The application of POE switches in the security industry has brought qualitative changes to the video surveillance system, POE switches. It can supply power to PoE terminal equipment such as wireless AP and network cameras through the network cable, and the transmission distance can reach 100m. It is easy to install and plug and play. It is very suitable for wireless city, security monitoring, and other industries.
The origin of POE What is POE
POE (PowerOverEthernet) refers to the power supply of some IP-based terminals (such as IP phones, wireless LAN access points AP, network Cameras, etc.) while transmitting data signals, it can also provide DC power supply technology for such devices. POE is also known as Power over LAN (POL, PoweroverLAN) or Active Ethernet (ActiveEthernet). Standard specification and maintain compatibility with existing Ethernet systems and users.
Two power supply schemes for POE – the POE standard delivers DC power to the transmission cable using Ethernet
POE compatible devices define two methods:
In a method called “Mid-Span”, which uses the unused wire pairs in the Ethernet cable to transmit DC power (ie 1236 line sequence for data, 4578 line sequence for power), the corresponding EndpointPSE supports POE-capable Ethernet switches, routers, hubs, or other network switching devices.
Another method is the “End-Span method”, which is to transmit DC power on the core wire used for data transmission at the same time, and its power transmission adopts a frequency different from that of the Ethernet data signal (that is, the 1236 line sequence transmits data and electricity). MidspanPSE is a specialized power management device, usually, put together with the switch. It has two RJ45 jacks corresponding to each port, one is connected to the switch with a short cable, and the other is connected to the remote device. It is foreseeable that End-Span will be rapidly popularized. This is because the Ethernet data and power transmission use a common wire pair, thus eliminating the need to set up a dedicated wire for independent power transmission. The RJ-45 socket is particularly significant.
The working process of POE power supply
When the PSE power supply end device is arranged in a network, the POE power over the Ethernet working process is as follows.
- Detection: At the beginning, the PSE device outputs a small voltage at the port until it detects that the connection of the cable terminal is a receiving end device that supports the IEEE802.3af standard.
- PD device classification: After detecting the PD of the receiving end device, the PSE device may classify the PD device and evaluate the power loss required by the PD device.
- Start power supply: During the startup period of a configurable time (usually less than 15μs), the PSE device starts to supply power to the PD device from a low voltage until a 48V DC power supply is provided.
- Power supply: Provide stable and reliable 48V DC power for PD devices to meet the power consumption of PD devices that do not exceed 15.4W.
- Power failure: If the PD device is disconnected from the network, the PSE will quickly (usually within 300-400ms) stop supplying power to the PD device, and repeat the detection process to detect whether the terminal of the cable is connected to the PD device.
Advantages of using Power over Ethernet
- POE only needs to install and support one cable, which is simple and space-saving, and the device can be moved at will. (easy and convenient)
- Cost-saving. Much live equipment, such as video surveillance cameras, etc., needs to be installed in places where it is difficult to deploy AC power. POE saves money and time by eliminating the need for expensive power supplies and the time it takes to install them.
- Like data transmission, POE can supervise and control the device by using the Simple Network Management Protocol (SNMP).
- The POE power supply end device will only supply power to the device that needs a power supply. Only when the device that needs a power supply is connected, the Ethernet cable will have voltage, thus eliminating the risk of leakage on the line. (safety)
- A single UPS can provide power for all related equipment when the power is cut off. (centralized power supply)
- Users can automatically and safely mix all original equipment and POE equipment on the network, which can coexist with existing Ethernet cables.
- Make network equipment easy to manage. Because when the remote device is connected to the network, it can be remotely controlled, reconfigured, or reset.
- In a wireless local area network, POE can simplify the task of RF testing, and the access point can be easily moved and connected.
- Reduce the material cost and labor wiring cost of the power line.
- Reduce later maintenance and repair costs.
How to troubleshoot when POE cannot supply power
① First determine whether the powered device supports the POE power supply
Not all devices support the POE power supply. I have received such an after-sales service once. A customer said that his surveillance camera was connected to our 8-port POE through a network cable without being connected to the power supply. Not working. After plugging in the camera, everything works fine. Hearing the situation reported by the customer, I first asked the customer to check whether his camera supports the POE power supply. Sure enough, his device does not support the POE power supply.
② Check whether the power of the powered device exceeds the maximum power supplied by the port of the POE switch
There are two types of port power supply for POE switches, one is 15.4W and the other is 30W. If a 15.4W POE switch is used, but the power of the powered device is greater than 15.4W, the power supply will be unstable or unable to supply power.
