Comcast claiming router backfeed voltage through ethernet cable to modem


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Old 09-17-18, 10:45 AM
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Comcast claiming router backfeed voltage through ethernet cable to modem

Son is having intermittent issues with wi-fi streaming calls and videos in a bedroom next to the room with his wireless router (computer is about 15 ft away and on the other side of a wallboard/stud wall. Wifi signal strengh on his phone app is good (-50--60 dB on both 2.4 and 5 GHz in the bedroom). Had Comcast techs come in and the first tech claimed that there was voltage being backfed to the cable from somewhere, but he couldn't find it (I don't know what "cable" he was referring to...coax or ethernet, nor do I know how he measured the voltage). A second tech came out and in my son's words waved a device over the cables and got a signal over the Cat6 ethernet cable between the router (Archer C2600) and the cable modem (Aris 6141). Tech disconnected the ethernet cable from the router and signal went away. He reconnected the ethernet cable to the router and disconnected it from the cable modem and signal was present. His conclusion was that the router was bad and needed to be replaced.

Being skeptical, I decided to do a test. Presuming that the device the 2nd tech waved over the wires was a Non Contact Voltage detector, I went through my network with my TackLife DM09 meter in NCV mode and found exactly what the tech found. A voltage signal on the ethernet line to the cable modem from the router. But then I checked all the other ethernet lines and they also triggered the meter as long as they were connected to a router (I use several routers as remote wifi access points). Even if I disconnected the remote access point routers from the LAN, any ethernet cable plugged into a router triggers the NCV signal. Even without an ethernet cable, the ethernet sockets in the router triggered a NCV signal.

So, with all that background, is it normal for ethernet cables to show a NCV.signal (by the way, neither he nor I are running anything using Power Over Ethernet on these cables)? Is NCV over an ethernet cable described somewhere so that when the Comcast tech returns my son can point him to a document online that debunks Comcast's evidence of a defective router. Finally, details on the first tech's visit are fuzzy, but it seems that he may have seen some voltage on the coaxial cables coming into the apartment. I presume that if he did, that is evidence of a bad cable ground or stray signal getting into the coax upstream of my son's apartment. Yes/No?

Sorry to have been so long winded, but I wanted to include as much hard detail as I could to help with the diagnostics.
 

Last edited by bkspero; 09-17-18 at 10:47 AM. Reason: spelling
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Old 09-17-18, 10:56 AM
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Well, IIRC, the ethernet cable specifications are they are not grounded, which is intentional. Grounding risks creating ground loops when devices up to one-hundred feet away, plugged into different outlets, are conencted, which IIRC would be sufficient to cause an unacceptable level of noise in ethernet cables.
Some Cat6 does have exterior shielding, but it's not usually grounded.

More likely, data cables are running parallel to electic wires and getting induced voltage?
 
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Old 09-17-18, 11:08 AM
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Do you have another router or spare router to try just in case the router already there was defective?

Can you run temporary cables on the surface between the modem and the router and between the router and the computer to try out just in case the cables in the wall were too close to power wires?

I still believe that it is okay and even desirable to ground the equipment to your house ground. Temporarily (can become permanently later) run a #12 or #14 wire from the panel neutral bus bar or the grounding electrode conductor to the ground rod. Connect the other end using jumper wires or daisy chaining to the chassis or the coax jack shell of each piece of equipment.
.
 
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Old 09-17-18, 11:21 AM
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Originally Posted by bkspero
Son is having intermittent issues with wi-fi streaming calls and videos
Eh, sounds like it could be a Comcast throttling issue.

Have him test the connection speed with a few speed/latency websites.
https://forums.xfinity.com/t5/Your-H...s/td-p/3056103

See if there's a difference between speed reported by Comcast/Speedtest and 3rd party sites.
 
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Old 09-17-18, 01:57 PM
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the first tech claimed that there was voltage being backfed to the cable from somewhere, but he couldn't find it
That is the silliest thing I ever heard of.

The cable system should be at zero ground potential. If it's not.....it is their problem. I see more and more installations where the incoming cable is not correctly grounded and causing all kinds of problems.
 
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Old 09-17-18, 02:07 PM
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I can't imagine what use a NCV meter would have on Ethernet wiring. If might signal it might not but that doesn't tell you anything because an operational network is supposed to have current in the cables. NCVs are not finely tuned meters they are only for an "oops check" before you cut through something that might be live. Other than that they tell you nothing useful.

