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Author Topic: Actual sync significantly lower than max attainable  (Read 5257 times)

BorisTheSpider

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Actual sync significantly lower than max attainable
« on: January 22, 2016, 05:20:00 PM »


Hi,

I'm fairly technical but quite new to tweaking VDSL. Due to needing to relocate for work, I've had to split my home network, so my main fileserver and router (pfsense) are hosted remotely at my mums place, and my kit at my residence (pfsense router also) connects to that site via a site-to-site openvpn link.

As such, I really want to tweak the upstream at the hosted site, since that determines the speed at which I can access stuff on my fileserver (my own downstream here is more than adequate). The VDSL modem at the hosting site is an unlocked Openreach (Huawei) HG612 B3, running Software V100R001C01B030SP08 and Firmware A2pv6C038m.d24j - the line is TalkTalk.

I'm currently seeing:

Max:   Upstream rate = 30116 Kbps, Downstream rate = 83260 Kbps
Bearer:   0, Upstream rate = 20000 Kbps, Downstream rate = 59997 Kbps

I'm not sure how far from the cabinet the hosting site is. SNR down is 8.7db, and SNR up is 12.4db - I understand from the reading that I've done that these SNRs are quite decent, albeit not fantastic, and that I should probably be able to achieve more like the full, max-attainable 30MBPs upstream.

In fact, when I first installed it (yesterday) I did see the sync at pretty much max-attainable (not sure that it was fully 100%, but it was very close and at the time I took no real notice because of that), then a few hours later at home I saw that my VPN connection had dropped - the VDSL had basically locked up, such that the HG612 hadn't renegotiated a new sync but the line was inoperative, presumably because the sync was unsustainable. Power cycling the HG612 restored connectivity, and that's when the actual sync dropped down, presumably because of the DSLAM detecting the errors.

My feeling is, based on the SNR figures I've pasted above, that probably the max achievable downstream is too unstable, and in any case, as I explained, I'm not overly concerned about that - the 60MBPs being achieved is adequate, but since theres over 12db of SNR on the upstream side, I'm hoping that the line will be able to sync eventually at something like 60/30 or even 60/25 and remain stable.

All I've done so far, apart from some monitoring using the dslstats program, is reboot the HG612 at 14:00 today, which I understand is the time of day when I should expect it to sync fastest, but it resync'd at exactly the same speeds. I know the target SNRM is fixed with VDSL (at the DSLAM?) and that I can't do anything (other than maybe contacting the ISP) to change it. I don't want to start repeatedly resyncing the line since I know vaguely that DLM might lock me into an even lower speed profile if it detects the line going up and down regularly, but I don't know how often I'd have to reboot the modem for that to happen.

I also understand that the sync achieved may not just depend on rebooting the modem, but may also change if I remade the IP connection (ie. rebooted the pfsense box) - I don't actually understand how/why that could be the case - surely the modem is just a bridge, and the only thing that should matter in terms of sync speed would be the modem - if I've misunderstood I'd appreciate some clarification here.

I'm a bit limited in the troubleshooting I can do since what I definitely don't want to end up doing is destabilizing or bricking the remote site to the extent I have to physically visit it to fix the problem - it's quite a long way from my home, and there is no tech assistance or support available there, so if I brick it I'm in for a long journey.

Here's the rest of my data from the HG612, any suggestions are welcome, particularly relating to whether it's realistic to expect to achieve a better upstream than 20MBPs and if so how I should go about it.

xdslcmd info --stats
xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason:   0
Last initialization procedure status:   0
Max:   Upstream rate = 30091 Kbps, Downstream rate = 83024 Kbps
Bearer:   0, Upstream rate = 20000 Kbps, Downstream rate = 59997 Kbps

