PoE things: the maximum transmission distance of PoE power supply technology

One question that can often be seen at network technology forums at home or abroad is: How far can PoE be transmitted using Cat 5/5e/6 cables? This is indeed a problem that is often encountered when using PoE technology. In this article, PoE switch and solution provider Feng Runda will answer in detail the "PoE maximum transmission distance."

The key factor in determining the transmission distance

To answer the question of PoE's maximum transmission distance, it's important to figure out what the key factors in determining the maximum distance are. In fact, the use of standard Ethernet cable (twisted pair) to transmit DC power can be transmitted very far, this distance is far greater than the transmission distance of the data signal. So, the biggest distance of data transmission is the key.

Network cable data transmission distance

Friends who know more about the network know that the twisted pair has an "insurmountable" transmission distance of "100 meters." Whether it is 10M twisted pair transmission rate, or 100M transmission rate of the Category 5 twisted pair, or even 1000M transmission rate of the Category 6 twisted pair, the farthest effective transmission distance is 100 meters. In the comprehensive wiring specification, it is also clearly required that the horizontal wiring cannot exceed 90 meters, and the total length of the link cannot exceed 100 meters. In other words, 100 meters is a limit for wired Ethernet. This limit is the link length from the network card to the hub device.

What is the maximum distance of 100 meters?

What caused the twisted pair's 100-meter transmission distance limit? This requires a deeper look at the deep physics of the twisted pair. The transmission of the network is actually the transmission of the network signal on the twisted pair. As an electronic signal, when it is transmitted in the twisted pair, it must be affected by the resistance and capacitance, which leads to the attenuation and distortion of the network signal. Attenuation or distortion of the signal to a certain extent will affect the effective and stable transmission of the signal. Therefore, the twisted pair has a transmission distance limit. So how do you calculate the upper limit of 100 meters?

Category 5 UTP and UTP Category 5 UTP are mainly used for computer networks. According to the 100Base-TX specification of Fast Ethernet, the communication rate is 100 mbps, and the time it takes for 100 mbps Ethernet to transmit one bit of data (also known as “bit time”). ) can be calculated as follows:

1 bit time=1/100mbps=10ns

Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection), which is a carrier sense multiple access technology with collision detection (Carrier Sense Multiple Access/Collision Detection) to share the communication channel. When it is introduced and exchanged, it still leaves Do not open this technology. A device is connected to both ends of a link. The two devices may send data at the same time, causing a collision. In this case, the collision domain is 2. Conflicts will lose packets. In order to avoid packet loss due to collisions, Ethernet uses collision detection and retransmission back-off techniques. In order to do backoff and retransmission, it must be ensured that one end can detect a collision before transmitting a packet. The minimum frame length of Ethernet is 64 bytes, ie 512 bits. Calculated at a rate of 100 mbps, transmitting 512 bits takes 512 bits * 10 ns = 5120 ns.

Data information is transmitted in the network, and delays are generated when passing through different components. The delay of the five types of UTP is 5.56 ns/m. When designing Ethernet, it is required to comply with a relay rule. This rule is also called the golden rule or 5-4-3-2-1 rule. This rule applies not only to 10 mbps Ethernet but also to Fast Ethernet. This rule requires that the ring collision delay must not exceed 512 bits, and for a 100 mbps transmission rate, it is 5120 ns. In the ring, the network elements include cables, relay units, MAUs, and DTEs. Adding their delays and multiplying them by 2, the ring delay is obtained, and the circle collision diameter can also be calculated. According to this theory, the longest distance that can be transmitted to ensure that a minimum frame is transmitted before it is transmitted can be calculated. This is why the link span is limited to 100 meters.

When it exceeds 100 meters, because the conflict cannot be detected in time, the packet that has been destroyed due to the conflict is transmitted and is received by the receiver. This packet is forced to be discarded because it fails the verification. At this time, the mechanism for the retransmission is not Is activated, it will cause the loss of the package. When the transmission rate is lower than 100 mbps, the limit of 100 meters can be appropriately relaxed in practical applications. It must be stated that although this actually has an effect, it does not conform to the standard. In the certification test, it must be explained, otherwise there will be some problems, such as product warranty.

The maximum cable distance during actual construction

From the above, we can see why the maximum length of the network cable should not exceed 100 meters when using PoE. However, in actual construction, in order to ensure the quality of the project, generally take 80-90 meters.

Impact of Different Network Cables on Power Transmission over PoE

Category 5 (Cat 5) is the most common standard network cable on the market today, but the quality varies greatly among different manufacturers. Especially in the domestic price-oriented environment, many manufacturers use copper wire to reduce costs. Replacement with copper-clad iron and copper-clad steel led to a decrease in the transmission distance of the network cable, and even the phenomenon of network instability and packet loss, and equipment manufacturers often backfired. Therefore, if you want PoE to play its best role, you must use a good quality network cable.

Cat 5e: Compared to Category 5 twisted pair, Category 5e twisted pair offers less attenuation and crosstalk, providing a more solid network foundation to meet most application requirements (especially supporting Gigabit The Ethernet 1000Base-T cabling) has brought convenience to the installation and testing of the network and has become a better solution in current network applications. The transmission characteristics of the Super Category 5 cable are the same as those of the normal Category 5 cable, but the Super Category 5 cabling standard specifies that all 4 pairs of the Super Category 5 cable can achieve full duplex communication.

Category 6 (Cat 6): The transmission frequency of this type of cable is from 1MHz to 250MHz. The PS-ACR of Category 6 cabling system should have a large margin when it is 200MHz. It provides twice as much as SuperFive. Class bandwidth. The transmission performance of the six types of cabling is much higher than the super five standard, and is most suitable for applications with a transmission rate higher than 1 Gbps.

An important difference between Category 6 and Category 5 is that it improves performance in terms of crosstalk and return loss. For a new generation of full-duplex high-speed network applications, excellent return loss performance is extremely important. The basic link model has been eliminated from the six types of standards. The wiring standard adopts a star topology. The required wiring distance is: the length of the permanent link cannot exceed 90 meters, and the length of the channel cannot exceed 100 meters.

There is no strict difference between the transmission distance of Category 6 and Super Category 5 cables, that is, the maximum transmission distance of a single segment is 100 meters. Of course, the six types of lines can be appropriately increased in the transmission distance. The so-called 100 meters refers to the related technical indicators such as the inability to meet the 1000M bandwidth beyond, and brings about problems such as a decrease in speed.