PoE, Comcast Modems, and Service Outages

Blake Hudson blake at ispn.net
Tue Mar 29 21:59:05 UTC 2022

On 3/29/2022 3:24 PM, Joe Greco wrote:
> He's got graphs showing it every 24 hours?  Liar, liar, pants on fire,
> lazy SOB is looking for an excuse to clear you off the line.  Where the
> heck does this "24 hour" cycle even come from?  What SNMP OID is there
> for "ghostly PoE build-up"?  What crontab is there that would clear out
> such buildups in the router's daily run?  What capacitor would store up
> juice for precisely 24 hours?  What's the mechanism here?  CURIOUS MINDS

Taken at face value, I assume the tech would be looking at historical 
signal graphs (we keep them for cable networks for each CM) that record 
stats like FEC, SNR, and signal strength. For aerial runs it's common to 
see some change throughout the day due to warming and cooling. These 
look like waves with peaks and valleys around 4PM/4AM and generally 
affect all customers in a service area equally. Sometimes there will be 
a device at a customer premise that causes interference with a CM, 
something like a motor or tool. These could absolutely be on a 24hr 
cycle (think of a programmable thermostat kicking on the blower fan in 
your HVAC at the same time every day).

As Joe said, there's no SNMP MIB for PoE buildup. There are well 
documented MIBs for DOCSIS to cover standard signal level, quality, or 
similar. The cause of that signal strength or quality can be myriad. 
This Comcast tech has likely climbed the ladder of inference several 
steps too far.

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