Link Power State:   L0
Mode:         VDSL2 Annex B
VDSL2 Profile:      Profile 17a
TPS-TC:         PTM Mode(0x0)
Trellis:      U:ON /D:ON
Line Status:      No Defect
Training Status:   Showtime
      Down      Up
SNR (dB):    8.7       12.4
Attn(dB):    16.4       0.0
Pwr(dBm):    5.8       5.8
         VDSL2 framing
         Bearer 0
MSGc:      14      150
B:      39      236
M:      1      1
T:      64      5
R:      14      16
S:      0.0212      0.3771
L:      20368      5410
D:      1853      1
I:      54      255
N:      54      255
         Counters
         Bearer 0
OHF:      8301735      1847070
OHFErr:      23901      2
RS:      2125058410      4036109
RSCorr:      7799843      5350
RSUnCorr:   645199      0

         Bearer 0
HEC:      149994      0
OCD:      4409      0
LCD:      4409      0
Total Cells:   1303676501      0
Data Cells:   6341585      0
Drop Cells:   0
Bit Errors:   0      0

ES:      64      2
SES:      35      0
UAS:      26      26
AS:      11314

         Bearer 0
INP:      5.00      0.00
INPRein:   0.00      0.00
delay:      10      0
PER:      1.36      6.15
OR:      117.41      202.87
AgR:      60114.80   20203.27

Bitswap:   5066/5068      8/8

Total time = 3 hours 9 min 0 sec
FEC:      7799843      5350
CRC:      23901      2
ES:      64      2
SES:      35      0
UAS:      26      26
LOS:      0      0
LOF:      0      0
LOM:      0      0
Latest 15 minutes time = 9 min 0 sec
FEC:      15005      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:      295736      97
CRC:      978      0
ES:      6      0
SES:      2      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
Latest 1 day time = 3 hours 9 min 0 sec
FEC:      7799843      5350
CRC:      23901      2
ES:      64      2
SES:      35      0
UAS:      26      26
LOS:      0      0
LOF:      0      0
LOM:      0      0
Previous 1 day time = 0 sec
FEC:      0      0
CRC:      0      0
ES:      0      0
SES:      0      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
Since Link time = 3 hours 8 min 33 sec
FEC:      7799843      5350
CRC:      23901      2
ES:      64      2
SES:      35      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
#



xdslcmd info --pbParams
xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason:   0
Last initialization procedure status:   0
Max:   Upstream rate = 30070 Kbps, Downstream rate = 83024 Kbps
Bearer:   0, Upstream rate = 20000 Kbps, Downstream rate = 59997 Kbps

Discovery Phase (Initial) Band Plan
US: (6,31) (882,1193) (1984,2770)
DS: (33,857) (1218,1959) (2795,4083)
Medley Phase (Final) Band Plan
US: (6,31) (882,1193) (1984,2770)
DS: (41,857) (1218,1959) (2795,4083)
     VDSL Port Details        Upstream        Downstream
Attainable Net Data Rate:       30070 kbps          83024 kbps
Actual Aggregate Tx Power:          5.8 dBm            5.8 dBm
====================================================================================
  VDSL Band Status   U0   U1   U2   U3   U4   D1   D2   D3
  Line Attenuation(dB):   0.1   19.1   27.4   N/A   N/A   11.2   23.8   37.8
Signal Attenuation(dB):   0.1   19.1   27.3   N/A   N/A   14.2   23.4   37.8
        SNR Margin(dB):   11.9   13.4   12.1   N/A   N/A   8.7   8.7   8.7
         TX Power(dBm):   -5.9   -25.3   5.4   N/A   N/A   8.3   7.4   7.4
#


Line attenuation (dB)       U0   U1   U2   U3   U4   D1   D2   D3
21 Jan 2016 22:21:49      0.1   19.2   27.4   N/A   N/A   11.1   23.6   37.8
22 Jan 2016 14:01:48      0.0   0.0   0.0   N/A   N/A   11.2   23.8   37.8
22 Jan 2016 14:02:18      0.1   19.1   27.4   N/A   N/A   11.2   23.8   37.8


Stats recorded 22 Jan 2016 17:11:49

DSLAM/MSAN type:           IFTN:0xb206 / v0xb206
Modem/router firmware:     AnnexA version - A2pv6C038m.d24j
DSL mode:                  VDSL2 Profile 17a
Status:                    Showtime
Uptime:                    3 hours 10 min 3 sec
Resyncs:                   0 (since 21 Jan 2016 22:21:45)
         
