FTTC First 10 days?

Hi,

I had FFTC installed on the 13th Jan. The first 5 days the line was syncing with an attainable rate of over 90/30 giving me close to the full 80/20 bandwidth with almost no errors including HEC

I left the country Sat evening and since then the attainable rate has dropped by a third and the speed by almost 40% on downstream and the HEC error rate went pretty high for a while.

I can see on my BQM that the average latency has doubled at 4 AM this morning so I guess that is the line management kicking in a new profile as the SNR went from just over 3db to 6 db

Is this normal. I thought in general the line management improved things over the first 10 days not made them worse?

Could me not using the line for the second 5 days of the training period for other than for a small amount of remote access be a factor?

Finally can anyone explain very simply what causes HEC errors. The Wikipedia explanation is head bending

Thanks

Stuart

Apparently with FTTC lines start as fast as they can go and the DLM works down from there depending on errors, performance and dropped connections.

Sounds like you had a noise surge somewhere that has affected the line. Not using the line should make no difference to the DLM at all.

Thanks Ian.

It's strange that it has happened when I am not there. Possibly the neighbours central heating all kicking in for the cold snap I'm missing here in India! Although we are all detached houses and not especially close

My internal wiring is very simple twisted pair to master socket from the modem and just one hardwired internal phone extension to a small PBX with high quality cable which the BTOR engineer checked out

There is no training period as such on FTTC. As Ian says, it starts off as fast as it can go, and any major adjustments are made during the night of the second day of use, unless it goes completely haywire during the first two days. In which case they are made earlier.

Does FTTC speed increase again after a period of stability or is that it now and I'm stuffed with dramatically slower speeds?

The speeds can recover, mine dove to 15 Mbps at one point, then settled after a few weeks back at its more normal 20 Mbps (long line)

In reply to a post by stuartfletcheruk:

Does FTTC speed increase again after a period of stability or is that it now and I'm stuffed with dramatically slower speeds?

Mine used to bounce in and out of interleaving. Typically it took 14 days for it to be removed. Thankfully since a modem firmware upgrade it's stayed disabled.

Out of interest what modem are you using?

I could not use the supplied TalkTalk super router as it doesn't support modem only mode

I have a Billion 8800N which seems to get good feedback for line stability

The OR engineer kindly gave me an ECI MK3 modem which at some point I was planning to put on the modded firmware that shows line stats

In reply to a post by stuartfletcheruk:

I left the country Sat evening and since then the attainable rate has dropped by a third and the speed by almost 40% on downstream and the HEC error rate went pretty high for a while.

A drop in speed of 40% in one go is probably enough for your ISP to be able to call out an Openreach engineer to investigate.

However, I guess it really depends on what the current estimated speed range is, and where your speed now stands relative to that.

I can see on my BQM that the average latency has doubled at 4 AM this morning so I guess that is the line management kicking in a new profile as the SNR went from just over 3db to 6 db

Yes, it probably is DLM.

In the FTTC cabs, DLM leaves the target noise margin at 6dB; it never changes. When DLM intervenes, it normally turns on FEC and interleaving; The former causes you to lose 10-20% of your bandwidth, while the latter causes you to lose 8-10ms of latency.

DLM can intervene further, if previous interventions have not proven effective enough. Then you might lose more latency, and a higher proportion of your speed (I've seen up to around 30% at worst).

Is this normal. I thought in general the line management improved things over the first 10 days not made them worse?

DLM's job is to improve quality, by sacrificing speed and latency.

You should see DLM as having 2 jobs: the first is to establish a stable, quality connection quickly: this it does within 2-3 days since installation. Its second job is to keep monitoring that quality is maintained.

The reason the second part is required is because VDSL2's biggest downside is that it suffers from the effects of crosstalk - noise from other subscribers. Because of this, you can expect the noise levels to only ever increase after the initial installation - randomly - and, hence, expect that error rates increase. This has the dual effect of reducing speed and, eventually, triggering DLM, which then reduces speed further, and adds latency. A double whammy.

There are two solutions:
a) vectoring, a technical solution requiring a BT rollout which might be a few years away.
b) G.INP, a new technique that DLM can use instead of FEC/interleaving, saving that extra drop in speed and additional latency. BT are rolling this out now, so we don't have much experience.

For now, you should expect that DLM will keep watch over your stability, and that you'll lose speed.

Finally can anyone explain very simply what causes HEC errors. The Wikipedia explanation is head bending

I understand the ADSL and VDSL2 protocols fairly well, and thought that HEC errors only applied to a protocol carried by ATM (ie original ADSL only) and PPPoA. The error indicates a problem in the ATM cell addresses.

In FTTC, we use ethernet as the lower protocol layer, and PPPoE; HEC shouldn't apply.

The thing to watch is the ES rate, and see how many accumulate over 24 hours.

Many thanks for the informative and interesting reply. It is appreciated