③ Detect whether the power supply of the POE switch exceeds the total power supply under the working state of full load
In addition to the power supply of a single port, the POE switch also has a total power supply. The total power supply of some 8-port POE switches on the market is 90W, and the power supply of a single port is 15.4W. If the power supply of each port is used to the peak, 8 ports are 8×15.4=123.2W. In this way, the total power of the switch will be exceeded, and the power supply will naturally have problems. Therefore, the use of POE switches should be properly matched, and the power receiving equipment with high power should be used together with the power receiving equipment with low power.
How to calculate POE power supply distance
Only for IEEE802.3af/at compliant PoE devices
① The influence of the network cable on the power supply distance
First, let’s look at the requirements for Cat5e network cables in the IEEE802.af and IEEE802.3at standards below:
| Parameter | Symbol | Units | Type 1 value | Type 2 value |
| Nominal highest DC current per pair | I Cable | A | 0.350 | 0.600 |
| Channel maximum DC pair loop resistance | R Ch | Ω | 20.0 | 12.5 |
| Minimum cable type | UTP per 14.4 and 14.5a | Class D |
Note: Type 1 value and Type 2 value refer to the requirements of IEEE802.3af and IEEE802.3at respectively.
It can be seen from the table that the equivalent impedance of a 100-meter cat5e network cable must be less than 12.5 ohms for IEEE802.3at, and less than 20 ohms for IEEE802.3af.
Conclusion 1: The PoE power supply distance is closely related to the network cable. The lower the impedance of the network cable, the longer the transmission distance. If the power supply is to be far enough, the network cable must first be of high quality.
② IEEE 802.3af/at standard for PoE power supply voltage requirements
PoE output voltage range is 44-57V, PD receiving voltage range is 36-57V, 802.3af/at standard; refer to the following:
| Parameter | Symbol | Unit | Min | Max |
| Output voltage | V port | V dc | 44 | 57 |
| Load regulation | V | 44 | 57 |
| Parameter | Symbol | Unit | Min | Max |
| Input voltage | V port | V dc | 36 | 57 |
| Input average power | P port | W | 12.95 |
Conclusion 2: The PoE power supply distance is required to be long, and the output voltage must be high.
Summary: If the PoE power supply distance is far enough, the network cable must meet the standard, and the output voltage of the PoE power supply product should be as high as possible within the standard (44-57V).
For example, Bithoo PoE products use an output voltage of 55V, the equivalent impedance of a Cat5e 200m network cable that meets the standard is 25 ohms, the AF standard current is 300mA, U=0.325=7.5V, 55V (PoE output) -7.5V ( Attenuation voltage)=47.5V (200m network cable load terminal voltage), according to P=UI, the final result is 200m network cable load terminal power P=load terminal voltage UAF standard current I=47V*0.3A= 14.1W, enough The camera or wireless AP is working.
Of course, a strong network not only depends on the long enough PoE power supply distance but also the network transmission quality must be guaranteed. According to IEEE regulations, the network cable transmission exceeds 100 meters, and the transmission quality is not guaranteed. However, in some cases (under cost pressure), for example, when installing a camera, we will exceed more than ten meters. Here is a little trick: it is not too much to exceed In many cases, manually set the connection rate of the camera to 10M. For a 2 million pixesl camera, the rate of 10M is enough, and the problem is very likely to be solved.
About the POE switch power
PoE the total power of the PoE switch is a very important indicator, which is directly related to how many cameras can be brought. For a 24-port PoE switch powered by 400W, the total power of the PoE switch after removing the loss is about 370W.
Under the IEEE802.3af standard, it can supply 24 ports (370/15.4=24), that is, it can supply power to 24 cameras at the same time, that is, full power supply. However, if it is calculated according to the IEEE802.3at standard with a maximum power supply of 30W per port, it can only supply power to 12 ports at most at the same time (370/30=12).
In actual use, the maximum power consumption of many ordinary network cameras is relatively low, basically no more than 15W. If each PoE port reserves the power supply according to the maximum power (such as 30W), there will be some ports that use PoE power. It is not finished, and some ports cannot divide the power. For example, some PoE switches support dynamic power allocation to avoid this situation. When purchasing a PoE switch, check whether the switch supports dynamic power allocation, so that each port only allocates the power actually used, so that the power supply of the PoE switch can be used more efficiently.
POE power supply stability
From a technical point of view, the technology of PoE has been developed for many years and is currently in a very mature stage. However, due to the cost pressure of the current monitoring market, the quality of the selected PoE switches or cables is too low, or the solution design itself is unreasonable, resulting in a particularly large maintenance workload for projects using PoE power supply. stable point of view.
The network monitoring project is different from the ordinary network-integrated wiring. The data transmission volume is very large, the power is high, and it requires uninterrupted work around the clock. The use of quality-assured PoE equipment and cables is the guarantee the stability of the entire system.
