PoE, Comcast Modems, and Service Outages

Blake Hudson blake at ispn.net
Tue Mar 29 22:41:09 UTC 2022

Generally anything inside the customer premise (including wiring) is the customer’s responsibility. If your coax runs across a fluorescent light fixture, that’s on you. If your coax is RG59, it’s on you to replace with RG6 quad shield. 

Maybe the cable operator will work with you, maybe not. I’ve had some techs replace splitters or make custom length user cables. 

And yes, the RF stats I mentioned are all on the HFC interface of the cable modem (or the CMTS). Customer facing Ethernet stats may not even be tracked by Comcast. 


> On Mar 29, 2022, at 5:29 PM, Aaron de Bruyn <aaron at heyaaron.com> wrote:
> 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.
> -A
> On Tue Mar 29, 2022, 09:59 PM GMT, Blake Hudson 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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