Tech Primer
PoE technology allows DC power supply and data transfer of IP phone, wireless AP, IP security cameras to be implemented over network cables simultaneously, with the existing cable infrastructure unchanged.
A complete PoE system includes two parts: PSE (Power Source Equipment) and PD (Powered Device). PSE is responsible for the management, classification of power, detection and power supply of the PD, which converts the power provided by the PSE for terminal devices.
Most of the PoE devices comply with the 802.3af or 802.3at ratified respectively in 2003 and 2009.
The IEEE standardized version of PoE supplies up to 15.4 W of DC power (minimum 44 VDC and 350 mA) to each device. Only 12.95 W is assumed to be available at the powered device as some power is dissipated in the cables. An updated standard in progress referred to as PoE Plus or 802.3at introduces increased available power. Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.
PoE standards have defined two implementation modes for powering device: A and B, Mode A uses the same two pairs used for data transmission in 10Base-T and 100Base-TX, permitting the provision of both power and data over only two pairs in such networks. Polarity of the 48V power is immaterial as this is allowed for by the PD to allow for crossover cables and automatic RX/TX detection. Mode B uses the spare pairs of 10Base-T and 100Base-TX cable to provide power; therefore, it requires a 4-pair cable assembly.
The PSE decides whether mode A or mode B shall be used, not the PD. some PDs are compliant for either mode.
There are two types of PSEs specified by IEEE 802.3-2008: end-spans and mid-spans. End-spans are usually called PoE switches for short, which are Ethernet switches that have the power over Ethernet transmission circuitry. Mid-spans are power injectors that add power to the connection without interrupting the data flow between the Ethernet switch and powered device. Choosing between an end span implementation or mid span usually comes down to cost. For example if a switch is scheduled to be replaced anyway, on a system using or moving to PoE, it makes sense to replace the switch with a POE enabled switch. If the upgrade purely requires power adding for a single or small number of devices connected to a non PoE switch, then a mid-span solution is likely the more cost effective. In general end-spans are found in new installations and mid-spans in retrofit situations.
Other Resources
- Power over Ethernet
- What is Power Over Ethernet?
- Power over Ethernet (PoE) Midspan -The Smart Path to Providing Power for IP Telephony
- Power over Ethernet: How High Can You Go? The 60W Standard Is Nearly With Us.
- Highly integrated, most reliable PoE PD solution with versatile programmability
- Power-over-Ethernet goes green
- IEEE802.3at High Power POE+ Class Programming
- The Power of PoE
- PoE Basic principle
Glossary:
| PoE | Power Over Ethernet: A relatively new technology that allows PoE-enabled Ethernet networked devices to receive power (as well as data) over existing CAT-5 Ethernet cable without the need to make modifications to it. |
| PD | Powered Device: A powered device is a device powered by a PSE and thus consumes energy. Examples include wireless access points, IP Phones, and IP cameras. The IEEE 802.3af standard specifies a maximum power usage of 12.95 W. |
| PSE | Power Sourcing Equipment, it is a device (switch or hub for instance) that will provide power in a PoE setup. Maximum allowed continuous output power per such device in IEEE 802.3af is 15.40 W |
| Midspan | Midspans are power injectors that stand between a regular Ethernet switch and the powered device, injecting power without affecting the data. |
| Endspan | Endspans are Ethernet switches that include the power over Ethernet transmission circuitry. |
| IEEE 802.3af | an IEEE standard (ratified in June 2003) that defines the transmission of power over Ethernet infrastructure. Also called Data Terminal Equipment (DTE) Power via Media Dependent Interface. |
| Ethernet | Ethernet, an IEEE 802.3 standard, is the most commonly installed computer local area network technology. Ethernet uses a bus or star topology and relies on the form of access known as Carrier Sense Multiple Access with Collision Detection (CSMA/DC) to regulate communication line traffic. |
| CAT-5 | Category 5 cable is the standard type of UTP cable used for horizontal Ethernet wiring. CAT-5 cable contains eight conductors, of which four are used for data communication in 10BaseT and 100BaseTX Ethernet. |
| Access Point | A wireless LAN device that interfaces with a wired Ethernet LAN. |
| RJ-45 | Registered Jack Standard 45. The RJ-45 is a single-line jack for digital transmission over ordinary phone wire, either untwisted or twisted. It is the interface of Ethernet standards 10BaseT and 100BaseT. The connector has eight connector points and can accommodate up to eight wires. |
| DC/DC converter | An electronic device that converts DC power at one voltage to DC power at another voltage (may be higher or lower voltage). The input and output sides of the converter typically are galvanically isolated from each other. |
| Switch | A networking device that connects multiple computers on a LAN so they can communicate with one another, the rest of the network and the internet. Unlike a hub, users connected to a switch do not share the available bandwidth. Each switch port runs at the full bandwidth of the switch port. A switch keeps track of MAC addresses attached to each of its ports and switches data to the intended recipient. |
| Hub | A connection device for networks that allows multiple segments or computers to connect and share packets of information. Hubs merely reproduce and re-send data pulses and do not switch the data based on its address or content. |
| LAN | Local Area Network. A communications network connecting a group of computer, printers, and other devices located within a relatively limited area (such as a home, office or small building). A LAN allows any connected device to interact with any other on the network. A LAN in turn often connects to other LANs, and to the Internet or other WAN. |