Application of POE Power Supply in Security System
The application of PoE networking in the security industry mainly has the advantages of simplified wiring, energy saving, flexibility, safety and convenience. As we all know, in addition to transmitting video signals through network cables, general network cameras must also provide sufficient power for them all day long. In actual construction, it often occurs that the installation site of the camera has to be redesigned because a stable power supply cannot be provided. As a result, redesigning the installation site may lead to monitoring dead spots and make monitoring meaningless.
- Flexibility, PoE only needs to install and support one cable, which is simple and space-saving, and the device can be moved at will;
- Cost-saving, video surveillance cameras, etc., all need to be installed in places where it is difficult to deploy AC power supply, PoE makes it no longer need expensive power supply and time spent installing the power supply, saving cost and time;
- Intelligent power supply, DC power supply is like data transmission, PoE can supervise and control equipment by using Simple Network Management Protocol (SNMP);
- Eliminate the hidden danger of power supply. The PoE power supply end device will only supply power to the device that needs a power supply. Only when the device that needs a power supply is connected, the Ethernet cable will have voltage, thus eliminating the risk of leakage on the line;
- The advantage of a centralized power supply, a single UPS power supply battery can provide power supply for all related equipment when the power is cut off;
- Device compatibility advantage, users can automatically and safely mix previous devices and PoE devices on the network, which can coexist with existing Ethernet cables;
- Make the network equipment easy to manage, when the remote equipment is connected to the network, it can be remotely controlled, reconfigured, or reset;
- In the network camera LAN, PoE can simplify the test task, and the access point can be easily moved and connected.
Application of PoE Power Supply in Wireless Network
PoE switches can be deployed between aggregation switches and wireless APs, or between ONUs and wireless APs in a PON network. By choosing 8-port, 16-port, and 24-port PoE switches with different port densities, they can be deployed in hotspot areas with different user densities. Among them, the networking mode of aggregation switch + PoE switch is very suitable for deployment in areas with dense users. At present, this method is mostly used in the construction of WLAN projects in colleges and universities. The PoE switch solution has a wide range of applications and a large number of deployments and is currently the mainstream choice of many engineering companies. It is widely used in wireless AP and network camera networking. The solution adopts AP+AC mode to facilitate centralized management and uses PoE power supply to save labor costs.
POE power supply cable selection
The safe transmission distance of POE power supply is 100 meters, and it is recommended to use more than Category 5 copper network cables.
The POE IEEE 802.3af standard requires the output power of the PSE output port to be 15.4W or 15.5W, and the received power of the PD device after 100 meters of transmission must not be less than 12.95W. Calculated according to the typical current value of 802.3af of 350ma, the resistance of the 100-meter network cable must be is (15.4-12.95W)/350ma = 7 ohms or (15.5-12.95)/350ma = 7.29 ohms.
The standard network cable naturally meets this requirement, and the IEEE 802.3af poe power supply standard itself is measured with the standard network cable. The reason for the problem of POE power supply network cable requirements is that many network cables on the market are non-standard network cables and are not produced in strict accordance with the requirements of standard network cables. The non-standard network cable materials on the market mainly include copper-clad steel, copper-clad aluminum, copper-clad iron, etc. These cables have high resistance and are not suitable for POE power supply. POE power supply must use an oxygen-free copper network cable, that is, a standard network cable.
How to choose a POE switch
① How much power needs to be supplied to the device:
The output power of the PoE switch is different depending on the standard adopted. For example, the maximum power consumption of IEEE802.3af is not more than 15.4W. Due to the loss of the transmission wire, it can supply power to devices with a maximum power consumption of not more than 12.95W. A PoE switch that follows the IEEE802.3at standard can supply power to devices with a maximum power consumption of no more than 25W.
② How many devices can be powered at most:
An important indicator of a PoE switch is the total power supplied by PoE. Under the IEEE802.3af standard, if the total PoE power supply of a 24-port PoE switch reaches 370W, then it can supply 24 ports (370/15.4=24), but if it is a single port according to the IEEE802.3at standard, the maximum The power supply is calculated at 30W, and it can only supply power to 12 ports at most at the same time (370/30=12).
③ Number of interfaces required:
Whether with fiber port, with or without network management, rate selection (10/100/1000M).
Can a POE switch be a secondary switch?
PoE only solves the problem of power supply and will not affect the signal. If you want to use a multi-layer switch, you only need to let the switch closest to the camera supply power to the camera, and the switches behind do not need to have anything to do with PoE.
PoE power supply technology has high requirements on wires. It is recommended that in monitoring projects, it is not worth saving costs on wires.
