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We all know that "PoE switches" play an important role in wireless network coverage or wireless monitoring engineering. Although it is not as eye-catching as wireless APs, bridges, etc., if there is a problem in the actual application process, it will directly affect the entire wireless project. No matter what kind of PoE switch, how good and how powerful the performance is, can't avoid some problems. After all, it is also a machine. In order to take some precautions into consideration before construction and reduce the maintenance of subsequent projects, the following is a list of common problems in the use of PoE switches in the use of weak current engineering and related common sense points!
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First, the power supply distance
Everyone knows that the power supply distance of a PoE switch is generally 100 meters. In fact, the PoE power supply distance is determined by the data signal and the transmission distance. The transmission distance of the data signal is determined by the network cable.
1, network cable requirements
A PoE switch can only be implemented through a network cable. The network cable is a medium and a carrier. The network cable is not good. In order to obtain a good power supply effect, it is necessary to use a standard network cable, that is, Category 5 network cable, Super Category 5 network cable, and Category 6 network cable.
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Three network cable appearances
The lower the impedance of the network cable, the farther the transmission distance is. Therefore, the quality of the network cable must be guaranteed first. It is recommended to use the super five network cable. The transmission distance of the ordinary Category 5 data signal is about 100 meters. There are two types of PoE standards: IEEE802.af and IEEE802.3at standards. They have different requirements for the super five types of network cables, and the differences are mainly reflected in the equivalent impedance. For a super-five network cable of 100 meters, the equivalent impedance of IEEE802.3at must be less than 12.5 ohms, and IEEE802.3af must be less than 20 ohms.
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Super 5 network cable
2, PoE standard
PoE switches need to comply with the power supply standard, there are two, IEEE802.3af and IEEE802.3 at. The power supply of 15.4W and 30W is defined separately, but the actual power supply is 12.95W and 25W due to the loss during transmission.
When using a PoE switch that supports the IEEE802.3af standard, the power of the powered device cannot exceed 12.95W. When the PoE switch of the IEEE802.3at standard is used in the same way, the power of the powered device cannot exceed 25W.
Second, choose a standard PoE switch
1, non-standard PoE
There are some non-standard PoE switches on the market. These PoE switches do not have automatic detection function. The power supply equipment does not detect PoE power supply before power supply. The power supply equipment may be burned when power is supplied. Therefore, there is a certain security. Risk, when you want to use it, be sure to match the voltage.
It is generally recommended to use as few non-standard PoE switches as possible to avoid losing more.
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2. How to determine which are fake PoE devices
The fake PoE switch is different from the non-standard PoE switch mentioned above. The fake PoE device only merges the DC power into the network cable through a PoE combiner. It cannot be powered by a standard PoE switch, otherwise the device will burn out. Therefore, in engineering applications, not only standard PoE switches but also standard PoE terminals are selected.
3. Security measures for standard PoE switches:
1. The power supply terminal (PSE) and the power receiving terminal (PD) can dynamically sense and adjust the power supply voltage.
2. Effectively protect the receiving end (usually IPC) from being burnt by electric shock (other aspects include short circuit, surge protection, etc.).
3. It can intelligently detect whether the terminal supports PoE and does not supply power when accessing a non-PoE terminal.
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Non-standard PoE switches are usually designed to save costs and do not have the above security measures.
Third, the switch cascading problem
1, switch cascade
The number of layers cascaded by the switch involves the calculation of bandwidth, a cascade of 100 Mbps switches, and an effective bandwidth of about 60 Mbps. If each switch is connected to a monitoring device with a total rate of 15 Mbps, that is, the bandwidth of a single switch is 15 M, then 45/15 ≈ 3, which can cascade three switches.
The number of layers in the cascade of switches involves the calculation of bandwidth. A simple example is:
If a switch with a 100M network port is cascaded to the center, the effective bandwidth is 45 Mbps (bandwidth utilization ≈ 45%). If each switch is connected to a total of 15 Mbps monitoring devices, that is, the bandwidth of a single switch is 15 M, then 45/15. ≈3, three switches can be cascaded.
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Why is the bandwidth utilization approximately equal to 45%?
The actual Ethernet IP packet header accounts for about 25% of the total traffic, and the actual available link bandwidth is 75%. In practice, the reserved bandwidth is considered to be 30%, so the insurance estimated bandwidth utilization is 45%.
2, access and uplink ports
The ports of the PoE switch have access and uplink, which are used to better differentiate services and simplify maintenance, thus specifying different port roles.
Access port: As the name implies, it is the interface (IPC, wireless AP, PC, etc.) that directly connects to the terminal.
Uplink port: A port connected to the aggregation or core network. Generally, the interface has a higher rate and does not support PoE.
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Fourth, select the PoE switch according to the power of the device
How much power is used by the device, and then corresponding to how many power PoE switches are selected. If your device's power is below 15W, then you can choose a PoE switch that supports the 802.3af standard. If the power is greater than 15W, you need to select the 802.3at standard high-power switch, otherwise it will not move. Of course, your device needs to support PoE. If PoE is not supported, you need to prepare a PoE splitter.