I suspect (and hope) the tech was talking about stray voltage on the coax shield. This suggests a problem with the grounding of the building's electrical service OR the cable distribution equipment, as the building ground is at a different potential than the ground at Comcast's pole. It can be a difficult problem to troubleshoot, but in general would involve an inspection of the building's electrical panels and earth grounding electrodes. If this is an apartment building with multiphase service it can also suggest a potential phase imbalance which creates excess neutral current. It's very hard to say without a close examination. It could be a bad/corroded coax termination somewhere in the line.

The best next step might be to see if other tenants in the building have Comcast service and if they have similar problems. It could help to narrow down whether it's a building electrical problem or something specific to the modem or cable components in your son's unit.

While troubleshooting it's also a good idea to plug your laptop directly in to the cable modem via Ethernet to eliminate all the potential problems that could be introduced the the router. It proves they are either delivering proper service to the modem or not and eliminates the blame game on internal components.
 
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Old 09-17-18, 02:24 PM
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Originally Posted by PJmax
If it's not.....it is their problem.
Unless there's some way to (A) find a problem and (B) convince the customer to pay to fix the problem.
 
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Old 09-17-18, 02:29 PM
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A cable installation is supposed to include a ground block connected to the panel service ground or to some other KNOWN good ground. I see the ground wire pushed into the ground. A green wire wrapped around a faucet handle. These methods are not acceptable and should not be tolerated. You don't see a FiOS installation done like that.
 
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Old 09-17-18, 02:41 PM
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You don't see a FiOS installation done like that.
Eh, this IS a Comcast installation.
 
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Old 09-17-18, 03:37 PM
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Yes Hal.... this is a Comcast installation done by a sub-contracted installer
"paid-by-the-drop" to get it done as quick as possible. This crap needs to be stopped.
 
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Old 09-18-18, 06:27 AM
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Thanks, all. My question focused on the NCV measurement because the of my surprise about getting a signal from all of my ethernet cables. These are devices that are intended to detect the electric fields from relatively high voltage (120V) 60 Hz sources. Not GHz sources at what I expected would be millivolt levels. And my NCV meter is not the most sensitive. I need to get within a couple of inches of the wire for it to trigger at 120 VAC. So why is it triggering on the ethernet cable.

I found one link that, if it is correct, could explain what I'm seeing:

https://electronics.stackexchange.co...ge-tester-work

It says that frequency does trade off for voltage. So a very high frequency low voltage ethernet signal could trigger a NCV tester if its amplifier is capable of handling the higher frequency. That is what he can use to rebut Comcast's diagnosis of a bad router based on NCV testing his ethernet cable.

As for other suggestions. I should have made it clearer that this is a high rise apartment building. He is unable to access the service panels or the building wiring. It would have to be done by Comcast or the building maintenance staff. This begs the question of interference from wifi in adjacent apartments. That is an issue on the 2.4 GHz band, but the 5 GHz channel that he has set is in the clear. We worried that the bedroom being on the other side of a wall might badly hurt the strength of the 5 GHz signal, but as I mentioned, the signal in the bedroom is strong (-55--60 dB).

He's tried a couple of old routers as replacements and they performed worse than his current one. That's why he bought the current one when he moved into this apartment...to replace the old ones. His next step would have to be to buy another new router, or wait for my next visit and have me bring another one with more up to date technology than his old routers. He's trying, I think, to avoid both (paying for a new router, or having me visit ).

I don't know that its throttling. Performance in the room with the router is ok. About what Comcast advertises when tested with Speedtest.net. Since the system works ok there, he hasn't bothered with testing using ethernet cables.

I also don't think it is from data cables being routed near power cables within the apartment. The coax, cable modem, and ethernet cable to the router are on one side of the table where the router is located. The AC power cables come in from the other side to a power strip. The only power runs to the cable modem and router side of the table are low voltage DC lines. I guess it's possible that the coax is routed near power cables before it gets into the apartment....that's what Comcast was supposed to test and find. If, when they return, he will have them retest the coax cables coming into the apartment, and he now knows enough that if they find voltage in the shields, Comcast has to get rid of it.

That leaves what else could be the cause. Why with a seemingly strong remote signal, is streaming intermittent in the bedroom? Maybe it isn't Comcast's fault. Maybe there is something else going on. He and I will keep digging. Thanks for all your thoughts.
 
