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[All About Switches - Maintenance Tips] Tell You About PoE

Latest reply: May 2, 2018 05:45:31 2493 2 0 0 1

Tell You About PoE

Recently, I was asked frequently about PoE power supply. So, I summarize information you may need to know about PoE and post it here.

1         Basics

1.1         Origin of PoE

Communication terminals, such as telephone, camera, and POS machine, must be powered by a power source to work. The following problems occur:

l  There are often no accessible power outlets for communication terminals mounted in corridor or ceiling.

l  If there are multiple communication terminals, managing them separately is time-consuming and inconvenient.

To address these problems, scientists developed a simple, easy-to-manage, centralized power supply mode — Power over Ethernet (PoE).

1.2         PoE Implementation

The PoE technology is used on the wired Ethernet and is most widely used on LANs. PoE allows power to be transmitted to terminals through data transmission lines or idle lines. This technology provides power over a 2.5GE Base-T, 1000Base-T, 100Base-TX, or 10Base-T Ethernet network, spanning a distance of up to 200 m.

1.2.1        PoE System


A PoE system has three roles:

l  1 — PoE power module: provides power to role 3.

l  2 — power-sourcing equipment (PSE): determines the way to provide power to role 3.

l  3 — powered device: served by role 2 and role 3. Communication terminals act as role 3.

1.2.2        Wire Sequence in an Ethernet Cable

As defined in IEEE standards, PSEs are classified into midspan and endpoint PSEs:

  • In midspan PSEs, the PoE module is installed outside of the device.
  • In endpoint PSEs, the PoE module is integrated into the device.

Huawei PSEs are endpoint devices. Endpoint PSEs are more widely used than midspan PSEs and are compatible with 2.5GE Base-T, 1000Base-T, 100Base-TX, and 10Base-T interfaces.

Endpoint PSEs can work in Alternative A (line pair 1/2 and line pair 3/6) and Alternative B (Line pair 4/5 and line pair 7/8) power supply modes according to different copper line pairs.

  • In Alternative A mode, power is transmitted over pairs of lines that transmit data.

The PSE provides power to the PD over copper line pairs connected to pins 1 and 2 and pins 3 and 6. Pins 1 and 2 use the negative voltage and pins 3 and 6 use the positive voltage.

10Base-T and 100Base-TX interfaces use copper line pairs connected to pins 1 and 2 and pins 3 and 6 to transmit data, and 1000Base-T interfaces use four line pairs to transmit data.

DC power and data frequency are independent. Therefore, power and data can be transmitted over a single pair of lines.

  • In Alternative B, power is transmitted over idle pairs of lines.

The PSE provides power to the PD over copper line pairs connected to pins 4 and 5 and pins 7 and 8. Pins 4 and 5 use the positive voltage and pins 7 and 8 use the negative voltage.

In general, standard PDs must support both modes, whereas PSEs only need to support one mode.S5720-14X-PWH-SI-AC, S5720-16X-PWH-LI-AC, S5720-28X-PWH-LI-AC, S5720I-SI, S5730HI, S5730SI, S5730S-EI, S5732-H, and S6720SI provide PoE++ power supply and support both Alternative A and Alternative B. Other models support only Alternative A.


2         Hardware and Software Requirements

Software of PoE switches provides the PoE function without a need for license. A PoE interface is enabled with PoE by default. So, I'm only talking about the hardware requirements here.

2.1         Modular Switches

Among Huawei modular switches, only the PoE models of the S7700 series support the PoE function, while the S9700 and S12700 series do not support this function. An S7700 switch needs the following before it can provide the PoE function:

l  PoE power module, which is installed in a PoE power slot. It is the source of power in the PoE system.

l  PoE line card, which is installed in an LPU slot. PDs are connected to the PoE line card.

The PoE card can start and register successfully when the following conditions are met:

1.         It has a dual in-line memory module (DIMM) installed in its DIMM slot or a PSE daughter card has been installed in the card.

2.         The chassis is a PoE model.

3.         The PoE power modules are supplying power to the chassis normally.

The following table lists the S7700 series’ support for PoE line cards and PoE power modules.

PoE Chassis

Max. Number of PoE Line Cards

Number of PoE Power Slots

PoE Power Module Supported

S7703 (PoE chassis)


1 (marked PoE)

l  800 W AC power module

l  2200 W AC power module

l  2200 W DC power module

l  3000 W AC power module


S7706 (PoE chassis)


4 (marked PoE1, PoE2, PoE3, and PoE4)

S7712 (PoE chassis)


S7703 PoE



S7706 PoE




2.2         Fixed Switches

Many fixed switches support the PoE function. You can identify a PoE switch from the PWR/P or PWH/U field in the product name printed on its panel.

Fixed PoE switches may use built-in or pluggable power modules.

The PoE switches using pluggable power modules have two power slots. If you want a switch to provide PoE power supply, you must install one or two PoE power modules in the switch.

3         Output Power


PoE interfaces can work in compliance with 802.3af or 802.3at. Some models support only 802.3bt. In these three standards, the maximum output power is different.

l  When a PoE interface works in compliance with 802.3af, it provides the maximum output power of 15.4 W.

l  When a PoE interface of a fixed switch works in compliance with 802.3at, it provides the maximum output power of 30 W.

l  When a PoE interface of a modular switch works in compliance with 802.3at, it provides the maximum output power of 37 W.

l  When a PoE interface works in compliance with 802.3bt, it provides the maximum output power of 60 W or 90 W.

A PoE interface determines the standard based on the maximum power consumption of the PD connected to it. If the maximum power consumption of the PD does not exceed 15.4 W, the PoE interface works in compliance with 802.3af. If the maximum power consumption of the PD exceeds 15.4 W, the PoE interface works in compliance with 802.3at.


The maximum number of PoE interfaces a fixed PoE switch can provide depends on the switch model, the number of power modules it has, and the PoE standard it complies with. I'm not listing the number of PoE interfaces that each model can provide. You can get the answer in the hardware description of your product.


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Created Apr 12, 2017 11:30:51

thank you
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MVE Created May 2, 2018 05:45:31

useful document, thanks
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