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

Created: Apr 11, 2017 20:34:31Latest reply: May 2, 2018 13:45:31 1679 2 0 1

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, like telephone, camera, and POS machine, must be powered by a power source to work. Then problems arise:

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

PoE enables terminals to receive power over Ethernet cables. This technology provides 0 V to -53 V DC power to terminals over a distance of up to 100 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 play role 3.

1.2.2 Wire Sequence in an Ethernet Cable

As we know, an Ethernet cable has four wire pairs: 1/2, 3/4, 5/6, 7/8. When a Huawei PoE switch powers a PD over an Ethernet cable, pair 1/2 acts as the positive pole, and pair 3/6 acts as the negative pole.

Here's supplementary information for you: Huawei switches support the Alternative A power supply mode defined in IEEE 802.3af, in which wire pairs 1/2 and 3/6 can transmit data while transmitting power. As DC power and data are transmitted on different frequencies, power and data can be transmitted over the same wire pairs at the same time. 10Base-T and 100Base-TX interfaces use wire pairs 1/2 and 3/6 to transmit data, while 1000Base-T interfaces use four line pairs to transmit data.

2   Hardware and Software Requirements

Software of PoE switches provides the PoE function without a need for license. You can run the poe-enable command to enable the PoE function. 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 (ES0D0G48VA00), which is installed in an LPU slot. PDs are connected to the PoE line card.


The ES0D0G48VA00 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.

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

Important note: Power modules of different power values or power supply modes (AC/DC) cannot be used together in the same chassis.

S7706 PoE chassis


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

S7712 PoE chassis



2.2  Fixed Switches

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

Fixed PoE switches may use built-in or pluggable power modules. The S2750 and S5700-LI series PoE switches use built-in power modules and do not support 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. When using two PoE power modules in a switch, ensure that the two power modules are the same model.

3  Output Voltage and Power

Each PoE interface provides 0 V to -53 V DC power output. A PoE interface can work in compliance with IEEE 802.3af or 802.3at. The two standards differ in the maximum output power.

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

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

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

3.1   Which types of PoE power modules do S7700 switches support? How much PoE power can they provide? How many PoE interfaces do these switches provide?

The following table tells you which PoE power modules the S7700 switches support and the maximum PoE power these PoE power modules can provide.

PoE Power Module

Max. PoE Power

800 W AC power module

800 W

2200 W AC power module

2200 W

2200 W DC power module


The following table tells you which S7700 chassis support PoE, the maximum PoE power they can provide, and the maximum number of PoE interfaces they support.

PoE Chassis

Max. PoE Power

Max. Number of PoE Interfaces

S7703 PoE chassis

2200 W (power redundancy not supported)

144 (a maximum of 3 PoE line cards)

S7706 PoE chassis

8800 W (6600 W in N+1 power redundancy mode)

288 (a maximum of 6 PoE line cards)

S7712 PoE chassis

576 (a maximum of 12 PoE line cards)

3.2  How much PoE power can a PoE power module in a fixed switch provide?

You will know the answer from the following table.

PoE Power Module

Max. PoE Power

250 W PoE power module

123.2 W

500 W PoE power module

369.6 W

580 W PoE power module

369.6 W

1150 W PoE power module

785.4 W

This table provides the maximum PoE power of a single power module. If a fixed switch has two PoE power modules, the maximum PoE power doubles.

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 manual of your product.


This post was last edited by 交换机在江湖 at 2017-04-11 20:37.
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user_2790689     Created Apr 12, 2017 19:30:51 Helpful(0) Helpful(0)

thank you
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wissal  Visitor   Created May 2, 2018 13:45:31 Helpful(0) Helpful(0)

useful document, thanks
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