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Old 09-18-18, 01:27 PM
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A little hard to follow what you are doing but finding induced AC voltage on an ethernet cable is not uncommon and shouldn't be causing any problems. This issue is caused more and more by switch mode power supplies that run at high voltages.

The data signal is carried on twisted pairs of wiring and doesn't rely on any ground.

If you are having a wireless issue.... that could be due to many wireless routers running in the same apartment building.
 
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Old 09-18-18, 02:44 PM
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Originally Posted by bkspero
Son is having intermittent issues with wi-fi streaming calls and videos in a bedroom next to the room with his wireless router (computer is about 15 ft away and on the other side of a wallboard/stud wall.
In an apartment building, one common problem is that EVERY router defaults to WiFi "channel 6" for 2.4gh wifi, and a few channel for 5gh wifi.

A free app called "wifi analyzer" shows which channel your router uses, and what channels everyone ELSE uses. If you do see that eveyone's on the same channel, then you want to change the router setting to use a quiet section of the frequency spectrum.
 
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Old 09-18-18, 08:42 PM
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Originally Posted by Hal_S View Post
In an apartment building, one common problem is that EVERY router defaults to WiFi "channel 6" for 2.4gh wifi, and a few channel for 5gh wifi.

A free app called "wifi analyzer" shows which channel your router uses, and what channels everyone ELSE uses. If you do see that eveyone's on the same channel, then you want to change the router setting to use a quiet section of the frequency spectrum.
He did this when he moved in and checks it periodically. The 2.4 GHz band is so heavily used that there are no clear channels, but the wider and less popular 5 GHz band had (and has) lots of openings, and he has his router set to a clear area. That probably explains why streaming performance in the bedroom on the 2.4 GHz band is even worse than the 5 GHz band despite the greater wall-penetration capability of a 2.4 GHz signal. He was initially concerned that the signal strength on the 5 GHz band would be low in the bedroom, but as I mentioned in earlier posts, it is good (in the area of -50--60 dB).
 
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Old 09-18-18, 08:58 PM
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Originally Posted by PJmax View Post
A little hard to follow what you are doing but finding induced AC voltage on an ethernet cable is not uncommon and shouldn't be causing any problems. This issue is caused more and more by switch mode power supplies that run at high voltages.

The data signal is carried on twisted pairs of wiring and doesn't rely on any ground.

If you are having a wireless issue.... that could be due to many wireless routers running in the same apartment building.
Thank-you. Until your post I was unable to find any information one way or the other indicating whether or not is was expected to find detectable AC voltage on an ethernet cable not running PoE. If I understand what you are saying (wrote), AC voltage that is always present in the output of ungrounded switch mode DC power supplies (as used by just about all mass market routers and switches) passes through the router/switch and shows up at the RJ45 sockets of the devices (and then to the ethernet cable). This is normal and not evidence that any router or switch is defective. Do I understand correctly? Thanks again.

As for wifi congestion as a cause of poor streaming, see the note above this one. His 5 GHz signal is in in a clear channel on that band.
 
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Old 09-19-18, 05:58 PM
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Hey bkspero - boy did you luck out today; I found your post just idly browsing the forum after posting about a mirror I'm trying to wire in to my wall in my new condo.

Anyway - I'm a 5 year Comcast tech veteran. I worked for the actual company as a CT4 doing exactly the kind of calls you've had your son handle twice now - this was before I started working in tech around 2012 or so. Now, I do DevOps for a custom cluster / server company.

First thing's first - we need to characterize the problem. "Intermittent issues with wifi calling and video streaming" is pretty broad; is he trying to use an internet based service like Netflix and Skype? Or does he have some kind of local media player that's choking on things from another location in his unit?

Second - wifi is just straight up terrible for doing anything other than ****ing around on your phone. Run a physical ethernet line through the wall - a single 1/8inch drill bit hole is easy to hide and no one will be any the wiser when he moves out with some simple wall putty. Having a solid physical connection for PCs or consoles and then using Bluetooth or some other more local wireless standard for headsets and hands free is the way to go.

Third - there absolutely could be a problem with the service itself. You need to get any PC on his network to run PingPlotter to yahoo.com or google.com or whatever and watch it over a few days - it will perform traceroutes to each network hop from the PC it's running on and tell you where loss or latency is happening. This will shine a big fat light on Comcast or your internal network wiring to figure out if there is loss or latency. PingPlotter is free, and you can have it run to multiple destinations just by opening multiple instances of it.

