Power over Ethernet (PoE) basics and beyond

(edn.com)

141 points | by voxadam 6 days ago ago

90 comments

  • sema4hacker an hour ago

    POE switches can have more jacks than they can provide power for. I have five pan-tilt-zoom (PTZ) cameras but learned they can need up to 40 watts each, preventing plugging them all into a particular POE switch I had. Chaining a couple of switches solved the problem, be beware how much power your devices need to consume.

    • ifwinterco 3 minutes ago

      Is it actually likely that all 5 would be drawing full power at once though?

      It's normal in household wiring (at least here in the uk) for circuits to be somewhat undersized based on the concept of "diversity" (i.e. it's highly unlikely every socket on the circuit will be drawing 13A simultaneously)

  • skopje 5 hours ago

    PoE is awesome. My custom home security system is all CCTV PoE with a gstreamer backend running on four-core fanless linux box. Way to go. Complete control. No batteries, no wares spying on me, no personal data getting scraped by big guys. (Cloud connectivity sucks because I have segmented mp4s and jogging through them hurts but I only care for events after they happen, not while they happen.)

    • skulk 5 hours ago

      Got any recommendations on what cameras to get? The market is absolutely flooded with cheap shitty cloud-connected all-in-one cameras making it hard to find good, simple products.

      • foobarkey 2 hours ago

        The cheapest (~15 USD bullet, 20 USD dome) PoE cameras on AliExpress (focal length is pretty much the most important parameter to look at, depending on the fov you want) hooked up to a Unifi NVR. Skip all the vendor manuals, setup steps, and apps - adopt them directly to Unifi Protect.

        I put them on separate vlan where they get no outbound network connectivity.

        For cases where you want things like facial detection or license plate detection (automatic doors/gates) get a Unifi AI though and those things cost, but for normal perimeter/room monitoring the cheap ones are very good

        • vulkoingim 5 minutes ago

          I would argue sensor size is what's most impotant to look for in a camera.

          Have a look at this thread [1] I have bookmarked. I found it quite informative. I already got some cheap cameras and set them up, but I wish I would have found it earlier. The ones I got are 4MP with 1/3" sensor and perform absolutely terribly in night setting.

          [1] https://ipcamtalk.com/threads/getting-cameras-with-the-right...

      • skopje 36 minutes ago

        https://www.cctvsecuritypros.com/

        Blue-line domes, the $240 ones. Four of them. I'd get more but do not know how to make outside routing look neat. i have one bullet and i don't like it and don't use it, i prefer the wide-angle domes with ir.

      • mlsu 2 hours ago

        I got a lot of 6 Axis cameras from eBay, it was around $200. I think they took them off a school or something but they were in great shape. They look great and have no spyware etc because it’s an industrial company. I recommend getting some industrial surplus because they tend not to have all the bloatware and have significantly better weatherproofing, casing, etc, even if the optics are the same as the consumer units.

      • lights0123 5 hours ago

        Any of those that mention ONVIF or RTSP will do if you put them on a LAN without internet access

        • ssl-3 6 minutes ago

          Aye.

          ONVIF is the (now quite old, but still very relevant) standard for interfacing IP cameras locally on a network.

          A cheap-but-performant ONVIF camera on an isolated VLAN (or a physically-isolated network; I won't tell anyone) can be a thing of beauty that is also completely unable to call home to some mothership in the clown.

          I'm frankly very surprised that I don't see it mentioned here more often when discussions of cameras arise.

          https://en.wikipedia.org/wiki/ONVIF

        • kalaksi 36 minutes ago

          Not my experience. I've tried several such cameras and most of them are underpowered and suffer from very low fps or are fine when there's no movement but with movement the fps drops drastically essentially making the camera close to useless.

      • stargrazer 5 hours ago

        reolink, also look at the frigate nvr software, they have a list for decent recommendations

        • aivisol 39 minutes ago

          Reolink with Synology NAS using their native Surveillance app. All stored locally, no cloud. One issue with Reolink I haven’t solved is that it is unable to detect approaching cars in the night. Departing cars work fine though. Otherwise no complaints.

