dmt tones

Hi, My Zyxel router does line quality analysis and I've noticed that my Talktalk connection (very slow and unstable) only uses the first 128 tones (symbols). I tried this router on two other Talktalk lines, one uses only first 128 the other all 256 tones (fast connection). I queried this with Talktalk 2nd. level who said "tones, dunno about them mate" Anyone have any insight into this, it would seem to indicate that my line cannot support higher frquencies to me?

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it would seem to indicate that my line cannot support higher frquencies to me?

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That's right. It's perfectly normal behaviour - high frequencies get attenuated more, and this can make the signal too low to be usable on longer lines.

Thanks cahaddras, that was my assumption BUT (there's always a but). It seems odd to me that tone usage cuts off at 128, there is no bit loading after 128, I live 1.8km from local llu point (2.8km cable length). One of the other sites I tried the router at is a similar distance as is the third site that uses all 256 tones. My question is, is this restricted at the dslam but the isp provider as a global parameter and if so why? (bad cable plant?)

It's not just the line length and attenuation that matter - it's also the noise. The DSLAM doesn't limit the tone range, although I guess it's possible you have an obscure fault of some kind. You would need to post some line stats or DMT tool charts to get a better idea.

Edit - it's possible the modem is accidentally negotiating G.Lite instead of G.DMT, although it shouldn't if it's working correctly. You may be able to disable the G.Lite option if your modem allows this.

Edited by cahaddras (Wed 28-Mar-07 20:16:42)

Here you go,
The poor loading per tone is due to the line now being "capped" @ 512 to increase snr because it keeps dropping. Thats Talktalk "fixing" it.
It has been reported and tested by Openreach as "no fault found" on numerous occaizyxel2602> wan adsl linedata near

noise margin downstream: 18 db
output power upstream: 11 db
attenuation downstream: 48 db
carrier load: number of bits per symbol(tone)
tone 0- 31: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 32- 63: 02 33 22 24 34 22 32 03 33 01 34 34 44 33 43 33
tone 64- 95: 42 34 32 33 22 43 34 23 24 32 21 10 22 11 22 02
tone 96-127: 22 11 10 11 20 11 10 10 00 00 00 00 00 00 00 00
tone 128-159: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 160-191: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 192-223: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 224-255: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 256-287: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 288-319: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 320-351: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 352-383: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 384-415: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 416-447: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 448-479: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 480-511: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

zyxel2602>

sions.

That's dying out at tone 100, not 128, plus it's dying gradually rather than an abrupt cutoff. It's generally very low, but then that's to be expected with a 512kbps sync.

What was the problem you reported that caused TT to set a 512kbps sync?

The problem reported to Talktalk was loss of sync anything up to 20 times a day (I use Trapmon a free snmp monitor) when allowed to "free run, it connects at 2-3mb/s but upstream snr has always been an issue, at this speed it is gnerally less than 6db (and falling). At this speed the 128 cut off is more pronounced. The router is in adsl2 mode.
Big clue! had the same problem with previous ISP pipex after free upgrade to 1mb/s. The only common kit is the copper (or aluminium) Local Loop, I'm guessing here but (as an ex BT eng with 20+) I think it's a line fault. However "it tests ok". (wet finger??). I've not seen any other reference to "tone capping" but I beleive a global parameter can be set?

Aside from the fact that you've not provided any evidence for 'tone capping', there's no link between this idea and the actual problem of excessive resync.

TT have made a perfectly reasonable change. The line's unstable at 2-3Mbps and OpenReach have reported the line's sound, so they've dropped the speed to a stable one. You're apparently picking up lots of noise which TT are not responsible for solving.

It's a bit late now, but did you try eliminating faults on your extension cabling, bad filters, missing filters, etc? Did you try the modem on the test socket behind the master faceplace?

Line test results are from master socket all internal wiring 'dis'd off. I never suggested there was a link between "tone capping" and excessive resync, I just wondered if any one else had experience of this. Ofcom giudlines disagree, openreach have a responsibility to supply a connection that can sustain a "speed" that you should expect at this distance. Anyway the point of this testing is a)since the copper doesn't terminate on BT kit anymore HOW can they test it? b)In this broadband age continuity is not enough, BT Openreach nead to demonstrate line characteristics to qualify "No Fault Found", they have failed their customer (Talktalk) in not providing this, why do Talktalk not pursue this?

re:...why do Talktalk not pursue this?

Talk Talk are very new to the Broadband market.....and I suspect you've already answered the point yourself.

