What is PoE?
Power over Ethernet (PoE) is a way of passing electrical power along with data through a connected Ethernet cable instead of the usual AC cable. This allows a single cable to provide both data connection and electrical power to devices such as fixed RFID readers that support this feature. This approach can make the installation of your fixed RFID readers much easier and more flexible for the case that AC outlets are not readily accessible.
Unlike standards such as Universal Serial Bus (USB) which also power devices over the data cables, PoE allows much long cable lengths. Power may be carried on the same conductors as the data, or it may be carried on dedicated conductors in the same cable. There are several common techniques for transmitting power over Ethernet cabling with two of them have been standardized by IEEE 802.3. They are titled “Alternative A” and “Alternative B”.
Ethernet cables such as 10BASE-T or 100BASE-TX have four pairs of cables. Since only two of the four pairs are used for data, power may be transmitted on the unused pairs of the cable. In the IEEE standards, this is called “Alternative B”.
Power may also be transmitted on the data conductors by applying a common-mode voltage to each pair. Because twisted-pair Ethernet uses differential signaling, this does not interfere with data transmission. The common mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer. This is similar to the phantom power technique commonly used for powering audio microphones. In the IEEE standards, this is referred to as “Alternative A”.
In addition to standardizing existing practice for spare-pair and common-mode data pair power transmission, the IEEE PoE standards provide for signaling between the power sourcing equipment (PSE) and powered device (PD). This signaling allows the presence of a conformant device to be detected by the power source, and allows the device and source to negotiate the amount of power required or available. Up to 25.5 watts can be available for a device.
Following are the terminologies used in the PoE network:
- PD – Powered Devices: PDs are end devices that can accept the power transmitted over Ethernet Cat-5 cable.
- PSE – Power Source Equipment: PSEs add power to the Ethernet cable.
- Midspan device: These are power injectors that stands between the standard ethernet switch and the end device.
- Endspan devices (PoE Switches): These are Ethernet Switches that includes the circuit to inject power to the ethernet cable.
Why use POE in your RFID implementation?
Specifying Power over Ethernet can bring many possible advantages to an RFID installation:
- Time and cost savings – by reducing the time and expense of having electrical power cabling installed. Network cables do not require a qualified electrician to fit them, and can be located anywhere.
- Flexibility – without being tethered to an electrical outlet, devices such as RFID readers can be located wherever they are needed most, and repositioned easily if required.
- Safety – POE delivery is intelligent, and designed to protect network equipment from overload, underpowering, or incorrect installation.
- Reliability – POE power comes from a central and universally compatible source, rather than a collection of distributed wall adapters. It can be backed-up by an uninterruptible power supply (UPS), or controlled to easily disable or reset devices.
- Scalability – having power available on the network means that installation and distribution of network connections is simple and effective.
Got any great ideas on PoE its use with fixed RFID reader applications? Let us know in the comments!