Bearer 0
OHF: 0 37089
OHFErr: 0 113
RS: 2598366576 1668918
RSCorr: 4782889 0
RSUnCorr: 0 0
Bearer 1
OHF: 2103034 0
OHFErr: 0 0
RS: 25235673 0
RSCorr: 117 0
RSUnCorr: 0 0

Retransmit Counters
rtx_tx: 2264 456546
rtx_c: 268815 1309864
rtx_uc: 686069 2846007

G.INP Counters
LEFTRS: 6078 7462
minEFTR: 79999 19999
errFreeBits: 882998535 576987952

Bearer 0
HEC: 0 0
OCD: 0 0
LCD: 0 0
Total Cells: 901795069 0
Data Cells: 52209580 0
Drop Cells: 0
Bit Errors: 0 0

Bearer 1
HEC: 0 0
OCD: 0 0
LCD: 0 0
Total Cells: 0 0
Data Cells: 0 0
Drop Cells: 0
Bit Errors: 0 0

ES: 1689 15447
SES: 97 144
UAS: 149 110
AS: 33781

Bearer 0
INP: 48.00 0.00
INPRein: 0.00 0.00
delay: 0 0
PER: 0.00 6.13
OR: 0.01 33.91
AgR: 80614.82 20033.74

Bearer 1
INP: 4.00 0.00
INPRein: 4.00 0.00
delay: 3 0
PER: 16.06 0.01
OR: 95.62 0.01
AgR: 95.62 0.01

Bitswap: 24409/24409 1279/1279

Edited by Bobby_Valentino (Mon 06-Jul-26 11:41:22)

A jump from 7000 to 79999 would be enormous. Did you have a typo on the first and should have been 70000? If so then it is possible you are benefitting from most lines and voice moving to FTTP.

That's an incredible increase (in the true sense of the word incredible!).

The only reason I can think of for such an increase would be your line being moved to a different FTTC cabinet, but that doesn't seen credible either!

Either way, I hope it stays like that and you benefit from the improved speed.

In reply to a post by ian72:

A jump from 7000 to 79999 would be enormous. Did you have a typo on the first and should have been 70000? If so then it is possible you are benefitting from most lines and voice moving to FTTP.

No typo hence this very post! I'm quite surprised myself with this huge jump in sync speed albeit with a very low SNR downstream margin!

I would be thinking of some massive problem being removed, such as another circuit running along the same route being turned off [neighbour gone on holiday? got starlink or mobile broadband?] or some extraneous earth connection being removed or some tidying up in a junction box. Or some combination of these or similar scenarios.

In reply to a post by DFScale:

I would be thinking of some massive problem being removed, such as another circuit running along the same route being turned off [neighbour gone on holiday? got starlink or mobile broadband?] or some extraneous earth connection being removed or some tidying up in a junction box. Or some combination of these or similar scenarios.

Well the biggest change has been the downstream SNR target. It now sits at 1.6 dB (hence the extra/higher sync speed). This is very low indeed. The target I've seen previously has always been fluctuating around the 4.0 dB mark.

Edited by Bobby_Valentino (Mon 06-Jul-26 12:42:59)

Your line attenuation figures are similar to mine when I had FTTC and normally it would sync at around 70,000 Kbps that is why your 7000Kbps is very hard to believe, have you really had an FTTC connection running at less than 8000 Kbps which is the old ADSL max top speed, surely you would have noticed and got something done about it.

For some reason the link will not show up but it is in the following thread of yours
FTTC Interleaving question 10th May 2025

The above post confirms my suspicions as you had a sync speed of 72140 Kbps at that time.

I can suggest a possible reason for the increase to full sync speed as it happened on my line. Whenever there was local power cut which was fairly often in a semi rural area with overhead distribution lines my Huawei HG612 modem would come back on and sync at 79999 Kbps with 1.6db SNRM. Other modems and routers on the cab must have been slower starting up so there was lees crosstalk for a short time. My line would run at this rate for months with hardly any errors, which was lucky as I wanted to avoid restarting the modem and getting a lower sync speed.

Edited by Realalemadrid (Mon 06-Jul-26 13:38:06)

Yeah, some dreadful bit of line plant replaced maybe …. or as someone mentioned, a big noise source removed.

I’d be a tad worried with such low SNR .

Have you checked https://www.broadbandchecker.btwholesale.com/#/ADSL/... to see what cabinet you are on? Do you remember what cabinet you were on and has it changed?

In October 2019 when I first got upgraded to FTTC from an EO Line I was connected to Cabinet 20 in Old Montague Street.

A couple of years ago my FTTC cabinet then got connected to Cabinet 7 in Wentworth Street under the same Exchange BISHOPSGATE here in Central London. This happened completely out of the blue and I did experience a slightly lower line attenuation. But it didn't really make much difference to my speed as I was already receiving 80/20Mbps at that time despite a cabinet that is 0.3 mile further away.

After that due to cross talk my speeds dropped slightly to mid 70Mbps and only occasionally went back up to 80Mbps but dropped back down again.

Eventually of-course I upgraded to Community Fibre this March as I found it was a no-brainer even with CGNAT.

Has there been an increase in FTTP roll-outs in your area? Have you noticed whether more customers from your cabinet exchange switched to an Altnet? This will improve cross talk. Though, the difference shouldn't be as substantial as what you are noticing!

Keep in mind that 1.6dB does not mean your SNR will be able to maintain it long term! I was also getting this sort of dB with ADSL2+ and achieving a max speed of 18Mbps! But this did not last for more than a few hours before the connection would drop-out constantly re-syncing to 3dB.

In my case my line was so faulty that even with a 6dB SNR margin the connection could not be maintained for more than 3-4 days. I needed to manually contact my ISP to cap the SNR to 9dB to maintain stable sync. But this would drop my speeds to 12Mbps.

Anyway, your situation may be different under FTTC. But such a low decibel is extremely fragile and has a very high risk of a re-sync!

As my previous post seems to have been ignored by yourself and other posters and you have not confirmed that your reported 7000Kbps speed was actually a typo making the entire thread pointless and wasting people's time.

Here is the post you made on 10th May 2025

https://forums.thinkbroadband.com/fibre/t/4776597-re...

Just confirming that the line stats show a sync speed of 72140 Kbps.