What does PoE power supply mean? Why does a surveillance camera need just one network cable without a power outlet? I measured it with a multimeter, and the non-standard PoE pitfall really burned the network card

When

installed surveillance cameras at home, the technician pulled a network cable from the weak current box to the courtyard wall. Once the camera was installed and the connector plugged in, the footage appeared. My dad watched from the side, stunned: Where's the power supply? How does it light up without a power source? I couldn't answer at the time, but I knew this was called PoE, the power fed by the switch. Later, to figure out how this thing drains power inside the network cable, I dug out a multimeter and measured every port of my switch, and even helped Old Wang downstairs diagnose a machine that had burned out the network card. This article explains PoE from the inside out. People installing surveillance cameras, panel APs, or wireless coverage—these things are bound to run into you sooner or later.

network cables still run power: 100Mbps drives idle chips, gigabit plays with 'hitching a ride'

let me start with an old saying I wrote before: In my article about the connector, I mentioned that POE power supply uses the four spare wires of 4578, and when the camera is cut with four cores, the power cuts off. This is only half true; it refers to the situation with 100Mbps networks. Among the eight-core network cables, wires 1, 2, 3, and 6 are the ones that do the work, relying on them to transmit 100Mbps of data; Wires 4, 5, 7, and 8 were idle in the 100Mbps era, so PoE simply fed DC power into these two pairs of idle lines, with data as data and power as power, without interfering with each other. This method has an official name: Mode B. Most 100Mbps switches with PoE ports at home use this route.

Here's

question: all four gigabit network pairs are transmitting data, with no spare pairs—where does the power go? The answer is "ride." Gigabit PoE overlays DC power onto data signals and sends them together. The technical term is common-mode power supply, similar to condenser microphone microphone power supply—although data and power run on the same line, one stands on the signal side and the other on DC, and when they reach the device end, they each go their separate ways. This is why Gigabit PoE devices must have all eight cores fully connected—any single one will be damaged. In my about connectors, I advise everyone not to be easy and to use four-core cables. In the gigabit era, four-core cables are half-broken, and pairing them with PoE is just a complete failure.

the most basic yet effective way to inspect the surveillance cables in the yard is to check the negotiation speed: the camera page shows 1000M, indicating all eight cores are fully connected, with stable PoE and data; If it shows 100M, be alert—either the cable has damaged the four cores, or the middle section has broken the four-core wire. My family had been plagued by mice for the first couple of years. I always thought contact was bad, but later we found out the negotiation dropped to 100 meters. Following the clues, we finally found the dented section of the wall line.

rules measured with a multimeter: no handshake, no electricity

I've always wondered: since there's electricity in the PoE port, if I accidentally plug in the laptop's network port, will it burn the computer? For this, I specifically dug up the multimeter to measure it. When the probe was pushed into the free port of my standard PoE switch, the reading was strange—not a stable 48V, but fluctuating between 2V and 10V, as if it were poor contact. Plug in a PoE camera and measure again—wow, around 48V, solid and steady.

later researched, I realized that the pulsating reading was the answer. Before standard PoE power supply, there is a three-step handshake: the switch first sends a small voltage of less than 10 volts to the line, then checks if the device on the other side has a characteristic resistance of about 25 kΩ—this is the 'code' that PoE devices come with from the factory; If you get it, ask how many watts you want to consume (this step is called grading); Only when everything matches can the full 48V power be released. Ordinary computer network cards don't have this code, so if the switch can't detect it, it never gives power. So a standard PoE port is safe to insert into a computer—at worst, it won't respond. This method for power supply is similar to the method I used in the article on optical modem power adapters. Keeping a multimeter at home costs a few dozen yuan can save half the network issues and detours.

Saving Three Handshakes on Non-Standard PoE, the Cost Is the Network Card: Old Wang's Lesson

downstairs, Old Wang runs a small supermarket. Not long ago, installing surveillance cameras was cheap, so the package he bought included a "non-standard PoE" switch costing just a few dozen yuan. What is this thing? It's like cutting everything in three steps, with a constant voltage on your mouth. Whether you're plugging in a camera or a computer, just plug in the power first. Lao Wang wanted to use his laptop to enter the camera backend and change the password. When he plugged in the network cable, the laptop's network port didn't smoke, but the network card was instantly doomed. After-sales service checked and said the port was burned, and the motherboard wiring was affected. In the end, I spent over thirty yuan to buy a USB network card to keep it alive, and the onboard network card in the machine was basically useless.

this is the biggest pitfall of non-standard PoE: it looks exactly like standard PoE, but is more than half cheaper, with the product page just a line of small text saying "non-standard." There are two ways to identify them. First, look at the product page parameters. Proper standard PoE must be boldly labeled as 802.3af or 802.3at, 802.3bt, which are IEEE standard numbers, and writing them means you are following the handshake process; If it only says 'PoE power supply 48V' without mentioning the standard number, it's almost certainly non-standard. Second, use a multimeter to measure the air port: if the reading jumps between 2 and 10 volts, it's a standard PoE, which is 'interrogating'; The first measurement is a steady 48V or 24V and is non-standard, so it doesn't care at all. Non-standard cameras aren't completely unusable—all the cameras on hand are non-standard models, the wiring is fixed and unmovable, just okay enough to work; But as long as you can plug in ordinary devices like computers, routers, or set-top boxes on this cable, don't skimp on these dozen yuan. Old Wang's machine is still connected to the supermarket cash register, and every time I see it, I want to persuade him to replace it.