I queried this with Talktalk 2nd. level who said "tones, dunno about them mate"

Probably don't have much idea about anything to do with broadband.


For what it's worth; my ZyXEL gives the data below

I have a very good line; synch at 8128 with 5.5-6.1 throughput, and 'SamKnows' states;

You are approximately 679 metres from the exchange (straight line distance).


noise margin downstream: 13 db
output power upstream: 19 db
attenuation downstream: 13 db
carrier load: number of bits per symbol(tone)
tone 0- 31: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 32- 63: 00 00 00 00 23 44 56 67 88 89 9a aa ab bb bc cc
tone 64- 95: 0c cc cc cc cc cc cc cc cc cc cc cc cc cc cc cb
tone 96-127: bb bc cc bb bb bb bb bb bb bb bb bb bb bb bb bb
tone 128-159: bb bb bb aa aa aa aa aa aa aa aa aa aa aa aa aa
tone 160-191: aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
tone 192-223: aa aa aa aa aa aa aa aa aa aa aa a9 99 99 99 99
tone 224-255: 99 99 99 99 99 99 99 99 99 99 99 99 99 88 76 43
tone 256-287: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 288-319: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 320-351: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 352-383: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 384-415: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 416-447: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 448-479: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
tone 480-511: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

As I said, the ISP doesn't have any responsibility to resolve noise issues unless these are being caused by a fault with the cable itself. There is no 'expected' speed for ADSL - if the line happens to be picking up lots of noise because of the local environment then there's nothing anybody can do about it. In any case very many noise issues are caused by the customer's own wiring, which is solely the customer's responsibility.

More generally, you've gone for a cheap broadband provider and so you really cannot expect for them to provide anything but the most basic service.

thanks for that gipsydog, I guess what I'm really asking here is is this "tone capping" which has an abrupt cut off @ 128+ a fault or a restriction by my ISP does anyone else with routers capable of tone analysis experience these problems?

Perhaps your modem negotiated G992.4, aka ADSL2 Lite? As with ADSL Lite, this only uses 128 tones. I've never personally heard of this happening, but I imagine that on an ADSL2 system like TT-LLU it may occur in the same way that occasionally you see fallback to G.Lite on ADSL1.

Thats exactly how the connection behaves, like g.dmt(lite) my Zymon software monitors my router which will not connect as g.dmt(lite) it will only connect as G.DMT or multimode (adsl2). Because the drop in tone usage is "vertical" ie no 128+ tones in use (but reasonable usage up to 128) at all I,m thinking this is either deliberate (ISP setting) or there is a filter of some description on the line. Since I have seen this on another talktalk llu line (and another which uses all 256 tones) on three different exchanges I'm very baffled. I'm wondering if I have a faulty microfilter somewhere (could be any one of them?)

The Zyxel certainly supports G.992.4, but it sounds as though you have no way of controlling or even showing whether it chooses G.992.3 (full ADSL2) or G.992.4 (lite ADSL2). I guess it's possible that TalkTalk have made a mistake configuring their DSLAMs to only support G.992.4 for some lines, but I still think it's far more likely that the router is getting confused by line conditions during mode selection, especially as you're reporting bad noise problems anyway. As you've seen the problem on two lines you may have a router with faulty auto-sense that's prone to selecting G.992.4 when it shouldn't.

All I can suggest is that (a) you try the usual methods of tracking down noise problems, the first of which is to try the test socket on your master to eliminate internal wiring entirely, and/or (b) you try a different router. Problem is, whilst your line is fixed at 512kbps you can't easily tell whether this helps, so I think you first need to get TalkTalk to put the cap back up to 8Mbps.

My Zyxel 2602hw-61 is controllable either thru telnet or "zyxmon" application, however after a lengthy chat with talktalk 2nd level who did understand this time, the problem has been isolated to faulty microfilters, it seems that my filters (purchased from ebuyer) all block tones from about 550khz upwards! they have now been replaced with a bt active filtered faceplate and I now get 4mb+ and a full pattern of tones. The reason this has been difficult to isolate has been multiple faulty filters which has disconnected individually before but not all at once, any guy's thanks for the interest, happy surfing (I know I am)

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my filters (purchased from ebuyer) all block tones from about 550khz upwards

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If that was the case then they would not be ADSL filters at all, as these just provide low-pass filtering for the phone socket. The modem socket is just wired straight through to the line.

Far more likely that they were just sub-standard. They were letting interference through or abnormally loading the line, confusing the modem.