Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Serene Wing]

Hello experts,

I've got an odd problem with my ADSL2+ connection using a TP-Link TD-W8950ND router. and I'd really appreciate it if anyone can offer some insights into what it might be.

The connection's always been pretty stable in the past, syncing somewhere in the 3-4Mbps range. I went away for a fortnight at the end of October and switched off the router. When I came back, I found it will now only sync at a perfectly consistent 232Kbps.

The really odd thing is that the router reports an attainable rate of 3-4Mbps, and a huge SNR margin of around 28dB. It's as if it's negotiating a good rate with the DSLAM, then agreeing only to use the lower rate. My understanding is that BT will only wind this margin back to a maximum of 15-18dB, so I don't think that SNR margin limiting can be the reason.

I left it a couple of days to see if things would improve, but they haven't. I've tried connecting directly to the test socket, resetting the router to 'factory default', disabling ADSL2+ in favour of ADSL2, turning SRA Enable on and off - all to no avail. I haven't been able to try an alternate router, which would be nice because I'm pretty sure there's no physical fault with the line.

The only other thing that might be relevant is that my area is supposedly getting FTTC sometime in the next two months - I wondered if there might be some odd activity going on at the exchange around now.

Any ideas?

[broadstairs]

Would be good if you could get a full set of stats from your modem including attenuation which will give 'the experts' here a better chance to suggest a course of action. See here if you are not familiar with getting stats.

Stuart

[Serene Wing]

OK, that seems sensible. I found this table, though out of the bits I understand, nothing jumps out as obviously a problem. The attenuation is maybe a little high, but I would estimate I'm around a mile and a half from the exchange, perhaps even two miles:

Mode:   ADSL2+
Line Coding:   Trellis On
Status:   No Defect
Link Power State:   L0
 
    Downstream   Upstream
SNR Margin (dB):   27.4    6.2
Attenuation (dB):   46.5    26.9
Output Power (dBm):   12.8    0.0
Attainable Rate (Kbps):   3160    1152
Rate (Kbps):   232    1152
MSGc (number of bytes in overhead channel message):   41    50
B (number of bytes in Mux Data Frame):   0    22
M (number of Mux Data Frames in FEC Data Frame):   16    8
T (Mux Data Frames over sync bytes):   11    2
R (number of check bytes in FEC Data Frame):   16    6
S (ratio of FEC over PMD Data Frame length):   2.0000    5.0000
L (number of bits in PMD Data Frame):   128    304
D (interleaver depth):   16    4
Delay (msec):   8    5
 
Super Frames:   5959    5605
Super Frame Errors:   0    0
RS Words:   192554    76240
RS Correctable Errors:   0    0
RS Uncorrectable Errors:   0    N/A
 
HEC Errors:   0    0
OCD Errors:   0    0
LCD Errors:   0    0
Total Cells:   52580    400796
Data Cells:   68    883
Bit Errors:   0    0
 
Total ES:   0    0
Total SES:   0    0
Total UAS:   53    26

---

I also have this system log fragment (log was set to 'debug' level) but it's equally unrevealing. I was hoping for more details of the link negotiation:

Nov 10 16:00:59   user   crit   kernel: ADSL G.994 training
Nov 10 16:00:59   user   debug   syslog: rm -f /var/fyi/sys/dns
Nov 10 16:00:59   user   debug   syslog: rm -f /var/fyi/sys/gateway
Nov 10 16:01:08   user   crit   kernel: ADSL G.992 started
Nov 10 16:01:08   user   debug   syslog: rm -f /var/fyi/sys/dns
Nov 10 16:01:08   user   debug   syslog: rm -f /var/fyi/sys/gateway
Nov 10 16:01:12   user   crit   kernel: ADSL G.992 channel analysis
Nov 10 16:01:12   user   debug   syslog: rm -f /var/fyi/sys/dns
Nov 10 16:01:12   user   debug   syslog: rm -f /var/fyi/sys/gateway
Nov 10 16:01:19   user   crit   kernel: ADSL link up, interleaved, us=1152, ds=232

[roseway]

Those stats suggest that your speed is being capped for some reason. DLM wouldn't assign such a high target SNR margin. I suggest that you install Routerstats (if it works with your router) or Routerstats-Lite (which definitely does work with your router). You can then see how the SNR margin varies with time, to see if anything unusual is happening to the connection. If the connection is shown to be stable, and there aren't lots of large spikes of interference, then you will need to go back to your ISP and get the cap removed.

[gouledw]

unlikely anything to do with what's happening with fibre.  it could've just been a meagre short burst of noise which sent the connection mad for a short period of time.  have you waited the 5 days yet to get out of the banded profile?

[Serene Wing]

I haven't waited 5 days yet. Is the 'banded profile' that gouledw referred to the same thing as the 'speed cap' mentioned by roseway?

During reporting this to my ISP (PlusNet) I got deterred part-way through the process when they asked me to try an alternative router or risk being charged for a callout if that was the problem. It's a reasonable thing to ask but I haven't been able to get hold of one yet.

