# Moca works without POE filter - Do I still need to get one?



## The TiVo Dude (Jun 9, 2004)

So if Moca works just fine w/o the POE, do I need to add one for any reason?


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## sjsaliba (Mar 30, 2008)

My understanding is the purpose of POE filter is to prevent your media signals from transmitting back through your outside cable to your 'neighbors' cable.
It took my about a year researching basically how MoCa works and how to successfully setup in my home. It appears to be working fine and I have POE installed on the main cable line coming into my home.


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## dianebrat (Jul 6, 2002)

As sjsaliba says, it's not going to make a difference as to your network operation, but it blocks your data from getting out to your neighbors and causing issues with them. My dad did MoCA without a POE filter on cable and Comcast had complaints from the neighbors.

If you're on FiOS, you don't need a POE filter since the cable does not leave the building, it comes from the ONT.


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## k2ue (May 9, 2002)

Actually a POE filter will make a difference in your network operation -- it will strengthen the MOCA signal. Splitters are designed to have *high isolation between ports*, so when a MOCA signal needs to travel from a source on one output of a splitter to a destination on another port, there is rather large attentuation. Most of the MOCA signal will go back thru the splitter(s) in the direction intended by splitter design, *and wind up dissipated* (lost) in the line drop amp feeding your house. But when a a POE filter is present the MOCA signal reaching the filter is *reflected by the filter*, and propagates thru the system just like the cable signal, in the forward direction, reaching all points in the house with only splitting loss.

With a MOCA filter the path loss between any two points is only the sum of their individual splitting losses from the POE (the trip from source to POE, then POE to destination), but without it will be the isolation value between the ports (typically >20dB) *times the number of splitters to cross to reach the destination*! *And the wider band and higher quality your splitters, the greater the likely isolation (attenuation in this case)*.

So it is always good to have a MOCA POE Filter.


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## nooneuknow (Feb 5, 2011)

k2ue said:


> Actually a POE filter will make a difference in your network operation -- it will strengthen the MOCA signal. Splitters are designed to have *high isolation between ports*, so when a MOCA signal needs to travel from a source on one output of a splitter to a destination on another port, there is rather large attentuation. Most of the MOCA signal will go back thru the splitter(s) in the direction intended by splitter design, *and wind up dissipated* (lost) in the line drop amp feeding your house. But when a a POE filter is present the MOCA signal reaching the filter is *reflected by the filter*, and propagates thru the system just like the cable signal, in the forward direction, reaching all points in the house with only splitting loss.
> 
> With a MOCA filter the path loss between any two points is only the sum of their individual splitting losses from the POE (the trip from source to POE, then POE to destination), but without it will be the isolation value between the ports (typically >20dB) *times the number of splitters to cross to reach the destination*! *And the wider band and higher quality your splitters, the greater the likely isolation (attenuation in this case)*.
> 
> So it is always good to have a MOCA POE Filter.


I couldn't have said it any better, myself! :up:

It does make for a better MoCA network, to keep your frequencies isolated, stopping any from escaping outside your home (and stopping any that aren't yours from entering your home)


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## HarperVision (May 14, 2007)

sjsaliba said:


> My understanding is the purpose of POE filter is to prevent your media signals from transmitting back through your outside cable to your 'neighbors' cable.
> It took my about a year researching basically how MoCa works and how to successfully setup in my home. It appears to be working fine and I have POE installed on the main cable line coming into my home.


But what if you want to split your cable bill with your neighbor, so they can just throw a couple minis in their house to be a parasite off your Roamio Pro? 

Great explanation k2ue!


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## DCIFRTHS (Jan 6, 2000)

k2ue said:


> ...But when a a POE filter is present the MOCA signal reaching the filter is *reflected by the filter*, and propagates thru the system just like the cable signal, in the forward direction, reaching all points in the house with only splitting loss...


The POE filter is designed to reflect the signal? How does it accomplish this? I always thought these types of filters greatly attenuated the signal they were tuned too.


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## k2ue (May 9, 2002)

DCIFRTHS said:


> The POE filter is designed to reflect the signal? How does it accomplish this? I always thought these types of filters greatly attenuated the signal they were tuned too.


