How IP camera use Power over Ethernet

Same as all other electronic devices, IP cameras need power. IP camera is known as network camera. So the network is required for communicating. Power over Ethernet allows power and network transmit over single Ethernet cable. It eliminates the extra power cord to the camera. Using the PoE power system can save installation cost. It also meet NEC Class II and SELV regulations as the whole system uses 48V DC low voltage input.

How Power Over Ethernet work? There’s 4 twisted pairs in cat5e or cat6 Ethernet cable. Only 2 pairs are used for fast network (10/100mbps). It leaves 2 spare pairs twisted which will be used to carry the power. The power is injected to the those twisted pairs and send over to the IP camera. This is one way to understand the PoE, but it is not complete picture. We will come back to it a little bit later. In the typical PoE system, the power is located at the central together with the network switch. Technically it calls PSE (power supply equipment). When the power integrated into a switch. It becomes PoE switch, The PoE switch look no different from conventional switch. However, this switch can supply the power to the camera as well as network through single RJ45 cable.

As you know, network video recorder has to be deployed to record the footage in the CCTV system. If both power and switch are integrated to a network video recorder, the recorder is called PoE NVR. The PoE NVR has extra Ethernet ports at the rear panel. Simple connect those ports to PoE IP camera, the camera can got both network and power. It simplify the system installation. All the Ethernet cables will return to the PoE NVR. However, as the power is integrated to the recorder which will generate extra heat and increase the temperature inside the recorder. The consequences is it may shorten the use life. Either the power or switch failed, the PoE recorder need to be replaced.

If no PoE switch or PoE NVR are available in the system, use PoE injector also can inject the power to your network. The PoE injector doesn’t process the data, just adds the power to the twisted pair. The PoE injector is flexible device when only few channel of cameras need PoE power. This solution also can leverage your exist network infrastructure.

As we mentioned at the beginning, there are 2 spare twisted pairs in the Ethernet cable which has been taken for the power. In the practical application, PoE switch usually doesn’t use the spare pair to send the power. The power will be added to the data pair (12-36) and transmit with the network. At the front end, the IP camera receives the data and power on the same twisted pairs and split the power by using the network transformer. PoE has no problem to work with Gigabit network which all 4 twisted pairs have been occupied for high speed communication because of that. The truth is both data pair and spare are capable of carrying the power. If data pair are used to send the power, we call end-span PoE device. If spar pairs are used, we call mid-span PoE device. Most of the PoE switch use data pair to send the power today, Some of the PoE device such as PoE NVR or PoE injector still prefer the spare pairs for carrying the power.

Should We know which twisted pairs the PoE switch are sending the power before buying. After all, The PoE switch only will choose one way or the other to send the power, either data pair or spare pair. Fortunately, there’s IEEE802.3af/at standard. According this industrial standard, the IP cameras should adapt to either data pairs or spare pairs power way from the PoE switch. So no matter how PoE send the power, the PoE IP camera will still work as long as both side have followed IEEE802.3.

This industrial standard has provided important power negotiation protocol which is named as power signature. The PoE system is working with 48V DC voltage. Before the PoE switch release the power it has to be sure the front end device is cable of taking in the power, otherwise it may fire your front end IP camera. The power signature make sure it doesn’t happen. When the PoE switch release power, it starts from very low voltage, meanwhile it detect the feedback current from the IP camera. Later the PoE switch increase the voltage step by step up to DC48V. If the PoE switch doesn’t receive the correct feedback signal, it will stop releasing the power. What you sees is the PoE indicator LEDs keep flash on/off, but it never stay ON for long. It could be the IP camera is damaged which change the feedback signal or your IP camera doesn’t support standard PoE at all. Now what is the passive PoE? the passive PoE removes the power negotiation to save cost, it doesn’t detect the current feedback. The passive PoE will inject the power directly once the cable is plugged in. For your system reliability sakes, stay away the passive PoE unless you have to.

You may hear IEEE802.3af/at. The maximum power budget of IEEE802.3af is 14.4w and the maximum of IEEE802.3at 25.5W. The available remain power between these two standard are different. You may need higher power for some infrared IP camera or PTZ camera. If the PoE switch detects your IP camera require more the power than it can supply, for example, the infrared IP camera needs 16W, but the PoE switch is only comply to IEEE802.3af which is capable of supplying 14.4W, the PoE switch will stop working at night when the infrared is turned on. Also only both PoE switch and the PoE camera are comply with IEEE 802.3 at standard will have 25.5 power budget get in the system. The last thing is you may have read 30W IEEE 802.3at. That 30W refers to the power injecting to the cable. It is not the available remain power at the camera end. The power injected to the twisted pair are different from the available power budget at the IP camera. The voltage drop is considered for 100 meters transmitting. The power limited is set to 25.5w even the actually cable is only 1 meters.

Power over Ethernet has been developed over decade. It provides cost and time benefits to us. Some of the new technologies is evolving which allow to carry much more power such as IEEE802.3bt standard or transmit longer distance up to 3km PoE after integrated with fiber optical such as power fiber system. All those solution will benefit new IoT thing and IP cameras as well.

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