The article, in a popular question-and-answer format, talks about the key points when using power over PoE (Power over Ethernet). Differences between standards are given, information is given on protecting devices from surges and other useful things.
What is PoE?
PoE (Power over Ethernet) is a technology for supplying power to a client device via a twisted-pair Ethernet standard (usually a cat.5 cable with RJ45 connectors is used). The same cable is used for both data transfer and device power.
What devices are supported?
Power supplies can be:
- switches,
- routers,
- and other network equipment.
As client devices can be used:
- wire phones,
- video cameras,
- access points,
- various sensors and other peripheral equipment.
There are also devices for integrating with equipment that does not support
PoE.
What does it do?
As the poet Vladimir Mayakovsky wrote: "If the stars are lit, then someone needs it." Below are the benefits of using this technology.
Connecting devices in hard-to-reach places
For example, only two sockets are provided at the user's workplace: for the monitor and the system unit. Often these requirements arise not from a planning error, but are dictated by industry, regional and other standards for IT security, fire safety, labor protection, and so on.
Another example is if a video camera or access point is fixed under the ceiling, it can be difficult to extend the power cable there as well.
Food Administration
The second benefit is that PoE allows you to control the device by power, for example, temporarily turn off, turn on or restart (when it freezes, updates or otherwise).
This is convenient if you have to work remotely, or when devices are in hard-to-reach places.
This is especially useful when working with access points that can be located at a considerable distance or even hidden somewhere above a false ceiling.
Note. Almost all modern access points from Zyxel support PoE
and including new models with Wi-Fi 6 support: as the most "budget" and more advanced и
Figure 1 NWA802.11AX Dual Band 6ax (Wi-Fi 110) Access Point.
Simplify maintenance
In addition to ease of use, the use of PoE allows you to remove the headache in terms of purchasing and repairing power adapters, providing users with sockets, for example, through the purchase of PDUs (in other words, “splitter carriers”). Fewer nodes - fewer points of failure - fewer calls to technical support.
electrical safety
No matter what anyone says, but 220 Volts is a lot. It hurts, it kills. But 57 volts, which is the maximum for PoE, is also unpleasantly dangerous, but not so much. In some organizations, in order for the system administrator to do the work of an electrician, you need a special permit. It is regulated by the same industry and regional standards. And with PoE, they didn’t know anything like that. Weakness - she is a weakpoint.
Aesthetics
What do the technical staff need first? If only it worked. But some especially advanced "comrades" need it to be "beautiful" as well. For example, so that the "extra" wires do not hang down. Or that everything is the same color. And PoE gets rid of these most “extra” conductors. All sorts of inspectors, commissions and "big bosses" are especially sensitive to this.
What are the disadvantages of PoE?
Higher cost of devices
Indeed, it costs more. Especially if you take more or less proven equipment, and do not rely on "maybe", buying "inexpensive NoName solutions".
On the other hand, the principle “more expensive means better” does not always work. Therefore, hunting for an expensive brand only makes sense if there are additional requirements (there is a list of "permitted equipment").
But even with the high price of equipment with PoE, its price can be much lower than the organization from scratch of an additional branched cable system to power remote devices.
Power drop
When transmitting a low-voltage signal through thin wires, the efficiency, let's say, will not be very good. The farther from the power supply, the less electrical power will be left to power consumers. The rest is spent on resistance and heating of wires. With local power (not PoE), things are easier. I put the power supply into the outlet "and the energy went, went ..."
However, such distance problems can be solved by using special switches with increased signal strength, for example, Zyxel series [GS1300] () and .
Staff Qualification Requirements
Let's just say that while using PoE does not require great knowledge, some details
need to be mastered. Information on this issue can be found without much difficulty, although if a person has never worked with this technology, he will encounter some fragmentation and fragmentation of educational material.
PoE standards
For beginners, there may be some confusion. There are 3 generations
standard:
First generation PoE (IEEE 802.3af standard) provides up to 15,4W DC power to each connected device.