Also, do I really have to wait 5 days without a single disconnect in order for the banded profile to reset? There's been quite a bit of disconnecting going on as I move the router between sockets, try different line settings, ask BT to run the automated line test, etc. There'll be at least two more if I do manage to borrow a router from somewhere. Can the DSLAM not distinguish between drops due to noise and intentional shutdowns of the connection? It would seem to me that the line protocols should allow for an orderly commanded disconnect (analogous to a PPP shutdown) that doesn't affect line speed training, but I have no idea how it actually works in practice.

Thanks for the advice anyway. I will try Routerstats, but my guess is the problem originates in having the router switched off for a fortnight rather than noise on the line - certainly haven't noticed any random dropouts or instances of the SNR margin being significantly less than 28dB

[roseway]

I'm not sure how banded profiles work, but they are a sort of cap - a range of speeds within which the router has to sync. That certainly could explain the problem, but whether Plusnet are able/willing to override it I don't know.

[gouledw]

Serene Wing, look at this on PLusnet's website it tells you about banded profiles http://www.plus.net/support/broadband/speed_guide/broadband_terminology.shtml

[Black Sheep]

I can tell you now, nobody I have ever spoken with (and that's a lot of technical people), actually knows exactly how or why 'Banding' is applied to a line !! Using retrospective data viewing of a circuit, it can show as being as perfect a line as you could get. Graphical representation will show unfluctuating SNR, attenuation, MTBE's, etc etc ........ and still a banding will have been applied !!

We've resigned ourselves to the fact it's a glitch that just happens, and the way we get rid of it is to perform a 21CN circuit reset. This will take you back to default banding of 160Kbps-24Mbps, and both US and DS set to 'Fast path'. DLM will then make the neccessary adjustments over the ensuing stabilisation period.

[Serene Wing]

Well! Looks like your diagnoses of "banding" were spot on.

Plusnet called me today, but I was out walking off my Sunday dinner. Anyway, the connection was up again, and at a reasonable speed, when I got back home.

They sent me a little graph of my connection ups and downs - I can't post that, but here's what was in the message:

"Circuit is currently being tested - results to follow shortly.

The connection is dropping (2 drops in 24 hours, 15 drops in 72 hours)

<redacted name> - BOT - DSL Logged Faults
3:50pm, Sunday 11 Nov 2012
[internal]

WBC 160K - 288K Medium delay (INP 8 ) 6dB Downstream, UC Low delay (INP 0) 6dB Upstream (ADSL2+)


Copper Line Test - Pass
Network Incident Check - Pass
TAM Network Check - not available

<graph was here...>

Ellacoya: Service_Offer_54 OK

WP Connection Profile: Generic Speed 250 increased to 21meg

Upstream DSL Link Information   Downstream DSL Link Information
Loop Loss:   26.8   46.5
SNR Margin:   5.8   28.5
Errored Seconds:   0   0
HEC Errors:   0   
Cell Count:   251   6
Speed:   1112   232
 
Maximum Stable Rate (KBPS):       Fault Threshold Rate (KBPS):   
Mean Time Between Retrains (Seconds):       Mean Time Between Errors Upstream (Seconds):   
Indicative Line Quality:       Mean Time Between Errors Downstream (Seconds):

batch Id is 1-13877610648 Success
changed target to 9 and removed banding"

But of course the connection was dropping! - that was me trying out various line settings, reboots, moving the modem from one socket to another. Seems bonkers that the system treats every intentional disconnection as a reason to slug the speed a little more. Also seems bonkers that a switched-off router is treated as a rotten connection, if that's what happened.

The graph showed completely disconnected for the period I was off on holiday, so it must have definitely powered up afterwards in this 'banded' state. The first disconnection was almost a day later, probably around the time I was first thinking "this internet connection seems a bit slow, perhaps I need to reset the router"...

Thanks very much for the help. It looks as though the ISP can and will reset the "banding" if necessary, which is worth knowing. Pretty good service from Plusnet, assuming it wasn't their algorithms that stuffed the line up in the first place.

[Black Sheep]

The DLM had really banded you out of the game. That's the lowest setting there is !

Banding has been around, I'd say, for only a couple of years. Before we engineers had WHOOSH access reinstated (the ability to check classic Btw circuits), we learned about it the hard way !!
Hours and hours were spent  chasing ghosts, as you would get synch at point A, but not point B, with point B falling outside the banding due to attenuation. All the copper pair tests would pass, and you'd be going nuts wondering what the hell was going on, changing pairs etc etc .......

In sheer frustration, you'd end up ringing Btw for co-op, a lengthy process back in the day, and they'd go "Oh, whats this ?? It says there's a banded profile on the line, never heard of that before ?". As mooted, a simple reset of the circuit and all was well.
The first thing I do now is view WHOOSH, if the circuit will allow.

Glad you're back up and running as per the norm.