        • zer00eyz 5 hours ago

          This is the way.

          Reolink cameras are pretty good for what they are. Just dont buy into their NVR solution...

          Frigate also has some interesting applications to go along with it, see: https://github.com/mmcc-xx/WhosAtMyFeeder

          I also have YOLO on my to do list for the home cameras.

      • Cyberdogs7 4 hours ago

        I have built out several Amcrest systems. You have the many options for recording and access, that will allow remote access without going to the cloud.

    • benoliver999 an hour ago

      What switch do you use? I have PoE wifi APs throughout the house, but I bought an Aruba switch and it's super noisy tbh. Fine for me because it's in the basement, but I couldn't recommend it

      • ssl-3 24 minutes ago

        At one of our offices (it is not a large office), I have a 24-port Netgear POE switch running the show. If it has a fan inside (it may! there's cutouts for fans on the sides of the chassis but I have not looked inside), I've never heard it.

        It fit the price-performance curve for our needs several years ago when we eventually outgrew the previous Netgear POE switch...that was also apparently fanless, and that I installed in 2007.

        IIRC, it is a GS724TP. It's running a dozen cameras and some access points, and almost all of the rest of the ports are filled up with computers and printers and other Ethernet stuff. No issues at all to to report so far.

        (A used enterprise switch with serious fans may be cheaper and/or more featureful and/or more reliable, but do we need that kind of noise at home? We sure don't need it at that small office.

        I've also installed some fanless Cisco POE switches with big heatsinks (and dual power supplies, each fed from different sources) on some high-dollar projects where ultimate reliability was kind of a big deal, but... sheesh.

        If one of these installed Netgear switches dies in one of these low-risk environments, I'll just patch things up for now and get a replacement coming under warranty.)

      • skopje 32 minutes ago

        some netgear 8-ch poe switch. i don't recall. it's been on and running for about 8 years with no issues, way up near the ceiling of my garage, covered with dust. its plugged into a wrt1900 router i bridged via wifi to my main router.

    • yardstick 2 hours ago

      Do you upload events to a remote location? How is your storage box secured against theft? That’s my biggest concern for doing local cctv- if you are robbed, they’ll grab anything that looks of value.

      • skopje 34 minutes ago

        yes that is something you need to decide for yourself i'm ok with it. i push my segments up to an s3 bucket but yes if they find the box before the rsync i lose. oh well. there's much more valuable stuff in the house they'll probably go for first. i suspect junkies aren't that smart.

    • shadowpho 3 hours ago

      Poe is great for many things, but it’s not as efficient as direct connection (for both low and high power.)

    • dheera an hour ago

      Except when it isn't awesome. There are multiple PoE standards. Passive PoE, active PoE, PoE+, PoE++, PoE+++, 802.11af, 802.11at, 802.foo, blah blah.

      If they had just stuck with 12VDC and buildings had 12VDC wall sockets everywhere, everything would have been fantastic.

      I also had a PoE HAT for a RPi that smoked it. Never doing PoE again. 48V and 3.3V electronics probably don't belong within 10cm of each other.

      • varenc an hour ago

        I use a PoE "extractor" to power my RPi over PoE and it works great. The extractor does the negotiation and safely gets the normal 48V PoE power, then converts that to 5V outputted on a USB-C cable that powers the RPi. Extractor also has an Ethernet[1] passthrough port that goes into the RPi as well. A bit basic, but seems relatively error proof.

        [1] https://x.com/varenc/status/1961587127931867466

      • skopje 34 minutes ago

        I never had to know the difference. I have four cctv cams on ~100ft of cat5 each. didn't have to think about it, just plugged them in and they worked.

      • buccal an hour ago

        240V AC and 5V DC manage to live close in a charger without problems. Problems with quality does not depend on voltage. I love the concept of PoE with one exception that it requires constant 1W or similar load to work even if it is not needed for low power device.

