Choosing the Correct PoE for your Installation

Choosing the Correct PoE for your Installation

PoE (or Power over Ethernet) is the method of transmitting both power and data across the internal cores of an Ethernet cable and is ideal for getting power to hard-to-reach areas. There are many variants available and the type of PoE you use is ultimately decided by your equipment's capabilities.

1. Active PoE

802.3af/at ( PoE+) | 802.3bt ( PoE++)

Active PoE provides a safe alternative to passive PoE. When a device is plugged into an Active PoE switch, the product and the power source first perform a 'handshake' to ensure the device can accept the voltage being supplied by the switch or injector. This makes active PoE the safer PoE option for office or home environments where people may accidentally plug a computer into the PoE switch.

With active PoE, there are a variety of options, ranging from 15W - 100W, to power even the most power-hungry equipment.

802.3af supports a maximum of 15.4W.
Suited to low power devices such as IP phones or home AP's and is the most commonly used Active PoE.
802.3af PoE uses 4 of the 8 strands of the Ethernet cable in either a Midspan (Mode A) or Endspan (Mode B) configuration. This provides a 10/100Mbps (Fast Ethernet) data and power transmission. When using Gigabit PoE, depending on the manufacturer, either 4 or all 8 strands are used for data and power transmission.

802.3at supports a maximum of 30W.
802.3at offers a higher wattage than 802.3af and is therefore ideal for more power-hungry devices such as a PTZ camera or high power access point. Below is an example of the UAP-AC-SHD maximum power consumption, a device with this power requirement would need 802.3at.

Dimensions 220 x 220 x 48.1 mm
Weight 700 g
Weight with Mounting Kits 830 g
Network Interface 2x 10/100/1000 Ethernet Ports
Buttons Rest
Power Method 802.3at PoE+
Supported Voltage Range 44V to 57V DC
Power Supply UniFi Switch (PoE)
Power Save Supported
Beamforming Supported
Maximum Power Consumption 20 W

802.3bt supports a maximum of 60W on Type 3 and 100W on Type 4.
802.3bt is the latest commercial standard that utilizes all 8 strands of the Ethernet cable for power and data. This feature makes provision for higher power requirements and Gigabit data connections. Aside from single products, a common use for 802.3bt would be to provide a high wattage to a single unit such as the US5-FLEX and have that device power multiple units with smaller power requirements such as Cameras and Access Points.

The various forms of Active and Passive PoE utilise the individual cores of the Ethernet cable differently to provide power to your equipment.
Active PoE injectors and switches either support Mode A, Mode B or both, depending on the manufacturer. The mode supported determines which strands of the cable are used for power and data. If you are having trouble powering a device, check the mode supported by your device and ensure your PoE switch or Injector supports the correct mode.

2. Auto-sensing

Many PoE switches offer both passive and active PoE rolled into a single product. Items such as the Mikrotik RB-CRS328 offer both 24V passive PoE as well as 802.3af/at. Once connected, the switch will perform the 'handshake' with the device and determine what power is required. Once it has determined the product's capabilities, it will commence with supplying power.

Some switches provide both 24V passive and 802.3af/at but do not offer auto-sensing technology. These switches require the user to log into the switch and select the required voltage for each individual port on the switch. A good example of switches that operate like this is the Ubiquiti UniFi switches.

3. Passive PoE

Passive PoE is a simple alternative to active PoE, however, it does not have any of the built-in safeguards. Passive PoE accepts a wide voltage range but leaves the door open for possible damage to products not supporting PoE, or over-voltage when the incorrect power supply is used. With Passive PoE there is no 'handshake' and the PoE injector or switch simply supplies power to any device connected.

4. PoE Splitters

PoE splitters offer a simple 'hack' to power non-compliant devices via PoE. A splitter is made up of two components, an injector and a splitter. The injector accepts the power and data and combines them over the Ethernet cable. When reaching the non-compliant device the Splitter then splits the power and data into two separate connectors. A DC jack for power, and RJ45-male for data.

5. PoE Testers

With all the PoE variants out there, the chances of walking onto a new site and not knowing the power source of a random cable are fairly high. Rather than spending time trying to trace the cable back to the power source, or worse, causing voltage damage to equipment, invest in a PoE tester. They are typically available in many variants but Scoop stocks a very cost-effective keyring version that is simple to operate and gives immediate feedback.

Blog post by Abby Carelse

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