            Downstream   Upstream
Line attenuation (dB):     16.4      0.0
Signal attenuation (dB):   Not monitored      
Connection speed (kbps):   59997      20000
SNR margin (dB):           8.7      12.4
Power (dBm):               5.8      5.8
Interleave depth:          1853      1
INP:                       5.00      0
G.INP:                     Not enabled      

RSCorr/RS (%):             0.3642      0.7716
RSUnCorr/RS (%):           0.0301      0.0000
ES/hour:                   0      0



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gt94sss2

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Re: Actual sync significantly lower than max attainable
« Reply #1 on: January 22, 2016, 05:34:23 PM »

I'm not sure how far from the cabinet the hosting site is. SNR down is 8.7db, and SNR up is 12.4db - I understand from the reading that I've done that these SNRs are quite decent, albeit not fantastic, and that I should probably be able to achieve more like the full, max-attainable 30MBPs upstream.

Its not possible to get faster than a 20MB upload on an FTTC line at the moment - its the highest speed Openreach offer ISPs.

Not what you want to hear but the only scope you have for improving your line as such is on the downstream side where the gap between the 60MB you are getting and the 80MB its capable of (again capped by Openreach) is quite sizeable.
« Last Edit: January 22, 2016, 05:39:43 PM by gt94sss2 »
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BorisTheSpider

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Re: Actual sync significantly lower than max attainable
« Reply #2 on: January 22, 2016, 05:43:48 PM »


Thanks for the reply. So even though it's unbundled (TalkTalk) it's still using OpenReach equipment at the exchange, then presumably TalkTalk trunk the data from the exchange onto their network?

I must have seen what I wanted to see when I first installed it then, since I could have sworn I saw it sync higher, but I was in a hurry and I guess I just didn't look closely enough.

If I do decide to try to improve the downstream (which as I said isn't really a priority) what are my options to start doing that? Is the 8.5db or so I have of downstream SNR enough to be worth attempting to improve on the 60MBPs it's achieving?



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gt94sss2

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Re: Actual sync significantly lower than max attainable
« Reply #3 on: January 22, 2016, 05:59:13 PM »


Thanks for the reply. So even though it's unbundled (TalkTalk) it's still using OpenReach equipment at the exchange, then presumably TalkTalk trunk the data from the exchange onto their network?

Effectively, though the FTTC equipment is in a green Openreach cabinet near your house somewhere at which point it switches to fibre.

Quote
If I do decide to try to improve the downstream (which as I said isn't really a priority) what are my options to start doing that? Is the 8.5db or so I have of downstream SNR enough to be worth attempting to improve on the 60MBPs it's achieving?

Your line says:

Max:   Upstream rate = 30091 Kbps, Downstream rate = 83024 Kbps
Bearer:   0, Upstream rate = 20000 Kbps, Downstream rate = 59997 Kbps

which means that in theory you should get close to the 80MB maximum.

The target SNR of a line operating at 'maximum' is around 6dB - so not much scope from 8.5 but there must be a reason why your modem thinks its capable of higher - perhaps some local electricial interference - though I see you are on an ECI Cabinet so can't get G.INP at present
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WWWombat

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Re: Actual sync significantly lower than max attainable
« Reply #4 on: January 22, 2016, 06:14:33 PM »

Max:   Upstream rate = 30116 Kbps, Downstream rate = 83260 Kbps
Bearer:   0, Upstream rate = 20000 Kbps, Downstream rate = 59997 Kbps

I'm not sure how far from the cabinet the hosting site is. SNR down is 8.7db, and SNR up is 12.4db - I understand from the reading that I've done that these SNRs are quite decent, albeit not fantastic, and that I should probably be able to achieve more like the full, max-attainable 30MBPs upstream.

In FTTC, BT always use a target SNRM of 6dB. If you are getting something higher, it is either because
a) you are already sync'ed at the maximum speed for the package, or
b) because DLM has "banded" your connection to sync at a deliberately lower speed, or
c) You synced at 6dB, but there is now less background noise on your line.

