Any advantage to FTTC with LLU supplier?

Hi,

I've experienced progressive speed reductions on my FTTC connection over the last couple of months. My sync used to be a solid 56Mbps with 6dB SNRM and minimal errors but at the end of February the SNRM dropped to 3dB. The 56Mbps sync wasn't affected until three weeks later when I lost connection and had to reboot at the lower rate of 48Mbps.

The following day throughput had dropped to 9Mbps downstream and 0.12Mbps upstream so an engineer was sent. He reset the connection but the sync was lower still at 38Mbps with 6dB SNRM, sadly above the BT line speed estimate of 35Mbps. Several DLM-sensitive reboots later I was stuck on an exact 40,000Kbps sync despite there being no banding or cap.

A subsequent GEA test found an impaired junction but the engineer sent to fix that made no measurable difference. The next engineer checked the line back to the cabinet and determined that there was an AC imbalance on the dropwire. He changed the last 50m or so of overhead cable and downstream attenuation improved slightly from 26.2dB to 25.9dB but sync was back to 38Mbps again. After a couple of days there was a network initiated resync as G.INP was re-applied and I'm now on 44.7Mbps, so around 11.3Mbps down on what used to be normal.

There's now nothing apparently wrong with the line and I live in a rural area so have my doubts about the possibility of hordes of new users all coming online at the same time and eating up bandwidth. If the slowdown had been more progressive I'd have accepted the contention argument but I feel that there were sudden changes. Having tried three different routers and given DLM ample time to settle I've run out of options.

Maybe DLM isn't working as intended or has gained sentience and taken an arbitrary dislike to me. When I was on ADSL broadband with an LLU supplier it was a great relief to be free of BT profiles and over protective DLM which would limit me to painfully slow speeds for three days if offended. There are two LLU suppliers listed for my exchange but it's not clear whether they just provide a slightly more independent ADSL service or an FTTC connection which might let me have whatever speed the line is capable of without profiles and aggressive DLM. I don't want to switch to Talktalk and find that they are just reselling the BT product.

Edited by deleted (Sun 12-Jun-16 19:35:09)

DLM applies to all sellers of the FTTC service.

If the line is better the DLM will relent but it can take some weeks

Could be crosstalk. That's when the signals carried on a line 'leak' into other nearby lines. It's a known problem that particularly affects FTTC. It doesn't require significant numbers of users on a cabinet. It only requires that your telephone line run close to another line carrying FTTC.

The odds of that happening do increase as the cabinet provides service to more and more people but you could just be unlucky. Even with only two subscribers if their lines run close enough together for long enough they could both experience crosstalk interference. Conversely if your line happens to be at the edge of a bundle you could escape crosstalk effects even if your cabinet was fully subscribed.

The longer your line the more likely it is to pick up interference but any line can be affected. Pretty much all FTTC subscribers will be affected by it eventually to some extent but how soon and how badly is pretty much pot luck.

Edited by Andrue (Sun 12-Jun-16 20:17:46)

Reading your last couple of paragraphs, you seem to be blaming DLM. Is there a reason for this? That can be seen in detailed stats?

If you have G.INP activated downstream, and you aren't banded, then DLM is essentially as de-intervened as you could hope for.

A speed of 44.7 doesn't sound like a banded one. What value precisely? What is the SNRM just after a resync? Something very close to 6dB suggests no banding.

The rise in speed when retransmission activated, of 10%+ suggests that it took away old-style FEC+interleaving, but it is impossible to tell whether that was originally triggered by the faulty line, or just an increase in the error rate that tends to come along with increased crosstalk. Yes, crosstalk can happen in both big and small jumps, but if it pushes you over the (old-style) error threshold, you'd suffer the double-whammy of FEC grabbing an extra 10%+ of your speed.

Does your modem give you detailed error stats?

Thanks, I'll stay with my current ISP and try to restrain my itchy fingers from clicking the restart link for at least a month

In reply to a post by Andrue:

Could be crosstalk. That's when the signals carried on a line 'leak' into other nearby lines. It's a known problem that particularly affects FTTC. It doesn't require significant numbers of users on a cabinet. It only requires that your telephone line run close to another line carrying FTTC.

