PoE, Comcast Modems, and Service OutagesPyle

Abraham Y. Chen aychen at avinta.com
Thu Mar 31 14:40:26 UTC 2022

Hi, Colleagues:

0)    I would like to share a personal experience of a different setting 
to offer an angle for looking into this puzzling topic.

1)    During my graduate study, I was doing microwave experiments in the 
laboratory. On a six foot bench, I had a series (maybe a dozen or so) of 
waveguide components (made of metal, either solid copper or silver 
plated) connected together as the test bed. With a half dozen or so 
equipment attached to them at various points to serve as the energy 
source or sink as well as detection instruments. The setup exhibited 
randomly and unpredictable behaviors. After awhile, my advisor brought 
up the topic of "Ground Loops" that could introduce the interference in 
mysterious manners. This is a phenomenon whereby minuscule electric 
current flowing along metallic parts (even though they may appear to 
have no "resistance" in between) that are connected to the system common 
potential reference (the "ground") via slightly different paths. These 
could be very small resistance path between two points or high 
resistance leakage source. The resultant electric potentials among the 
subsystems of interest could be significant enough to affect the outputs 
of sensitive instruments.

2)    After much cut-&-try, including plugging all instruments into the 
same electric extension strip with no avail, I finally floated the AC 
power cords of all instruments (using three-to-two prong adapters) but 
kept only the most sensitive node in the system connected to the AC 
power source "ground". Although this was against the electric safety 
code, I finally got consistent results. With clear record of the 
configuration, I even could take the waveguide setup apart and then 
reassembled days later to repeat the same results. Years later, I 
applied the same philosophy to a smart-meter PCB design which got a 
better precision than the chip manufacturer's demon board could.

3)    So, it is possible that the site with the reported "PoE induced" 
issues may be somehow experiencing the above related phenomena. This 
kind of situations are almost impossible to duplicate at another site. 
It has to be diagnosed with pains-taking detective efforts, such as 
inserting isolation subsystems suggested by one colleague. Since this 
phenomenon takes a month or so to show up, discipline and patience are 
the virtue.

4)    On the other hand, a product that can build up certain "memory" of 
the disturbances like the above and to the point of requiring periodical 
power cycling to flush clear the issue is definitely a sign of defect 
design, based on my old school training.


Abe (2022-03-31 10:40)

On 2022-03-30 15:57, Joe Greco wrote:
> On Wed, Mar 30, 2022 at 05:52:06PM +0000, Livingood, Jason wrote:
>>>   Their crappy equipment needing rebooting every few weeks, not ridiculous.
>>> Their purchasing gear from incompetent vendors who cannot be standards
>>      compliant for PoE PD negotiation, tragically plausible.
>> Many customers buy their own cable modem. You can lease an Xfinity
>> device as well and those function pretty nicely these days but YMMV.
>> But typically a device reboot is a way to quickly solve a few
>> different kinds of problems, which is why techs will often recommend
>> it as an initial step (you can generally assume that there's data
>> behind what occurs when any one of tens of thousands of support reps
>> suggesting something to a customer - support at scale is data-driven).
> No one's doubting all of that -- support is best when data-driven, scale
> or otherwise.  But that's actually the issue here.  There's no data that
> I know of to suggest widespread PoE ghost current buildups, and, given
> the audience here, no one else has popped up with a clear "me too".
> PoE is typically negotiated by modern switches, 24v Unifi special
> jobbies aside, so it's all DC on cables that are already handling
> differential signalling.
>>>     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.
>> Could well be from noise ingress - lots of work goes into finding &
>> fixing ingress issues. Hard to say unless we look in detail at the
>> connection in question and the neighborhood node.
> No doubt.  There's huge amounts of room for problems to be introduced
> into last mile networks.  But, again, this isn't about general problems.
> This is about a tech claiming it's due to PoE, and that he's seen it
> often before.
> I certainly have a lot of sympathy for cable techs, but that doesn't
> mean I want to swallow any random garbage they want to blame issues on.
> Please just tell me it's the chipmunks getting feisty and nibbling on
> the copper if you want to feed me a line...
> ... JG

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