Upstream, you are at the maximum speed of 20Mbps. It won't go faster (perhaps you missed something; FTTC maxes out at 20Mbps), so no chance of 25 or 30 there; the extra SNRM there is just additional spare.

Downstream, your actual sync is a few bytes under 60Mbps ... which is such a nice, round number, that it would make me think that banding has occurred. However, I didn't think that 60Mbps was actually one of the banding ranges that BT uses - so I'm a little unsure. You also mention that your line was only activated yesterday - and it would normally take longer for DLM to choose to add banding.

In fact, it would normally take DLM at least a day longer to take any action at all .. and yet DLM has certainly intervened, because we can see settings of INP=5 and delay=10ms. Another anomaly is that these settings are higher than bare minimum (INP=3, delay=8ms) - and it would normally take DLM another day before it turned settings up from the minimum.

DLM's job is to monitor your line for instability - either too high an error rate (using the ES value), or too high a rate of resyncs. DLM is known to be able to take faster action if it thinks your line is very unstable ... so perhaps this is what happened - especially if you think the modem had locked up in some way.

(TL;DR section: Too many errors!)
Right now, you have a low rate of ES's (64 in 3 hours wouldn't worry DLM), but we should expect it to be low now: the intervention aims to swap all errors into being correctable FECs. However, the amount of uncorrectable FECs (RSUnCorr of 645199) seems high relative to the number of corrected FECs (RSCorr of 7799843): Whatever interference you are suffering, it is bad enough to defeat the interleaving+FEC protection too frequently. Likewise the number of CRC's (at 23901) is quite high - and the only good news is that they are concentrated into a very few seconds (hence just 64 Errored Seconds).

For comparison, my old line would not trigger DLM at all. It would see around 600-800 ES's per day, and 6,000-8,000 CRC's per day.

I get the feeling that your line, without DLM intervention, would be suffering from a very high level of errors. And that DLM hasn't yet got a handle on the settings it needs to eradicate them well.

This points you at some actions going forward - with the distinct aim to increase stability of your line.
a) Try not to resync your modem much; you'll likely make things worse in the short term
b) Don't worry about reconnecting the router (or rebooting it); that doesn't cause a resync, and is a better way to reconnect.
c) Keep DslStats going - but pay attention to the graphs showing errors: CRC and FEC errors, and the ES per hour. Look for patterns in the times of high error rates - does it coincide with heating, for example.

Judging by the attenuation figures, I'd say your line is around 350m from the cabinet, assuming the line is of 0.5mm copper. The figures are almost identical to my old line mentioned above.
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WWWombat

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Re: Actual sync significantly lower than max attainable
« Reply #5 on: January 22, 2016, 06:25:49 PM »

One thing I missed an explanation for:

When DLM has intervened, and turned on FEC and interleaving, it will make use of a proportion of your bandwidth (to send extra parity bits, that allows the error correction process to work) and will increase latency (interleaving spreads out your bits, so that a burst of noise concentrated on a few bits is actually spread out, giving the error correction process a chance to work).

In your case, the parity overhead consumes about 26% of your bandwidth (14 bytes in every 54), which is quite high. This overhead creates a gap between the actual sync speed and the "max attainable" value.

On an HG612, we also see something of a bug - when the actual sync speed drops due to DLM, the "max attainable" rises by about the same amount. When DLM removes intervention, the two will come back to meet in the middle. You might expect speeds to come back to around 70-72Mbps at best.

However, when DLM "only" turns on FEC and interleaving (but doesn't apply banding), we would still expect the SNRM to be 6dB still. The fact that you are getting 8.7dB will also affect the gap in the two speeds.
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WWWombat

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Re: Actual sync significantly lower than max attainable
« Reply #6 on: January 22, 2016, 06:28:47 PM »

Last thing:

There are probably lots of things you could do physically to the line to check it is in the best of health. However, there isn't much you can do until you get to your Mum's home, or send someone else.
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