The second generation of the IEEE 802.3at standard, also called PoE+, can deliver up to 30W of power per device. This standard is used to power more "gluttonous" consumers, such as Pan-Tilt-Zoom (PTZ) surveillance cameras and 11n wireless access points.
For ease of perception, the main differences are summarized in the table:
Parameters
PoE
PoE +
DC voltages on the powered device
36 to 57 V (nominal 48V)
from 42,5 to 57 V
Source voltage
from 44 to 57 V
from 50 to 57 V
Maximum PoE source power
15,4 W
30 W
Maximum power received by PoE consumer
12,95 W
25,50 W
Maximum current
350 mA
600 mA
Maximum cable resistance
20 ohm (for cat.3)
12,5 ohm (for cat.5)
Food classes
0-3
0-4
The third generation is described by the IEEE 802.3bt standard.
Devices of the third generation of PoE allow you to provide power up to 51 W over a single cable.
Note. To power devices using IEEE 802.3bt standard technologies. all eight conductors of a modern twisted pair cable (cat. 5 and higher) are involved, while for the first two generations only four can be dispensed with.
In terms of compatibility, PoE devices are backwards compatible - a more powerful 802.3bt power supply can be used for older PoE and PoE+ consumers (802.3af, and 802.3at).
Terminology: End-span and Mid-Span
end-span - a device that provides power supply from the beginning of the cable
line.
A classic example: an IP telephony switch provides power to a small network of fixed telephones within an office.
Another example is a video surveillance system in a small warehouse, where the video cameras are powered by a switch via PoE.
Typically, such systems do not provide additional devices to amplify the supply signal.
mid-span - when the power supply is connected not from the beginning of the cable line, but additionally between the switch and the end device. For example, a video camera is powered through an injector, which is switched on after a switch in an intermediate cross cabinet.
Some more terminology:
- PSE (Power Source Equipment) - supply equipment.
- PD (Powered Device) - powered device.
Can the power device understand which client device is connected: with or without PoE?
When it comes to End-span, for example, about a switch, everything happens not simply, but very simply. A power source, such as a switch with PoE ports, only enables power for that port if the connected device (such as an access point) supports PoE.
How does it work?
- At the beginning, a check is made: whether the client device supports power supply via PoE. A voltage is applied from 2,8 to 10V, the input resistance is determined. In the case when the obtained results can be considered satisfactory for powering over PoE, the power device proceeds to the next stage.
- The power device determines the required power to power the client device, for subsequent management of this power. Depending on the level of consumption, devices are assigned a class: from 0 to 4.
However, when it comes to low-cost Mid-Span devices that are switched on after ordinary network equipment (without PoE), everything is not so rosy here. In such cases, constant power with fixed parameters is usually supplied to the line, and the check for: “What device is at the other end of the line?” is not performed.
But what to do when you need to connect devices without PoE support, but there are no sockets for the power adapter?
For such situations, it is Passive PoE using PoE splitter.
In this case, the power supply does not poll the connected device and does not negotiate its power. Power is simply supplied through the free twisted pair conductors using a PoE splitter.
The PoE splitter splits the incoming twisted-pair signal into data and power (12V-24V). Thus, it becomes possible to supply power and integrate a device without PoE support into the existing infrastructure. With this connection method, it is necessary to carefully select the power of the power source and its consumer.
What if it's the other way around? Need to connect a PD (PoE client device) to a common network equipment?
To power client devices with PoE, you can use PoE injector, which is designed to supply additional power to the network cable.
The PoE injector has an RJ45 connector at the input and a connector for connecting to a power source. At the output, it has a single RJ45 connector with PoE.
The PoE injector accepts a standard network signal and injects power into the line for network connection, which allows you to connect a PoE device at the output.
Figure 2. Zyxel PoE injector
What are the cable requirements?
For connection when powered via PoE, a twisted pair cable of at least cat.5e is used.