Simple LC (Inductance/Capacitance) Filters are reflective, and attenuate by *refusing to accept energy* in the rejection band. The illustrative analogy would be a rope tied to a wall: if we whip the rope to create a wave, the wave moves down the rope, is refected by the wall, and comes back to our hand. Absorbative filters, which only absorb and do not reflect, are more complex, requiring internal resistors as well as LC elements, and are almost never encountered in a controlled impedance environment like cable distribution, for the simple reason that the signals reflected by simple LC filters are absorbed on the first bounce by the controlled 75 ohm impedance of the connected device(s) -- it is only necessary for *one end *of a transmission line to be terminated to control reflections. It is a common assumption that everything connected to a transmission line system would be matched to it (completely accepting/absorbing all signals sent to it), but that is not the case, some devices do their job by simple rejection.


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## nooneuknow (Feb 5, 2011)

k2ue said:


> Simple LC (Inductance/Capacitance) Filters are reflective, and attenuate by *refusing to accept energy* in the rejection band. The illustrative analogy would be a rope tied to a wall: if we whip the rope to create a wave, the wave moves down the rope, is refected by the wall, and comes back to our hand. Absorbative filters, which only absorb and do not reflect, are more complex, requiring internal resistors as well as LC elements, and are almost never encountered in a controlled impedance environment like cable distribution, for the simple reason that the signals reflected by simple LC filters are absorbed on the first bounce by the controlled 75 ohm impedance of the connected device(s) -- it is only necessary for *one end *of a transmission line to be terminated to control reflections. It is a common assumption that everything connected to a transmission line system would be matched to it (completely accepting/absorbing all signals sent to it), but that is not the case, some devices do their job by simple rejection.


I think this explains something that has been nagging at me for awhile (although has nothing to do with MoCA, since I already have gigabit infrastructure and Cat5e going to nearly every room). Although, I do utilize a MoCA POE filter at the drop (lateral-run entrance) point, since they came with my Tuning Adapter self-install kits, which were free.

Moving along, attenuators were my savior, way back to the TiVo HD, and even more important for my Premieres. I used to seem to be the only one preaching how great they were, and how people should use them. After sharing my findings with TiVo, they updated their knowledgebase articles to mention them.

Move forward a couple years, and SDV is being "deployed", meaning they were insisting we needed TAs, and right away, or lose many channels. I get them, set them up, and they merely gave Cox a way to read the signals remotely, for nearly 8 months, before they started shifting anything to SDV.

Ever since they deployed TAs, attenuators were stricken, by my Cox market, as an acceptable means to reduce signal strength. All techs were instructed to remove them, on sight, and instead utilize more splitters, capping off any unused ports with terminators (the VERY tamper resistant type, which I just happen to have the correct tool to remove, if I need to).

I've wondered what the harm is in attenuators, and what the gain was by installing more splitters. I think your explanation about reflection and termination, as opposed to absorption, answers that question. It's much better than a Cox tech/rep stating "All we know is that we are no longer allowed to install them, are to remove any we find, and resolve any excessive signal issues with splitters."


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## k2ue (May 9, 2002)

nooneuknow said:


> I've wondered what the harm is in attenuators, and what the gain was by installing more splitters. I think your explanation about reflection and termination, as opposed to absorption, answers that question. It's much better than a Cox tech/rep stating "All we know is that we are no longer allowed to install them, are to remove any we find, and resolve any excessive signal issues with splitters."


There is *no functional difference* between an attentuator and a terminated splitter being used as an attenuator, except that the conventional attenuator probably has flatter frequency response. The isolation properties of the splitter have no significance when used as an attenuator. The only two explanations for insisting on splitters are either: a) obsessive standardization, seeking to remove attenuators from their inventory, because they can simulate them with splitters; or b) surprising ignorance of transmissionline techniques by someone in a position of authority.

QAM stands for Quadrature Amplitude Modulation, and by its nature is sensitive to amplitude. So no piece of equipment handling a QAM signal should be driven to limiting -- the removal to the highest portions of the amplitude modulation. Accordingly, *unless the manufacturer of the equipment specifically states that it can handle signals larger than its Signal Strength indicator's range*, it is wise and prudent to reduce the signal strength to within that range, by any practical distortionless means -- attenuators or splitters.