15 watts, 30 watts, 90 watts: three generations standard with one table, and line losses must also be accounted for

standard PoE has three generations up to now, and the power difference is huge. Before buying equipment, look at this table:

note the difference between the second and third columns: the switch says 15.4 watts, but the device can only get 13 watts. So where does the extra 2 watts go? Online. When the current runs 100 meters, the network cable has resistance, and the voltage drop is absorbed by this part. The standard calculates this loss based on a 100-meter cable length, so the design power consumption of AF devices was kept below 12.95 watts. This design also reminds you of two things: first, the longer and less reliable the cable (like copper-clad aluminum), the more the device gets a discount. Don't choose equipment based on extreme power; Second, the cables must be oxygen-free copper. In my article how far network cables can be stretched I calculated the copper-clad aluminum resistance calculations, and the data was tough, not to mention stacking another power line.

the most common cameras at home: a standard camera with 5 to 8 watts, a slightly over 10 watts with a stronger infrared light, and a PTZ camera with heating (the outdoor kind in northern China) that can use 15 to 20 watts. In other words, an AF standard mouth-feed ordinary bolt is more than enough; feeding a heated PTZ camera is unreliable. It depends on the specific device specifications—before buying, just glance at the power consumption section on the camera specs page, don't just look at resolution.

single-port power and total budget are two different things: my 80-watt unit is on the books

this is the second cognitive pitfall I've ever fallen into. My eight-port machine has four ports with PoE. When I bought it, I only saw the line saying "15.4W per port" and thought every port was enough, so I plugged it in casually. Later, I discovered that the machine label had an even more critical number: total PoE budget 80 watts—all PoE ports in the machine combined must not exceed this number. My phone is quite generous. Cheap machines often label 50W or 65W for you. If all four ports are plugged into an AT device, each port uses 30W, totaling 120W. A 65W machine with a budget can't run from the third port onward. The camera either won't light up or keeps restarting, looking like the camera is broken, but the switch is out of battery.

The

formula for calculating the budget is simple: the combined nominal power consumption of all PoE devices does not exceed 70% of the total budget. Leaving a 30% margin is to prevent equipment from suddenly starting up with high current, and second, switches heat up badly under full load in summer—I mentioned in my post on how to choose a switch in PoE models heat up even more under full load, and if you have a weak current box, don't stack it up close. My current account book: three bolts in the yard—one 7W and two 5W—and one at the entrance with a 10W setup, plus a panel AP using 9W, totaling only 36W. I used less than half of the 80W budget, which is very stable. How to separate the network segments and store the footage for the most economical footage for surveillance — I calculated the entire account in the monitoring network planning ; If you have a NAS at home, you can directly go to the camera via RTSP to the NAS. That article NAS added the camera covered the details.

100 meters is data wire, 90 meters is electric: give a sequence at the end

also has two layers of distance from this red line. On the data side, a single 100-meter network cable segment is a line drawn by IEEE. I overturned it in the section on network cable distance, so I won't repeat it here. When it comes to power supply, be even more conservative: 100 meters is a passing line in theory; in practice, try to keep the cable within 90 meters, because the longer the cable, the greater the voltage drop, and the power received by the terminal device decreases. Data may still be running, but the camera may not be able to keep up with power supply at low temperatures or when infrared is fully enabled at night, resulting in 'everything working fine during the day, but dropping at night.' This is the hardest problem to detect, and nine times out of ten, it's due to insufficient power supply for long-term lines.

Give the order of the wrap. For those who want to implement PoE, follow this example: first count the devices, and copy the nominal power consumption of each PoE device from the parameter page and sum it up; Then choose a standard: a regular camera with an AF is sufficient; if you have a PTZ camera and a high-power AP, go for AT. If the product page states 802.3af/AT, do not touch it, only 48V non-standard ones; Then, for budget, the total device power consumption should be controlled within 70% of the switch's PoE budget; Wiring above Class 5 oxygen-free copper, with a 90-meter internal seam for single sections; Once everything is connected, take your phone into the camera backend to check if the negotiation speed is 1000M. If you casually pick up a multimeter to measure the empty port, the one that pops up is the good switch asking questions. If you've been standing there foolishly at 48V, return it early.