  • stego-tech 5 hours ago

    PoE is a godsend that should really be in more consumer devices and households, alongside structured wiring. An AppleTV, Chromecast, or NVIDIA Shield can easily fit within the envelope of PoE+, as can many enterprise-grade switches and WAPs (see UniFi as an example). Converting AC to DC once at the switch is more efficient (in resources and often, but not always, power) than including bulky PSUs for every device, while simplifying the ease of setup for end users (in theory).

    Whenever possible, I opt for PoE. It’s a damn shame it’s limited to a niche userbase given its myriad advantages.

    • toomuchtodo 3 hours ago

      It’s supported by the latest revision of Glinet’s Comet kvm over IP hardware, which was a cool upgrade imho.

      https://www.gl-inet.com/products/gl-rm1/

    • benoliver999 43 minutes ago

      All our desk phones at work are poe and they have pass through ports to allow connecting a PC to the LAN. Really neat to just have one cable that gives you a powered phone, and Ethernet access.

    • dfc 4 hours ago

      In my head enterprise grade switch has 48 ports with some >10g SFPs for uplink. What does enterprise grade mean to you? And what enterprise grade switches are poe powered?

      • greycol 4 hours ago

        It's such a wide field that it's hard to pin down. I agree if your thinking about what a business that isn't just a handful of people needs then we'd be looking at the kind of switch you're thinking of if it has standard office workers. But soon as you start talking about businesses that are manipulating central data (which keep in mind would probably include most primary business, design workers or anyone working with media not just software people) you're talking about a wide gamut of devices that you wouldn't really (at least traditionally) call consumer grade.

        Mikrotik website has a good selection of them and if you look at the other hardware types it'll be interesting in getting an idea of weird things you don't see in normal offices.

        https://mikrotik.com/products/group/switches

        Apart from obviously larger bandwidth options like 28qfsp 100gb (I'm unaware if mikrotik does them but 400gb is normal in some circles) there's things like reverse POE switches, media converter switches, and all sfp+ switches.

        Poe++ exists and you can use switches with it to power poe+ switches that will power poe switches. Or they can be used to power laptops or NUCS directly.

      • hackmiester 4 hours ago

        Arista 710P for instance. I don’t see what port count has to do with it, it runs the same OS and has the same capabilities as all their other switches. Cisco has a Catalyst 9k like this too.

        • dfc 3 hours ago

          In my head, one of the things that makes up an "enterprise grade" switch is 48 ports. Because "for the enterprise", in my opinion, evokes some idea of large scale deployment, not a mom and pop trinket store with one PoS cash register device and three company computers.

          What does enterprise grade mean to you?

          • kevvok an hour ago

            The smaller switches like the Arista 710P are meant for deployment out at the edge of the network where you want something small and quiet (e.g. at people’s desks or in conference rooms) to provide more ports without needing as many runs back to the network core where the big loud switches live. They’re still enterpise grade since they support enterprise features like centralized management, VLANs, QoS, IGMP snooping, etc.

    • ianburrell 4 hours ago

      There are finally PoE adapters that give USB-C power and USB Ethernet. Those should allow home theater devices to be powered and use wired Ethernet. But the ones I have found are expensive, but should drop if there is demand.

      • addaon 32 minutes ago

        I also haven’t found any bidirectional ones yet, even though the hardware (except maybe the negotiation chip if it’s pure PDE) should be able to support it. Would be really nice for development of PoE devices to just hook up one dongle to my laptop.

      • stego-tech 3 hours ago

        I already use splitters throughout my stack and devices, but that’s additional material and complexity cost compared to native PoE.