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## nooneuknow (Feb 5, 2011)

k2ue said:


> There is *no functional difference* between an attentuator and a terminated splitter being used as an attenuator, except that the conventional attenuator probably has flatter frequency response. The isolation properties of the splitter have no significance when used as an attenuator. The only two explanations for insisting on splitters are either: a) obsessive standardization, seeking to remove attenuators from their inventory, because they can simulate them with splitters; or b) surprising ignorance of transmissionline techniques by someone in a position of authority.
> 
> QAM stands for Quadrature Amplitude Modulation, and by its nature is sensitive to amplitude. So no piece of equipment handling a QAM signal should be driven to limiting -- the removal to the highest portions of the amplitude modulation. Accordingly, *unless the manufacturer of the equipment specifically states that it can handle signals larger than its Signal Strength indicator's range*, it is wise and prudent to reduce the signal strength to within that range, by any practical distortionless means -- attenuators or splitters.


I really can't find anything I disagree with, in what you said.

I can say that splitters provide a better, more consistent, overall level of attenuation from low frequencies, to high frequencies, while attenuators hit the frequencies harder at one end, while barely changing them at the other extreme.

This all started when they went to a 1GHz RF network, and I honestly can say that attenuators would often "fix" problem channels, and "kill" ones that were working fine. I do get a better balance, now. Before they went to 1GHz, attenuators were something I couldn't live without, and also without a whole plethora of values to use (often changing to -6dB ones in the winter, and -3dB ones in the spring/fall, as well as none required in 113 degree summers).

Cox refuses to fix the AGC (Automatic Gain Control) stress condition that is present on my end-of-the-line RF network interface, so rather than having their equipment compensate for the seasons, I have to do it myself.


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## k2ue (May 9, 2002)

nooneuknow said:


> I really can't find anything I disagree with, in what you said.
> 
> I can say that splitters provide a better, more consistent, overall level of attenuation from low frequencies, to high frequencies, while attenuators hit the frequencies harder at one end, while barely changing them at the other extreme.
> 
> ...


Attenuators designed for current cable use will have very uniform attenuation from near DC to 1GHz or above. But there are plenty of attenuators around that were designed only for Outdoor Antenna use, either VHF/UHF, or worse VHF only, and they will indeed have erratic performance over various channels on a modern cable system. And certainly any attenuator purchased or deployed before 1GHz cable systems were widely deployed should not be used, as they simply were not designed for the job. But cable companies have a plethora of well designed products suitable to their purposes available now. Perhaps the move to splitters was based on not having to ascertain the quality of every attenuator they encounter, especially if many were deployed in their system before they increased their bandwidth. That would make some sense, as splitters are normally well marked as to their bandwidth, whereas attenuators are normally not so marked..

Your cable signal should not vary significantly with the seasons, even at the end of the line. If it does there is something wrong the cable company should be fixing. Properly operating cable system equipment is moisture-tight and temperature stable.


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## nooneuknow (Feb 5, 2011)

k2ue said:


> Attenuators designed for current cable use will have very uniform attenuation from near DC to 1GHz or above. But there are plenty of attenuators around that were designed only for Outdoor Antenna use, either VHF/UHF, or worse VHF only, and they will indeed have erratic performance over various channels on a modern cable system. And certainly any attenuator purchased or deployed before 1GHz cable systems were widely deployed should not be used, as they simply were not designed for the job. But cable companies have a plethora of well designed products suitable to their purposes available now. Perhaps the move to splitters was based on not having to ascertain the quality of every attenuator they encounter, especially if many were deployed in their system before they increased their bandwidth. That would make some sense, as splitters are normally well marked as their bandwidth.
> 
> Your cable signal should not vary significantly with the seasons, even at the end of the line. If it does there is something wrong the cable company should be fixing. Properly operating cable system equipment is moisture-tight and temperature stable.


The attenuators I have been speaking of are not rated, other than the -dB ratings. They came off Cox trucks before 1GHz rolled. I always ask if they have any when they are out, and they usually just have the same ones, which they only have because they removed them. So, I think you are right in your assessment.

Your assessment of my AGC stress condition, and change of the seasons adjustments are also correct.

