Must Know: Common Engineering Issues for PoE Switches

The role of PoE switches in security engineering applications is that they are not large enough but they are small but indispensable. However, in the actual application process, problems are often not known. In view of this situation, some considerations should be taken into account before construction, and the amount of maintenance for follow-up projects should be reduced. Feng Runda Xiao Bian specifically arranged the following points.

One, power supply distance

The PoE power supply distance is determined by the data signal and the transmission distance, and the data signal transmission distance is determined by the network cable.

1, network cable requirements

The lower the impedance of the network cable, the longer the transmission distance, so we must first ensure the quality of the network cable. To purchase genuine products, we recommend the use of Cat5e cable (the outer cover is marked “CAT5e”). The transmission distance of the ordinary Cat 5 cable is about 100 meters. about.

Because there are two kinds of PoE standard: IEEE802.af and IEEE802.3at standard, they are different to the requirement of the super five kinds of network cables, different mainly is reflected in the equivalent impedance. For example, for a 100m Super Category 5 network cable, the equivalent impedance of IEEE802.3at must be less than 12.5 ohms, while IEEE802.3af must be less than 20 ohms. It can be seen that the smaller the equivalent resistance is, the longer the transmission distance is.

2, PoE standard

To ensure the transmission distance of a PoE switch, it depends on the output voltage of the PoE power supply. Within the standard (44-57V DC) as high as possible, the output voltage of the PoE switch port must comply with the IEEE802.3af/at standard.

Second, non-standard hidden trouble

The non-standard PoE power supply is relative to the standard PoE power supply. There is no PoE control chip in it. It does not detect this step. No matter whether the IP terminal supports PoE, it will supply power to it. If the IP terminal does not provide PoE power, it is very likely. Burn the network port.

1, less "non-standard" PoE

When selecting a PoE switch, try to select the standard and have the following advantages:

Power supply (PSE) and power (PD) can dynamically sense and adjust the supply voltage.

Effectively protect the power receiving end (usually IPC) from being burned by electric shock (other aspects include short circuit, surge protection, etc.).

It can intelligently detect whether the terminal supports PoE and does not supply power when accessing a non-PoE terminal.

Non-standard PoE switches usually save costs and do not have the above security measures. Therefore, there are certain security risks. However, it is not necessary to use non-standard PoE. When the non-standard PoE voltage matches the voltage of the powered device, it can also be used and the cost can be reduced.

2, no "false" PoE

A fake PoE device only uses a PoE combiner to merge DC power into a network cable. It cannot use standard PoE switches to provide power. Otherwise, the device will burn out. Therefore, fake PoE devices should not be used. In engineering applications, not only standard PoE switches should be selected, but also standard PoE terminals.

Third, the cascade problem

The number of cascaded switches involves the calculation of bandwidth. A simple example:

If switches with 100 Mbps network ports are cascaded to the center, the effective bandwidth is 45 Mbps (bandwidth utilization is ≈45%). If each switch is connected to a monitoring device with a total of 15 M bit rate, that is, it occupies 15 Mbit/s of the bandwidth of a single switch. 45/15≈3, 3 switches can be cascaded.

Why is the bandwidth utilization approximately equal to 45%? The actual Ethernet IP packet header accounts for about 25% of the total traffic. The actual usable link bandwidth is 75%. In practical applications, the reserved bandwidth is 30%. Therefore, the estimated utilization of the insurance bandwidth is 45%.

Fourth, port identification

1, access and uplink ports

The ports of the switch are divided into access and upstream ports to better differentiate services and simplify maintenance, thus defining different port roles.

Access port: As the name implies, it is the interface directly connected to the terminal (IPC, wireless AP, PC, etc.)

Upstream port: A port connected to an aggregation or core network. Generally, the port has a higher interface rate and does not support PoE.

2, Feng Runda PoE switch

Feng Runda's PoE switches include: normal access ports and uplink ports. Among them, the common access port supports PoE function, and the uplink port has several common RJ45 ports and SFP optical ports.

Knowledge Extension: How to Configure a PoE Switch for a Device

PoE switches have two power standards, IEEE802.3af and IEEE802.3at. The most common camera power on the market is 7W, and a standard PoE switch can be used. If the camera exceeds 16W, use the standard PoE switch. If the camera power is more than 30W, you need to use a dedicated PoE power supply module.

In addition, the total power of the PoE switch needs to be seen. For example, Fengrunda new PS504E, 1~4 ports support PoE power supply, support af/at standard, total power 60W, meet the mainstream camera power supply requirements in the market, in addition can also provide maximum 250 meters power supply distance and 3 kinds of work modes.

Under the IEEE802.3af standard, it can supply 4 ports (60/15.4=4), that is, it can supply power to 4 cameras simultaneously, that is, full-load power supply. If it is calculated according to the IEEE802.3at standard single-port maximum power supply of 30W, at most two ports can be powered at the same time (60/30=2).

Generally support the af/at standard PoE switch at the same time, its power supply power is self-adapting. After reading this article, if you don't know anything about the pro, please consult our technical engineers.