    • scottlamb 4 hours ago

      > Converting AC to DC once at the switch is more efficient (in resources and often, but not always, power)

      Can you expand on "often, but not always, power"? Here's my guess:

      * It's more efficient for the small stuff: little wall warts aren't very efficient I think in part because there's some no-load consumption for each. The switch pays that no-load cost once for many devices and has like an 80-plus gold or better PSU, hopefully. And then I think even cheap buck converters are like 95% efficient; they have some no-load consumption too but I think less than the wall warts? And even though this goes over 2 (or 4) tiny wires, at 48V–56V, the current is low enough that power loss is not bad because those wires are just for one small device, and P=I^2R.

      * It's less efficient for the big stuff: that P=I^2R starts to suck for the PoE case, and in the non-PoE case they're more likely to have efficient AC->DC conversion on their own. 90% efficient beats 90% * 95% efficient.

    • somerandomqaguy 3 hours ago

      Doesn't really make any sense to in that example.

      Where ever you're putting the TV you have to put in regular power anyways, so it's fairly tidy to just put the device's power cable parallel with the TV's power cable. WiFi will handle communication. On the other hand, NEC and CEC requires minimum of 2 inches gap for communication wiring to electrical so you're now you've got that minor complication.

      POE makes sense mostly when it makes sense to combine communication and power cabling. Corded phones, wifi access points, security cameras, small touch screen modules, etc. Not saying what you're doing can't work, but the added expense of installing parallel CAT6 everywhere doesn't seem worth it.

    • RulerOf 2 hours ago

      > An AppleTV, Chromecast, or NVIDIA Shield can easily fit within the envelope of PoE+

      I ended up buying a PoE extractor and barrel plug adapter for my Roku, and another extractor for my HDHomeRun.

      It annoyed the heck out of me that they had PoE running to them and still had to be plugged into a separate transformer.

    • morshu9001 4 hours ago

      Might be because it's scary. User plugs passive poe into something not expecting it, magic smoke

      • labcomputer 37 minutes ago

        Theory and practice and all that, but that shouldn’t happen regardless.

        A correctly-designed Ethernet interface is galvanically isolated at both ends to avoid ground loops, differing grounds, and other nasties over long distances.

      • ianburrell 4 hours ago

        Passive PoE is evil, standard PoE is safe since does negotiation.

        • morshu9001 4 hours ago

          Step 1 is to eradicate passive poe then

          • ianburrell 4 hours ago

            Ubiquiti has stopped selling anything with passive 24V PoE, and has lots of standard PoE. The risk is low since I think only worked with injector so no switches providing power to everything.

            • wolrah 2 hours ago

              > Ubiquiti has stopped selling anything with passive 24V PoE

              In their consumer "UniFi" product line. Pull up their store and switch over to the "UISP" product line. Most of the smaller wireless devices and consumer-tier CPE are 24v passive, most of the larger wireless devices, 60GHz bridges, etc. are 48v passive, a few devices in the middle support both, and standard "active" PoE is almost nowhere to be found. Even on product lines that weren't even dreamed up when modern standard PoE was ubiquitous.

              They say it's because the WISP crowd loves passive PoE as it can easily be wired to batteries on towers, and I get that, but that's no excuse for not also supporting standard-based PoE on the device end. There's no good reason for a product designed in the 2020s to force the installation of passive PoE where there was none prior.

              They demonstrated they can do both with most of the transition-era UniFi products. Support and encourage the use of standards, allow the use of non-standard but common alternatives where they make sense.

              • morshu9001 an hour ago

                Also my UniFi AP has passive poe cause it's just that old. Without researching, idk when that got fixed because nothing on the boxes tells you. Consumer tier means people will plug whatever fits.

          • ocdtrekkie 4 hours ago

            Passive PoE is already extremely rare today. Early Ubiquiti stuff probably is the most passive PoE stuff likely still in service.

    • wmf 4 hours ago

      Probably USB-C wall outlets will end up solving this instead.

      • greycol 3 hours ago

        Which is ok if done right, but if they're anything like the usb-a ones there'll be plenty that are continuously pulling much more power than they need let alone the danger of uncertified ones.