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5. How to configure a PoE switch for the device?
PoE switches have two power supply standards, IEEE802.3af and IEEE802.3at. The most common camera power on the market is 7W, using the af standard PoE switch. If the camera is more than 16W, you need to use an at-standard PoE switch. If the camera power is more than 30W, you need to use a dedicated PoE power supply module to supply power.
In addition, you need to look at the total power of the PoE switch. For example, ports 1~4 support PoE power supply, support af/at standard, and total power is 60W, which can meet the power supply requirements of mainstream cameras on the market. In addition, it can provide a maximum power supply distance of 250 meters and three working modes. Under the IEEE802.3af standard, it can supply 4 ports (60/15.4=4), that is, it can supply power to 4 cameras at the same time, that is, full load power supply. If it is calculated according to the IEEE802.3at standard single port maximum power supply 30W, at the same time, only two ports can be powered (60/30=2). Generally, the af/at standard PoE switch is supported, and its power supply is adaptive.
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Six, four connection methods of PoE switches
PoE switches have many limitations. For example, when the upper switch and the lower terminal do not support PoE, the traditional wiring method is adopted. It is well known that the traditional wiring method has high wiring cost and high labor cost, which is not conducive to subsequent maintenance. The following four methods of application of PoE switch power supply are discussed. After familiarizing with these four methods, the PoE switch can be used in any scenario. Sexually reduce troubles.
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1. Both switches and terminals support PoE.
In this way, the PoE switch is directly connected to the wireless AP and the network camera supporting the PoE power supply through the network cable. This method is the simplest, but also needs to pay attention to the following two points:
(1) Determine whether the PoE switch and the wireless AP or network camera are standard PoE devices.
(2) To carefully confirm the specifications of the purchased network cable, the quality of the network cable is critical. A poor quality network cable can cause the AP or IPC to be unable to receive power or restart.
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2. The switch supports PoE. The terminal does not support PoE.
The PoE switch is connected to the PoE splitter. The PoE splitter separates the power supply into data signals and power. There are two output lines, one is the power output line, and the other is the network data signal output line, that is, the ordinary network cable. The power output has 5V/9/12V, etc. It can match various DC input non-PoE powered terminals and supports IEEE 802.3af/802.3at standard. The data signal output line, that is, the ordinary network cable, can be directly connected to the network port of the non-PoE power receiving terminal.
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3. The switch does not support PoE. The terminal supports PoE.
The solution switch comes out to the PoE power supply, and the PoE power supply transmits power to the network line and transmits it to the terminal. This kind of scheme is conducive to expanding the original wiring network and has no impact on the original network.
4. The switch does not support PoE, and the terminal does not support PoE.
The switch of this solution comes out to the PoE power supply, then to the PoE splitter, and finally to the terminal.
Solution 3 and Solution 4 are suitable for the transformation of the traditional network, that is, the original switch does not support PoE power supply, but wants to take advantage of PoE power supply.
In summary, PoE can be used regardless of the scene, making it easy to take advantage of the various conveniences that PoE brings. In addition, it is important to choose a PoE switch. A good PoE switch can make the whole system more stable and easy to maintain and upgrade.
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7. Daily maintenance of the PoE switch
The PoE switch is powered by the network cable, which simplifies cabling. If there is a problem, the whole system will not work properly. After purchasing the PoE switch, how can it be maintained to make it work better?
(1) If long-distance power supply needs to pay attention to the quality of the network cable. The distance is no more than 100 meters.
(2) If there is a link up and link down phenomenon when the PD is plugged in, first check whether the port is powered normally by show pse int X. The physical layer has no problem checking the data. In many cases, the AP is not associated with the AC. s reason.
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(3) Note that if the port pse enable, show pse int finds the port status as fault, enter ps disable after the port mode, and then pse enable again, observe whether the power supply is successful.
(4) Confirm that the grounding of the AP is normal. (In other areas, when the power supply to the ungrounded AP occurs, the opening environment is very bad, the equipment is not grounded, and the voltage of the equipment casing is higher than the power supply voltage of the RJ45 port.) In the case of the case, try to power the AP in half-auto or force mode.
(5) When using a test instrument to test the performance of my PSE device, be sure to use auto mode
(6) For the case where the special environment needs to use AT support, be sure to use the force mode to power the PD.
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(7) Same as all switches. When the device is cascaded, the loopback detection function must be disabled. The configuration method is to enter the interface configuration mode and configure the loop-check disable command. If the power supply port (powered by the AP and the power supply of the AP port) has a regular up/down phenomenon, and there is always a loop in the uplink port detection, turn off the loopback detection function and check it. This problem is most likely to be connected to the AP. The functional mechanism of the device is related. For the time being, it can only be solved by closing the loopback.
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(8) In the case of grounding conditions, it is necessary to have a good grounding, otherwise the casing may have a certain voltage (there is a feeling of electric shock, but it will not cause harm to the human body). If the switch is not well grounded, the port up/down will be easy to occur.
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