G.dmt.bis v ADSL2?

Line has been unstable over Xmas using both ADSL2 and ADSL2+ modulations (Billion 7800N router) so yesterday I tried G.dmt and now the connection is rock solid.

> adsl info --stats
adsl: ADSL driver and PHY status
Status: Showtime
Retrain Reason: 1
Max: Upstream rate = 1312 Kbps, Downstream rate = 10624 Kbps
Channel: INTR, Upstream rate = 1024 Kbps, Downstream rate = 10144 Kbps

Link Power State: L0
Mode: G.DMT
TPS-TC: ATM Mode
Trellis: ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 6.4 10.0
Attn(dB): 34.0 17.0
Pwr(dBm): 19.8 12.3
G.dmt framing
K: 318(0) 33
R: 12 12
S: 1 4
D: 64 4

It appears from the stats that the connection is interleaved G.dmt.bis rather than interleaved ADSL2 (which I thought were the same, i.e. G.992.3?) The symptoms of the instability using interleaved ADSL2 and interleaved ADSL2+ were high error rates and a reduction in the upstream SNRM from 9dB going to below 6dB which would not recover until a resync. Quiet line tests were OK though.

The connection is TTB partial LLU (SMPF) and now together with using the Billion's G.dmt modulation setting there don't appear to be any issues. Any ideas why G.dmt.bis with almost identical sync rates and interleaving depth to ADSL2 should be better? Possibly not specifically a router fault since with other routers both ADSL2 and ADSL2+ modulations have been unstable...

Edit: with the Billion's G.dmt setting CRC, ES and bit errors, on the upstream, are no longer occurring when there is an incoming phone call.

Edited by 4M2 (Tue 16-Jan-18 19:50:03)

In reply to a post by 4M2:

Any ideas why G.dmt.bis with almost identical sync rates and interleaving depth to ADSL2 should be better?

Cuz you are not on G.DMT despite what the router reports. Max Downstream on G.DMT (G.992.1) is 8 Meg.

Thanks for your interest and the reply.

Just guessing but perhaps the Billion router's G.DMT modulation setting is for 20cn exchange kit and hence ADSL MAX - as you say up to 8Mbps downstream (and Annex A 448Kbps upstream.) Yet for some reason with TTB LLU exchange kit the sync (following "xDSL G.994 training") is G.992 a.k.a G.DMT.bis.

The line was unstable over Xmas which I thought might be temporarily due to Xmas lights but after the lights were taken down the instability continued both with ADSL2 and ADSL2+ modulation settings. So I thought I would try the D.GMT setting expecting, as you rightly suggest, ADSL1. However much to my surprise I got a very stable ADSL2 connection.

The PPP link did go down on one occasion shortly before the instability began so I guess it could possibly be an exchange issue rather than a router issue at my end...

ADSL Max isn't full G.DMT but BT's implementation of it with the upstream limited to 448K or 888K for ADSL MAX Premium (I think it was called) as opposed to G.DMT's true max of about 1.3 Meg, just like ADSL2.

I can't imagine a US router being built for a BT anomaly.

I was once on ADSL1 LLU & got max speeds of 8 / 1+ Meg.

I can't sus out the definition of G.dmt.bis. It's not in those router stats. Where do you see it & why do you think you are on it? Even if it's an offering of the router, the exchange would need to be capable of it. As the stats report G.DMT I would not take it that you were on anything but G.992.1. However that contradicts the speeds reported. Puzzling!

For example my router offers me ADSL2+ Annex M, the double speed upstream, but I can't get it even if I select it as the exchange kit isn't offering it.

Edited by XRaySpeX (Sat 20-Jan-18 23:14:18)

Sorry I neglected to mention that the router's system log reported "...Line 0: ADSL G.992 Started" which was the connection. Yet the stats report G.DMT although the sync speeds, interleaving, attenuations and SRNM's are virtually the same as the stats that were reported on ADSL2. Since the sync speeds etc. are virtually the same as the previous ADSL2 connection I've interpreted the current connection as G.DMT.bis. The big difference is the stability factor.

There isn't a way to select G.DMT.bis nor G.DMT.bisplus from the router's GUI, only the G.DMT setting is available. Alternatively I can select ADSL2 or ADSL2+ modulations, both of which appear to be currently unstable. It seems that by selecting G.DMT from the router's GUI I'm automatically getting some form of different ADSL2 connection than I did previously.

As I mentioned before I don't think there is a fault with the router since other routers when set to either ADSL2 or ADSL2+ also report instability, However i haven't tried an ADSL1 setting with them.

Edited by 4M2 (Sun 21-Jan-18 01:08:12)

But where are you getting the terms G.DMT.bis , G.DMT.bisplus from?

http://www.kitz.co.uk/adsl/adsl_technology.htm

Ah, they are just other terms for ADSL2 & ADSL2+ respectively.

You are running on ADSL2 with a max of 12 Meg Down. Dunno why your router's calling it G.DMT.

Well the router calling it G.DMT seems to have solved the issues What is particularly noticeable now is the lack of errors on the upstream, e.g. previously incoming phone calls would cause CRC, ES, SES and bit errors on the upstream. Quiet line tests were always OK though. Additionally, at other times, weird things happened on the upstream such as very high FEC rates which were higher than downstream FEC's, the upstream attenuation reducing to 11dB (from 17dB) whilst the downstream attenuation remained at 34dB and the upstream SNRM reducing to less than 6dB (from 9dB) and not recovering until a resync.

Makes me think there is an issue at the exchange, perhaps something to do with partial LLU (SMPF) - the line is now absolutely fine using the router's G.DMT setting, sync speed is adequate for my needs and throughput has been invariably very good.

I don't really want to get a SFI engineer to visit since with the ADSL2 and ADSL2+ settings it did look like a devilish intermittent fault that happened at random times...however there is something odd going on and it would be interesting to know what it is.

It's all a coincidence or a matter of timing!