        For those thinking about adding one they've grabbed off amazon and installing themselves, please do a bit of hunting and reading rather than just buying the first word soup brand cheapest ones. Also remember installing uncertified electronics in your walls is a good way to void your insurance if they're the cause of disaster and turn it into a legal battle even if they're not.

      • userbinator 3 hours ago

        Besides being reversible, USB-C is a horrible connector. Tiny contacts, no positive retention, and a massively overengineered standard that should've been broken up.

  • ww520 21 minutes ago

    It is also good on delivering power to AES50 snake stage box to microphones that needs fathom power.

  • Animats 2 hours ago

    Not clear on how multiple sources on the same cable work. Is that allowed? Is there a power break at midspan, or does power flow through? How are the regulators coordinated?

  • rmunn 5 hours ago

    Practical question for HN: How do you all label your PoE cables so that you don't accidentally plug the powered cable into a socket that wasn't expecting 48 volts on those pins and fry something? (I know the power injector is supposed to only deliver power when it's safe, but if all your devices work exactly as they should all the time, then I'd like to buy that bridge in Manhattan you're selling).

    Do you buy Ethernet cables of different colors and say "Yellow is reserved for PoE, all yellow cables should be assumed to have power on them"? Or do you slap a "48V" label on both ends of the cables you're going to use for PoE and the label is what warns you that this cable should only go into the PoE receiver, and not into an unpowered device? Or do you just not label your PoE cables any differently, and trust that the injector will never malfunction at the same time that you plug the PoE cable into the wrong device?

    • hackmiester 4 hours ago

      All 21,000 ports I administer have 802.3 standard PoE enabled at all times. Incidents of inadvertent powering are at zero. I think this is just a non problem.

    • Kirby64 5 hours ago

      Unless you’re using the “passive” PoE variants (ubiquiti sold these for awhile, for instance) that always has voltage on the pins, there is no risk. Negotiation is mandatory for the actual IEEE variants. Just use those and don’t worry about it.

    • varenc an hour ago

      With modern 802.3 spec compliant PoE: I don't worry about it. At least with all the switches I've used. Never ever had it send power to some device that wasn't expecting it.

      This is a bit analogous to USB-C PD power supplies, which can supply 12V/24V, but only do this when devices ask for it. I don't worry that my laptop's USB-C power supply will go rogue and send 24V to my earbuds.

    • generuso 3 hours ago

      From what I have seen, Ethernet ports always have a small isolation transformer for each twisted pair, between the connector and the PHY. Usually four of such transformers are combined in one small magnetics package. The insulation in the transformer is specified to withstand over a kilovolt of lightning induced voltage -- that's one of the purposes of such galvanic isolation.

      The data travels as the differential voltage in each of the twisted pairs, and is transmitted magnetically by the transformer to the secondary winding. The power is applied between different pairs, and in each pair appears as a common mode voltage. This is all stopped by the transformer, and in devices designed to support PoE, the PoE circuits tap the mid-point of the primary windings to access the supplied voltage.

      So at a first glance, it seems that if 48 volts is applied between the twisted pairs to a non-PoE device, this voltage would simply be blocked by the transformer. But since there is a widespread concern about this, there must be more to the story -- maybe somebody who actually worked with these circuits can explain why this is more complicated than it seems at first?

      Edit: Found an answer. It seems that at least some of the designs of non-PoE Ethernet jacks terminate the common mode signals to a common ground though 75 Ohm resistors. In this case, if the voltage were applied between the twisted pairs, the resistors would dissipate far too much power and would burn out. So there is definitely a concern with the dumb PoE injectors and at least some non-PoE devices. https://electronics.stackexchange.com/questions/459169/how-c...

    • supertrope 5 hours ago

      Always buy standards based equipment. 802.3af, 802.3at, 802.3bt. You can label cables and jacks with red lettering (“Passive PoE. will fry your laptop port. Really!”) but it only takes one mistake to let the magic smoke out.