I get these lines from Cox:

1. "We do not support TiVo"

2. "We will not re-engineer our network to accommodate your TiVos"

3. "You are at the end of the run, and the only customer connected to it. It is not worth re-engineering our network for one customer who uses equipment that we do not support"

4. "You can not force us to fix anything we don't want to"

After working my way up to high management, I was told this: "We do not have the power to force our network maintenance engineers to fix anything they don't want to. It is their call, and we can not override them".

One time, when the maintenance engineer in charge of my network segment rolled up, he nearly slammed my hand in the truck door, and then sped-off, as soon as I answered his question of what equipment I was using. The answer was "Your cablecards used in TiVo DVRs".

I complained to corporate, and hinted they were lucky I had good reflexes. I am now on some sort of blacklist of people not to engage, or talk to. Calls I place are supposed to be returned, but never are.

After three years of getting these same lines, and this treatment, I'm done trying. I can get nowhere. The FCC would not help me, nor could TiVo. Now you know why I'm so angry sometimes...


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## k2ue (May 9, 2002)

nooneuknow said:


> TAfter three years of getting these same lines, and this treatment, I'm done trying. I can get nowhere. The FCC would not help me, nor could TiVo. Now you know why I'm so angry sometimes...


If I were you I'd be using a DirecTV Tivo in a heartbeat, and the hell with cable. Even if you still need cable for internet -- they have to fix the line if it won't run a cable modem -- that IS supported equipment.


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## HarperVision (May 14, 2007)

k2ue said:


> If I were you I'd be using a DirecTV Tivo in a heartbeat, and the hell with cable. Even if you still need cable for internet -- they have to fix the line if it won't run a cable modem -- that IS supported equipment.


Ditto


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## nooneuknow (Feb 5, 2011)

k2ue said:


> If I were you I'd be using a DirecTV Tivo in a heartbeat, and the hell with cable. Even if you still need cable for internet -- they have to fix the line if it won't run a cable modem -- that IS supported equipment.


I've already placed all my eggs in the TiVo Roamio basket. Done and done. I'm stuck with what I have (for many reasons, which I'll spare everybody the details on).

Their idea of "fixing the line" is to swap out splitters to different values.

If that brings my cable modem and Tuning Adapters within the acceptable signal strength and SNR, their work is done (going by their playbook).

I think you've figured out I can do that myself. It's not worth the way they treat me, and setting aside a whole day for them to come, when I can do everything they can, except analyze the signal, say it shows AGC stress, tell me they will send maintenance, and then maintenance never comes, or leaves without doing anything (beyond the splitter swapping the techs already did).

*I've hijacked the thread. I did not intend to. Sorry everybody...*

I'll still be watching for on-topic matters, and will try to help and contribute, if I can.


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## DCIFRTHS (Jan 6, 2000)

k2ue said:


> Simple LC (Inductance/Capacitance) Filters are reflective, and attenuate by *refusing to accept energy* in the rejection band. The illustrative analogy would be a rope tied to a wall: if we whip the rope to create a wave, the wave moves down the rope, is refected by the wall, and comes back to our hand. Absorbative filters, which only absorb and do not reflect, are more complex, requiring internal resistors as well as LC elements, and are almost never encountered in a controlled impedance environment like cable distribution, for the simple reason that the signals reflected by simple LC filters are absorbed on the first bounce by the controlled 75 ohm impedance of the connected device(s) -- it is only necessary for *one end *of a transmission line to be terminated to control reflections. It is a common assumption that everything connected to a transmission line system would be matched to it (completely accepting/absorbing all signals sent to it), but that is not the case, some devices do their job by simple rejection.


Thank you so much for the explanation


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## The TiVo Dude (Jun 9, 2004)

And thanks to all for the great responses.

Search is my friend, but I was still confused. No longer.

I installed a POE filter yesterday.

Thanks again.


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## grey ghost (Feb 2, 2010)

Where did you get your POE filter? I've been like you, my Tivo worked without a filter, so I didn't see the need to put one on. I guess I really need to go purchase one.


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## k2ue (May 9, 2002)

grey ghost said:


> Where did you get your POE filter? I've been like you, my Tivo worked without a filter, so I didn't see the need to put one on. I guess I really need to go purchase one.


Just search on MOCA filter on ebay, or you can order one from TiVo.


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## HarperVision (May 14, 2007)

Or complain to your cable co and they may give you one. That's what I did. It helps their system and behooves them to have one installed at your location.


