PoE, Comcast Modems, and Service Outages

Aaron de Bruyn aaron at heyaaron.com
Tue Mar 29 22:29:15 UTC 2022

Thanks Blake,

As I understand it all that stuff is on the "cable provider" side of the
CPE and (within reason) it's up to the provider to deal with the signals
arriving on the cable side of the modem.
i.e. if it was a blower or something in our suite that was causing RF
interference, the provider might work with us to move the modem or the
cable run.


On Tue Mar 29, 2022, 09:59 PM GMT, Blake Hudson <blake at ispn.net> wrote:

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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