    • VanTheBrand 5 hours ago

      I completely avoid passive PoE. Not worth the risk. On the standardized active stuff I’ve never had any issues even when I’ve plugged it broken cables to unpowered devices.

    • leoh 5 hours ago

      What every engineer really should know!

    • doubleg72 5 hours ago

      Well the entire point of standards is so things work exactly as they should every time. I haven’t seen any issues with standards based poe

  • willis936 4 hours ago

    Other stuff engineers should know:

    - PoE endpoints should have isolation barriers, factor this into cost and size estimates

    - Don't skimp on TVS

    - ideal diode full bridge rectifiers are really cool and you should use them (in more power entries than just PoE)

    • dynode 3 hours ago

      Awesome I’m just doing a POE design now.

      - Was having a conversation today about isolation and grounding for POE (product has a metal case). Do you have a reference? Or standard?

      - TVS ahead of the bridge right?

      - Do you have a part recommendation or reference design for ideal diode POE?

  • RandomBacon 2 hours ago

    Question for anyone who might have a suggestion:

    I have a Ring home security system. I would like to get an offline home CCTV that only records when the alarm is set (either in Home or Away).

    A quick internet search does not show an API. I'm not sure Ring has a device that I could wire a relay (if that's the right thing) to.

  • brcmthrowaway 6 hours ago

    1. Why are IEC/Edison cables so thick if Ethernet can carry the equivalent power?

    2. How does PoE compare to Powerline Networking?

    • otterley 5 hours ago

      No Ethernet cable that I’m aware of is capable of carrying anywhere near 15A of current. Even type 4 (90W assuming 57V) is 1.5A.

    • skinner927 5 hours ago

      1. Assuming IEC refers to cables we plug a desktop PSU into mains/wall: IEC can carry up to 1800w vs 100w PoE++

      2. Powerline networking is considerably slower and less reliable than CAT5/6. Additionally, building code for running power lines is much more strict than low voltage CAT5/6

      • dlgeek 24 minutes ago

        Nerdsniped: You're describing a IEC 60320 C13 cable - they're technically only spec'd for 10A, which means you're looking at ~1200W, not 1800.

        (However, UL will list them for the full 15A -> 1800W, and I'm sure plenty carry that. And for that matter, I suppose you can get twice that in Europe on 240v...)

  • cyberax 6 hours ago

    What about PoE for 10G Ethernet? I see that there are some vendors (e.g. Ubiquity) that are offering devices with it, but I don't see it in the standards?

    • VanTheBrand 5 hours ago

      As of 802.3bt (PoE++) the standard includes support for “all standardized copper link speeds of up to 10GBASE-T.” The previous standard 802.3at (PoE+) added gigabit support.

      So any 10GbE (and 2.5GbE) PoE/PoE+ devices out there are technically not to spec (lots of these on Ali Express) but I believe the the Ubiquiti 10GbE stuff is all at least PoE++. [1]

      (They do have their own non spec labeled PoE+++ products though, which are really just “802.3bt Type 4” but they added another plus because that probably sounded better.) [2]

      [1] https://store.ui.com/us/en/products/us-xg-6poe, https://store.ui.com/us/en/category/accessories-poe-power/co...

      [2] https://store.ui.com/us/en/category/accessories-poe-power/co... , https://help.ui.com/hc/en-us/articles/115000263008-PoE-Avail...

    • zamadatix 5 hours ago

      I think every major vendor has had 10G PoE switches: Arista, Aruba/HPE/Juniper, Cisco, Extreme, and Fortinet for sure. The problem is there is little use case for 10G + PoE in the enterprise and even less for consumers. Ubiquity likes to tout it for the 10G APs... but, realistically, most are worried about airtime with APs, not 10G wired throughput from a single one when they have a thousand.

      As a result, it tends to be relegated to the "high end switch which has every feature those one-off customers demand but costs an arm and a leg as a result" model/family. E.g. the only ones I ever sold were to a hospital that wanted to have select switches have 10G for radiology workstations but also wanted to still be able to plug 1G APs in without having to think about the port types. Radiology was covering the cost, so they didn't care it was a waste of money.