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## dianebrat (Jul 6, 2002)

grey ghost said:


> Where did you get your POE filter? I've been like you,* my Tivo worked without a filter*, so I didn't see the need to put one on. I guess I really need to go purchase one.


This issue is never that you need one for your Tivo to work, it's to prevent your data signals from backfilling out of your home and affecting your neighbors networks. If you're MoCA is active and you don't have a POE filter, your data could be available to your neighbors, and it can cause trouble for their installations. It's never about your Tivo not working.


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## k2ue (May 9, 2002)

dianebrat said:


> This issue is never that you need one for your Tivo to work, it's to prevent your data signals from backfilling out of your home and affecting your neighbors networks. If you're MoCA is active and you don't have a POE filter, your data could be available to your neighbors, and it can cause trouble for their installations. It's never about your Tivo not working.


That's not going to be universally true: for MOCA to have a thruput of >100Mbs the path loss should not exceed 56dB, and it will choke about 6dB below that. Less than 50dB path loss will let it reach its max speed of about 200Mbs. So there is about a 12dB margin from hunky-dory to not working. TiVo is using MOCA at ~1100MHz, where cable loss is significant (~75 feet of (good) cable is equivalent to a 4-way split). High-grade splitters like cable companies use may have 30dB typical port isolation. So if you have a scenario of the TiVo on separate ports with 75 ft of line to each and a another 2-way split on the way to the TiVos, you would have 50dB path loss -- right at the limit for max thruput. Add any other liabilities (more splitters, old cable, poor connectors, RG-58, attenuators, etc) and you are going to lose thruput. Adding a POE filter would boost the MOCA level ~16dB if your first splitter is 4-way, or ~22dB if it is 2-way, so it can be helpful in those cases.


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## RickBrant (Sep 19, 2015)

k2ue said:


> the signals reflected by simple LC filters are absorbed on the first bounce by the controlled 75 ohm impedance of the connected device(s) -- it is only necessary for *one end *of a transmission line to be terminated to control reflections.


Pardon me for resurrecting this thread, but I don't see how this can work.

The cable is obviously far longer than a wavelength in the MoCA band (about 1 to 1.6 GHz; wavelength in the cable would be well under a foot). So the reflected signals are going to be arriving at the receiving modems in what can only be described as an unpredictable phase relationship with the direct.

If you only have two MoCA modems this wouldn't be much of an issue (assuming the filter was installed very close to the "outward-facing" modem). But with three or more it's easy to imagine situations where signals sent by one will arrive at another by both "direct" and "reflected" paths, these differing hugely in transit time.

Why wouldn't this corrupt the signals?

Example: Suppose we pick 1200 MHz and assume the cable velocity factor is 0.7, then the wavelength is about 0.36 m. (The exact numbers don't matter; I'll show why in a moment.) Suppose we have one modem "B" that's 12m from the "head modem" ("A") and then another modem "C" that's 10m beyond B. B sends something. It's 10m to C by the direct path; it's 34m by the reflected path, a difference of 24m. 24m divided by 0.36 is 66.666... Which means the reflected signal will arrive at C 0.666 wavelengths, or 240 degrees out of phase, with the direct signal.

That's a significant phase shift. If it was 180 out of phase it would be canceled if they were of equal amplitude.

Of course the MoCA signal is very wideband (per Shannon-Hartley it has to be to get good data throughput), so we'll actually be getting reflections at all manner of phase shifts from the direct. Which is why the numbers above don't have to be exact. No matter how the stations are spaced there will be interfering reflections over a significant part of the MoCA band.) I just can't see why this can be ignored.

I mean... I've never ever been in a reverberant room where the echoes improved intelligibility, let alone conveniently reinforced the sound that arrived by the direct path!


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## RickBrant (Sep 19, 2015)

Update: So, I got a MoCA filter and tried it. My configuration is similar to the one described in my post above: Three modems. The path from the first modem's coax "in" goes through at least 20m of coax to a splitter, modem "B" is about 2m from that splitter, then the other leg of the splitter goes via who-knows-how-much cable... but certainly at least 10m... to modem "C". 