Fourth - all cable modems have a diagnostics page. Usually that page is an IP address that ends in 100.1, and should be accessible even through a router. If your IP address is 192.168.1.2, for example, you should be able to get to your router at 192.168.100.1. A diagnostics page will tell you several things:
  • The Downstream signal strength for each channel in DBMV - Ideally, this is between -5 and +10, with 0 being the best.
  • The Upstream signal strength in DBMV - Ideally, this is between 32 and 40 - this one is the killer, 35/36 is best.
  • The upstream and downstream SNR (Signal to Noise Ratio) should be 33+ - it should NOT be lower than 30. If it is, "there's yer problem", their system is ****ed up.
  • These are not exact numbers - I've seen modems kick ass and take names connecting at -8 and 45 upstream - don't agonize / quibble over small changes, except the SNR - that absolutely must be above 30.
  • The system signal strengths change - you can't take one measurement and assume it's like that all the time. It's best to get multiple measurements and especially when you're experiencing some kind of service interruption / issue.
Not all diagnostics page are at that address - you should google around for your particular modem, it may also ask for a username and password which will either be on the device itself or documented online somewhere.

In any case, hope this helps - diagnosing network problems is tricky business, and not everyone at Comcast is as knowledgeable as I was.
 
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Old 09-24-18, 07:24 AM
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Originally Posted by Locane View Post
First thing's first - we need to characterize the problem. "Intermittent issues with wifi calling and video streaming" is pretty broad; is he trying to use an internet based service like Netflix and Skype? Or does he have some kind of local media player that's choking on things from another location in his unit?

Second - wifi is just straight up terrible for doing anything other than ****ing around on your phone. Run a physical ethernet line through the wall - a single 1/8inch drill bit hole is easy to hide and no one will be any the wiser when he moves out with some simple wall putty. Having a solid physical connection for PCs or consoles and then using Bluetooth or some other more local wireless standard for headsets and hands free is the way to go.

Third - there absolutely could be a problem with the service itself. You need to get any PC on his network to run PingPlotter to yahoo.com or google.com or whatever and watch it over a few days - it will perform traceroutes to each network hop from the PC it's running on and tell you where loss or latency is happening. This will shine a big fat light on Comcast or your internal network wiring to figure out if there is loss or latency. PingPlotter is free, and you can have it run to multiple destinations just by opening multiple instances of it.

Fourth - all cable modems have a diagnostics page. Usually that page is an IP address that ends in 100.1, and should be accessible even through a router. If your IP address is 192.168.1.2, for example, you should be able to get to your router at 192.168.100.1. A diagnostics page will tell you several things:
  • The Downstream signal strength for each channel in DBMV - Ideally, this is between -5 and +10, with 0 being the best.
  • The Upstream signal strength in DBMV - Ideally, this is between 32 and 40 - this one is the killer, 35/36 is best.
  • The upstream and downstream SNR (Signal to Noise Ratio) should be 33+ - it should NOT be lower than 30. If it is, "there's yer problem", their system is ****ed up.
  • These are not exact numbers - I've seen modems kick ass and take names connecting at -8 and 45 upstream - don't agonize / quibble over small changes, except the SNR - that absolutely must be above 30.
  • The system signal strengths change - you can't take one measurement and assume it's like that all the time. It's best to get multiple measurements and especially when you're experiencing some kind of service interruption / issue.
Not all diagnostics page are at that address - you should google around for your particular modem, it may also ask for a username and password which will either be on the device itself or documented online somewhere.

In any case, hope this helps - diagnosing network problems is tricky business, and not everyone at Comcast is as knowledgeable as I was.
Thanks Locane. The main problem is with streaming of Skype and Google chat video calls. The calls experience intermittent (nearly every call, but only for as little as 1, 5 or so minute instance to as much as frequent (every few minutes) repeated stumbling and poor performance). But it also happens with Netflix and Amazon video, it's just not as important with video streaming as it is with the calls.

I think he is prepared to test cable connection if none of the wifi fixes help.

I will point him to PingPlotter, and to the status page of the cable modem. He and I both have the same type, so the local IP address should be the same for both of them. The parameters you specified give us something quantifiable to work with.

Thanks again.
 
 

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