      • VanTheBrand 5 hours ago

        I find it useful in Broadcast Video Production (that’s where I end up using it most) and yeah with Wifi7 supporting > gigabit speeds I’ve seen some Wireless Access Points supporting it (though 2.5GbE Poe++ is more common there and practically speaking enough)

    • zer00eyz 4 hours ago

      My question is this: what would you plug into POE that would need 10gbe?

      • varenc 32 minutes ago

        Ubiquiti sells Wi-Fi access points with 10GbE uplink ports that are PoE powered: https://store.ui.com/us/en/category/all-wifi/products/e7

        In theory at peak throughput the access point might use close to 10 gigabit. But definitely more than 1G/2.5G.

      • MBCook 4 hours ago

        I had no idea it could supply 90 watts or so. At that point you could power some pretty powerful terminals.

        The new 14” MacBook Pro comes with a 70 watt charger. An M4 Air only gets a 35 watt adapter.

        Basically seems like enough power is available to run something pretty powerful.

      • fc417fc802 4 hours ago

        What if you need a small switch somewhere due to the runs otherwise being too long?

      • kkapelon 29 minutes ago

        Another smaller switch closer to the location of your devices.

      • wmf 4 hours ago

        A multi-radio WiFi 8 AP?

        • zer00eyz 3 hours ago

          You would likely need fiber to saturate it...

      • mrheosuper 4 hours ago

        an all nand-flash NAS ?

      • cyberax 3 hours ago

        I have a WiFi 6 AP that can saturate a 2.5GB link when I test it with two devices. So far, for me the peak speed for an individual device was around 1.6GB

        I have not yet tested WiFi 7 APs, but they are supposed to be even faster. The use-case for me is video editing over WiFi (I do have a 10GBe Thunderbolt adapter but hey, I like wireless).

  • mmaunder 4 hours ago

    Jeez top post on HN and there's a full overlay ad to "download a mac extension". This deserves a summary post to save others the click. Here's the "what every engineer should know" without the spam:

    PoE (Power over Ethernet) sends both DC power and data through the same twisted-pair Ethernet cable, allowing devices like IP cameras, wireless access points, and VoIP phones to run without separate power lines. The power is delivered by Power Sourcing Equipment (PSE) — either an endspan (built-in PoE switch) or a midspan (PoE injector placed between a non-PoE switch and the device). The powered device (PD) negotiates power via detection and classification before voltage is applied, preventing damage to non-PoE gear. IEEE 802.3af (Type 1) provides up to 15.4 W at the source, 802.3at/PoE+ (Type 2) up to 25.5 W delivered, and 802.3bt (Type 3/4) extends that to roughly 60–90 W using all four wire pairs. Engineers need to understand not just wiring, but also cable category limits, pair usage, power losses over distance, and heat dissipation — especially at higher power levels. Modern PoE designs must consider standards compliance, thermal management, and efficiency, as power density rises with new generations of PoE technology.

    • pwg 2 hours ago

      With UblockOrigin in default deny all javascript mode there was no overlay ad.

      • IncreasePosts 2 hours ago

        Running a browser with javascript disabled is great, I recommend it to everyone. More often than not, you get a better experience - more responsive, your battery lasts longer, fewer ads. And, if the site breaks for some reason, you just allow list the site and reload.

  • dang 2 hours ago

    [stub for offtopicness]

    • varenc 30 minutes ago

      you're a hero

    • leoh 5 hours ago

      I'm just, like, vibe-coding, man. Why do I need to know about PoE?

    • scuff3d 4 hours ago

      Not gonna lie, I'm kind of disappointed this isn't a Path of Exile tutorial for engineers.

    • elromulous 3 hours ago

      "every engineer"? Should a chemical or nuclear energy engineer have to know about PoE?

      • ThrowawayTestr 3 hours ago

        Sure. It's really useful if you're setting up a home security network.