Intermittent T3 Timeouts

[DeadSignal]

Hi all,

TL;DR - Shibboleet help please! Am I being a muppet or is this upstream?

For background - I'm a Linux analyst and CCNA qualified but know very little about DOCSIS.

I'm getting occasional T3 (and previous T4) dropouts plus a bunch of Post-RS errors. I understand this is bad! 🤔

VM Superhub 3.0
Hardware version : 11
Software version : 9.1.2109.200
Last reboot: 06/02/2023 ~18:40

My first concern is that the superhub was delivered without a coax cable so currently have no primary isolator (I have the PSU into the VM wall-box setup... I'd love to know how this works!)

I'm currently using about 5m of CT100 cable, kept from last year's VM subscription.

Secondly, I noted a kernel panic in the logs when I first booted it! This has since disappeared.

Here's the log. 23:43 was when my Twitch broadcast dropped for a few seconds:

Time Priority Description

11/02/2023 01:25:37	notice	LAN login Success;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;
10/02/2023 23:43:22	Warning!	Lost MDD Timeout;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;
10/02/2023 23:43:17	critical	SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;
10/02/2023 23:43:16	Warning!	RCS Partial Service;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;
10/02/2023 23:43:16	critical	SYNC Timing Synchronization failure - Loss of Sync;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;
10/02/2023 13:11:38	critical	No Ranging Response received - T3 time-out;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;
09/02/2023 15:49:5	Error	DHCP RENEW WARNING - Field invalid in response v4 option;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;
06/02/2023 18:43:51	notice	LAN login Success;CM-MAC=**:**:**:**:**:**;CMTS-MAC=**:**:**:**:**:**;CM-QOS=1.1;CM-VER=3.0;

Up/Down metrics in comments due to character limit.

TIA. Rich.

[Client62]

Power levels et al !

https://community.virginmedia.com/t5/Networking-and-WiFi/POWER-LEVELS-amp-SNR-A-TECHNICAL-PRIMER-upd...

[DeadSignal]

Well that was some quality reading! 🙂

OK, problems are still occurring but VERY rarely. This is the entire log since the last post. (Not sure why the 01:58 timestamp is missing a 0 from the seconds field 😁 )

# Ommitted MAC addresses
# CM-QOS=1.1
# CM-VER=3.0

Time                 Priority  Message
17/02/2023 07:41:58  critical  No Ranging Response received - T3 time-out
17/02/2023 03:23:38  Error     DHCP RENEW WARNING - Field invalid in response v4 option
16/02/2023 11:11:19  critical  No Ranging Response received - T3 time-out
14/02/2023 17:26:57  critical  No Ranging Response received - T3 time-out
14/02/2023 01:58:9   Error     DHCP RENEW WARNING - Field invalid in response v4 option

From the technical primer (Thank you @Client62 !) - I understand the following:

Power:
Upstream, 4 Channels Locked, 64QAM = Limits: +23 to 51 dBmv. Actual: +37. OK
Downstream, 24 Channels Locked, 256QAM = Limits: -6 to +10 dBmv. Actual: +3.0 to +4.8. OK

SNR / RxMER (Error Rate)
VM Limit: ≥ 34 dB. Optimal Limit: ≥ 34.5 dB. Actual 33.3. NOT OK

---

Regarding downstream SNR, don't let anyone tell you that for 256QAM modulation that anything less than 34.5 dB RxMER/SNR is acceptable.  There is considerable evidence in the forums that data corruption occurs when the SNR falls to 34 dB. - Technical Primer

---

I'm seeing 47 Post-RS correction logs on one channel only.
Pre RS is 7M - 19M (not sure how relevant this is).

I find it hard to believe that a carefully de-sheathed and terminated 0.5m cable is a contributing factor although I'm still awaiting an official replacement from VM.

My only other question was with regards to the function of the Primary Isolator. Given that the micronode outside is powered over the co-ax, could noise on the DC feed be causing this?

I assume the primary isolator is a simple ground-loop isolator, decoupling the DC component. Is that correct?

Thanks again,
Rich.

[Ashleigh_C]

Hi there @DeadSignal 

Thank you so much for your post and welcome to the community forums, it's great to have you here. 

Thank you so much to the community members for all their help and insight here so far. I have taken a look into this and I can see there are a few issues with the specs that will require an engineer to take a look. 

I will pop you a PM across now to arrange this, please keep an eye out for the purple envelope in the top right corner of your screen alerting you to a new message. 
 

[Ashleigh_C]

Hello again

Thanks so much for your private message and confirming your address, I have now booked you a visit for your connection issues – you can check the date and time via your online account here. If you have any issues with accessing your online account or unable to see your visit, please do let us know and we’ll pop you a message to confirm.

There will be no charge for this visit unless:
• The technician diagnoses the faults as not being caused by our network/equipment 
• The technician discovers that the fault or problem relates to your equipment
• The technician discovers that the fault or problem relates to any system that we are not responsible for
 

The technician will confirm during their visit if any of these instances apply, and if so, a £25 charge will be applied to your account. 
 

Please ensure there is someone over the age of 18 present at the time of the visit. If you need to change or cancel your appointment you can do this online or via the My VM app by 4pm the day before the appointment.
 

If you do miss an agreed appointment for any reason, a £25 missed appointment charge will be applied to your account on the day of the appointment