File transfers from a machine attached to modem "B" to one attached to modem "C" were completely unaffected. Not better, not worse. One might expect that if there was an improvement in signal strength there would be an improvement in throughput. However the throughput I'm seeing was already at the limit of the MoCA modems' Ethernet interfaces (100 Mbit/s) so improvement really shouldn't have been possible. On the other hand it didn't get worse either, so clearly my concerns about destructive interference were not realized here. 

There's no harm in leaving the filter in, and it might avoid complaints from the cable company, so I'll leave it in, but as far as I am concerned the claims of improved performance remain unproven. 

Sure wish I hadn't sold my spectrum analyzer - I'd like to have seen what's happening on the cable with and without the filter! (But that would have been its first use in many years, so...)


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## snerd (Jun 6, 2008)

From a purely theoretical perspective, if you could build a system with perfect splitters, then with no PoE filter in place the MoCA devices would not be able to communicate at all. I'll explain why using a simplified system using a single "perfect" splitter with say 20 meters of coax between the splitter and each of the (A,B) MoCA adapters. We'll assume everything is properly terminated so that there are no reflections anywhere in the system. Now when A transmits, when the RF signals reach the splitter, they go through the splitter and out toward the cable company, never to return. Since we've assumed the splitters are perfect, the "downstream" ports of the splitter have perfect isolation, which means that none of the upstream signal from MoCA A ever reaches MoCA B. Adding the PoE filter causes the signal to be reflected back down through all of the splitters so that the MoCA signals are kept in the house rather than leaking out to the neighbors.

Real splitters aren't perfect, having port isolation of maybe 25 dB to 30 dB, so when an upstream signal hits the splitter, a tiny fraction of the RF power from MoCA A will be coupled to the other port to be received by MoCA B. A splitter with 30 dB of port isolation will allow 0.1% of the upstream RF power to couple to the other port, while about 45% of the upstream RF power leaves the house, and the rest is absorbed (and wasted) in the splitter. The 45% figure is based on a 3.5 dB insertion loss for a 2-port splitter, since the insertion loss is the same for downstream and upstream signals. Adding a PoE filters mean that the upstream signal will lose 3.5 dB going upstream through the splitter, then a small loss (may 1 dB) as it is reflected by the PoE filter, then another 3.5 dB loss going back down through the splitter for a net loss of 8 dB plus loss in the coax. So overall the signal is 22 dB stronger than if no PoE filter is used.

There is still an issue with interference between the part of the signal that is due to imperfect port isolation having a different phase than the part of the signal that is reflected by the PoE filter, and I don't really know how the MoCA system resolves that problem.

The TiVO menus include Network diagnostics which show the PHY data rates and some numbers about signals levels. You might try looking at those numbers with and without the PoE filter to see how much difference it makes, if any. The difficulty is the MoCA is designed to operate essentially at full speed as long as the total loss is below about 50dB, so you might need several layers of splitter and some long cables before you see much degradation in performance.


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## RickBrant (Sep 19, 2015)

snerd: Thank you for your well-thought-out and -expressed answer.



snerd said:


> [...]So overall the signal is 22 dB stronger than if no PoE filter is used.
> 
> There is still an issue with interference between the part of the signal that is due to imperfect port isolation having a different phase than the part of the signal that is reflected by the PoE filter, and I don't really know how the MoCA system resolves that problem.


I think you've answered it! If the reflected singal is 22 dB stronger than what leaks through the splitter, then the leaked signal is pretty far down in the noise. 22 dB is a LOT of difference with modern (DSP) techniques, with a modulation as well-characterized as QAM.

Practically speaking, we have to conclude that obviously the MoCA demod can cope with it. If it couldn't, my throughput would have been much poorer without the POE filter.



> The TiVO menus include Network diagnostics which show the PHY data rates and some numbers about signals levels. You might try looking at those numbers with and without the PoE filter to see how much difference it makes, if any.


Ah, well, I... (looks down, draws circles with toe) I don't actually have any TiVO hardware. I'm just using NIM100 modems. I was directed to this thread for the tech details on why the POE filter is supposed to improve MoCA, independent of whether it keeps the cable company happy. I have some info on getting into the management interface of the NIM100 but I don't know if they have something that reads out SNR. I'll probably look at it someday but there's no pressing need atm.

Thanks again for the answer, this clears up a lot.


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