It's important. Conductors must be copper, not copper-plated, with a minimum thickness of 0,51 mm (24 AWG). The resistance in the conductors must not exceed 9,38 ohm/100 m.
Usually in practice it is recommended not to use cables longer than 75m, although the 802.3af and 802.3at standards say 100m is supported. In the case of Passive PoE, practical recommendations are even more pessimistic - the actual cable length for normal operation should not exceed 60m.
However, special switches, such as managed can support devices at a distance of 250m at a speed of 10Mb/s.
Figure 3. An illustration of how Extended Range works.
What is surge protection (SPD)?
In any extended electrical circuit, there is a threat of short-term impulses caused by charge accumulation (increase in potential difference - overvoltage) with subsequent discharge. The causes of short surge surges are listed below.
- A lightning strike in the vicinity of an object, including a lightning rod, causes an electrical impulse and electromagnetic disturbance, which creates an induced EMF in the cable.
- The accumulation of static electricity caused by air ionization and other external phenomena leads to the appearance of static voltage pulses that can destroy equipment.
- Overvoltage due to switching and equipment switching, for example, switching patch cords in a cross cable, turning on additional power devices, turning on and off a powerful load leads to transients in electrical circuits with sudden voltage surges of a pulsed nature, which can lead to equipment failure.
Note. Due to a number of reasons: a lightning strike near the object during a thunderstorm, as well as air ionization and accumulation of atmospheric electricity before a thunderstorm, this type of protection is sometimes called "lightning protection". This term should not be confused with the term "lightning protection" - that is, with protection against a direct lightning strike.
To prevent such threats, surge protection devices (SPDs) are used. There are two options for protection (SPD): purchase and installation of external devices and embedding protection in devices with PoE.
And finally, the answer to the question: which devices to choose?
Power supply selection
When talking about choosing a source device for PoE power, they mean end-span, and usually this is a switch. The switch is the most used option, they are used in IP telephony, and video surveillance, and when hanging access points, and when arranging all kinds of security system sensors, access control controllers, and so on.
It is important to consider several factors here:
- Top to bottom compatibility. That is, a more modern device that supports the latest IEEE 802.3bt standard can be used to connect and power older devices. But on the contrary - no.
- Distance of PD (powered devices). In addition to the length that is "here and now", it is worth thinking about the future. For example, if the warehouse territory is expanded, or the office is planned to be moved. It is better to lay some reserve of characteristics "for the future".
- Device management. In addition to the “log in” to the switch and manually turn off / on the power, there are other management options, for example, for video cameras.
- Protection against impulse surges (SPD) and other harmful factors.
Zyxel has switches that meet all of the above requirements. These are models of the new GS1350 series. We already This series was originally positioned as However, they can be used without problems in other cases, for example, to power phones, access points and other devices with PoE.
Figure 4. GS1350-26HP Dedicated Managed PoE Switch.
Unmanaged Series Switches are also a good choice. A selection of specialized switches from Zyxel can be seen in Figure 5.
Figure 5 Zyxel's portfolio of managed and unmanaged PoE switches.
Selecting a consumer device
Usually, when choosing end devices, they are guided by their consumer characteristics, for example, image quality when choosing a video camera, support for Wi-Fi standards when choosing access points, and so on.
However, the power supply also leaves its mark. It makes sense to consider the following factors:
- The economy of the device.
- Control capabilities.
- Price and quality.
Important! Despite the declared compatibility from top to bottom, you should not rely 100% on this possibility. In a good project, the power supply and consumers must support the same standard, preferably the most up-to-date, be fully compatible, and be purchased based on the use of new technologies, for example, Wi-Fi 6. Alteration of a whole piece of infrastructure, proudly called “upgrading”, is most often more expensive than some additional costs during the implementation phase.
Useful links
Wi-Fi 6 hotspots: ,
и
Dedicated Managed Series Switches and unmanaged on the Zyxel website
Source: habr.com