You may be right. Is there any test for that or is it something which is assumed if other tests don't find a specific fault?

In reply to a post by WWWombat:

The rise in speed when retransmission activated, of 10%+ suggests that it took away old-style FEC+interleaving, but it is impossible to tell whether that was originally triggered by the faulty line, or just an increase in the error rate that tends to come along with increased crosstalk. Yes, crosstalk can happen in both big and small jumps, but if it pushes you over the (old-style) error threshold, you'd suffer the double-whammy of FEC grabbing an extra 10%+ of your speed.

Does your modem give you detailed error stats?

I use DSLStats v5.8 . It's only been 37 hours since the last reboot so the data sample is small but here's an extract:

Total time = 1 days 13 hours 5 min 22 sec
FEC: 2410424 0
CRC: 0 337
ES: 0 239
SES: 0 5
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Latest 15 minutes time = 5 min 22 sec
FEC: 2809 0
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Previous 15 minutes time = 15 min 0 sec
FEC: 28275 0
CRC: 0 1
ES: 0 1
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0


Upstream seems to be getting the worst of CRC and SES errors and downstream the FEC. Current sync is 44.676Mbps and SNRM is 5.8dB. It seems a bit more volatile these days but only over a very small range, being briefly 6.5dB this morning

Edited by deleted (Sun 12-Jun-16 22:54:09)

There are 3 DLM stability options available to all ISP's most are known to choose the Openreach SPEED option , as default , but some choose other options, which may or may not may a detrimental difference to the sync speeds experienced due to the error rate thresholds being different between the 3 stability options,

The only other advantage / disadvantage between llu suppliers and BT Wholesale is that llu operators don't use the same network for back haul, and there is no BTW IP profile with BT Openreach GEA (LLU) services , so if you are close to the FTTC cab then you may see a slight increase in throughput if compared to a connection using BTW as the IP profile does take a small amount of potential throughput away from the EU

Apart from that possibly support standards but they are ISP specific and not LLU or BT ,

In reply to a post by tommy45:

... and there is no BTW IP profile with BT Openreach GEA (LLU) services , so if you are close to the FTTC cab then you may see a slight increase in throughput if compared to a connection using BTW as the IP profile does take a small amount of potential throughput away from the EU.

Slightly misleading there Tommy . There is no difference in GEA between BT Wholesale based ISPs and what are (at the exchange) LLU ISPs. There is no such thing as Openreach GEA (LLU). The data is simply handed over to or received from BTW or Sky/TT/Vodafone/Zen at the exchange and all treated the same from there to the user.

The loss on BT Wholesale is typically less than 3.4%, (most frequently 3.21% without G.INP and 3.31% with G.INP), of the theoretical throughput if there wasn't a transmission protocol in place. But there has to be. That below 3.4%,basically accounts for the overheads inherent in packaging the data for VDSL2 so ISPs know how much to send without losing packets at the modem.

ADSL2+ LLU suppliers have to have some figure within their own systems that do something very much like the IP Profile, else they would be dropping packets by the bucket-load at their MSANs when transmitting. They have to buffer it is some way, just like BTW does.

The same principle applies on GEA. A technical description can be found at SIN 498v7.1 Section 1.2.1. (Note for anyone reading in the future, google SIN498, as the version number updates quite frequently).

I agree that Sky and TalkTalk can often give slightly higher speeds than BT Wholesale based, but just blaming the IP Profile is over-simplistic. They too have a loss, not no loss.

Broadly, that's a huge rate of FECs downstream, but a very low level of CRC and ES. It means that you are suffering from a fair amount of noise, but it isbeing dealt with successfully.

The FEC process (much reduced when retransmission is active) is catching a lot of the flak. Unfortunately, these figures don't capture the behaviour of retransmission, so we don't see how much FEC is missing. But, because CRC is low, we know that little fails retransmission.

What we can probably guess is that your line suffered a lot of noise before, and (prior to G.INP), your line likely needed higher grade DLM intervention. Likewise when things were bad enough to drop speeds to 9/0.1.

If your line really did suffer few errors, then that has changed. Perhaps your old DSLstats graphs can tell you whether this came on gradually or suddenly.