Succeed with PoE
Table of Contents
Power over Ethernet (PoE) describes the concept of delivering electrical power along with data on an Ethernet network infrastructure. PoE was first developed to simplify the deployment of VoIP phones by not requiring an additional power source at the phone. Since then it has played an important role in the expansion of network-connected devices, particularly when such devices are mounted in areas where it is difficult or expensive to install additional electrical power outlets. The growth of Wi-Fi via powered access points and IP surveillance via powered cameras is enabled by PoE. With the projected growth of IoT devices, coupled with the newly ratified 802.3bt higher power levels, proper PoE operation will become more critical.
With a single cabling infrastructure delivering both data and power, a lot can go wrong without good design and verification practices. Thorough knowledge of the power and data communication specifications of the devices to be deployed as well as understanding the characteristics of the existing or new cabling infrastructure that will be used to interconnect the devices and the power sources is a must for a smooth deployment.
This paper describes PoE technology including the recently ratified IEEE 802.3bt specification also referred to as PoE++ or 4PPoE (4 pair PoE). It will answer the following questions:
- How does PoE work?
- What are the considerations when deploying PoE, particularly as the power requirements increase?
- Is there a standardized best practice to verify and troubleshoot the deployment?
Types of PoE equipment
Before diving into PoE, it is important to understand a few key terms:
|Power Source Equipment (PSE)||This is the device which provides the power. The PSE can be either End-Span or Mid-Span.|
|Powered Device (PD)||This is the device to be powered by the PoE system.|
|End-Span||End-Span power is usually a network switch that is providing the power from the end of a cable run.|
|Mid-Span||A mid-span injector is a device that provides PoE power in the middle of a cable run between the network switch and the PD.|
|Cabling||PoE uses twisted-pair cabling to connect between the PSE and the PD. The gauge and material of the cable and interconnecting hardware (e.g. patch panel) have impact on power loss.|
Figure 1 depicts End-Span and Mid-Span PSE configurations. End-span equipment is normally used on new installations where other switch upgrades are desired (e.g. going to 1000-BaseT). Deploying a PoE switch makes adding power to your network convenient and adds fewer points-of-failure and complexity than mid-span injectors.
Mid-spans are used when it is undesirable to replace a non-PoE switch and only PoE needs to be added to the network. When using a purely passive mid-span injector for data communication, the distance limitation between the switch and the PD still needs to be less than 100 meters. Some mid-spans can draw power from a PoE end-span device and serve as a signal repeater to increase the distance between the PD and the switch beyond 100 meters.
PoE Standards & Interoperability
PoE standards have evolved over time, progressively delivering higher power to meet the requirements of new applications. This has resulted in a complex PoE product landscape with both standards-based and pre-standard implementations. The numerous implementations vary in the features offered with respect to voltage levels, power levels, power management and classification. Because of the large variety of PSE and PD equipment in the market, the burden is on the consumer to select the right equipment and verify interoperability. PDs that require higher power, such as heated Power Tilt Zoom (PTZ) cameras for outdoor surveillance, have power requirements that change between idle and active states. A successful PoE deployment requires the installer to understand this landscape and verify the maximum power required by the PDs.
Figure 2 shows the four defined types of PoE from the IEEE Standard. The new IEEE 802.3bt standard provides the highest maximum power level suitable for powering kiosks and lighting. There are also non-standard, ad-hoc PoE implementations such as 12 or 24 VDC power injection for vendor-specific security cameras and access points.
|Feature/Standard (PoE Type)||IEEE 802.3af (Type 1 )||IEEE 802.3at/PoE+ (Type 2)||UPOE/802.3bt (Type 3) PoE++||802.3bt (Type 4) PoE++|
|Output power of PSE [W]||15.4||30||60||90|
|Power at PD [W]||12.95||25.5||51||71.3|
|Output voltage at PSE [V]||44 - 57||50 - 57||50 - 57||52 - 57|
|Voltage at the PD [V]||37 - 57||42.5 - 57||42.5 - 57||41.1 - 57|
|Max current/pair [mA]||350||600||600||960|
Figure 2: PoE standards overview.
PoE Deployment Considerations
The overall benefit of PoE is to facilitate the ease of deployment of network connected devices. When deploying a PoE system the power delivery, types/classes, and power management must be considered.
PoE uses two or four twisted pairs in a standard Ethernet cable to supply DC power to PoE-enabled devices. PoE is transmitted on the data conductors by applying a common-mode voltage to each pair. Because twisted-pair Ethernet uses differential signaling for data transition, this does not interfere with data transmission as long as it follows the rules:
- The PoE is carried over twisted-pair cable via RJ45 connector following the pairing scheme defined under IEEE 802.3 Ethernet standard.
- The voltages on the two conductors within a pair are the same level and polarity.
- PoE is subject to the same distance limitations as standard cable channel: 100 meters or 328 feet.
If only two of the four pairs are used to deliver PoE, and the pairs are 1-2 and 3-6, the IEEE standard refers this delivery scheme as Alternative A. Since only two of the four pairs are needed for 10BASE-T or 100BASE-TX, power may be transmitted on the unused conductors of a cable, e.g., 4-5 and 7-8. In the IEEE standards, this is referred to as Alternative B. PoE can also be used on 1000BASE-T and 10GBase-T Ethernet, which uses all four pairs for data transmission. Higher power 4 pair PoE deploys all four pairs for both power and data. Figure 3 shows the details of how power is delivered over the pairs. The PSE determines the pairs over which power will be delivered.
|TIA/EIA-568 T568B Termination||TIA/EIA-568 T568A Termination||10/100 Mode B||10/100 Mode A||1000
|1||White/Orange||White/Green||Rx+||Rx+||DC+||TxRx A+||TxRx A+||DC+||TxRx A+||DC+|
|2||Orange||Green||Rx-||Rx-||DC+||TxRx A-||TxRx A-||DC+||TxRx A-||DC+|
|3||White/Green||White/Orange||Tx+||Tx+||DC-||TxRx B+||TxRx B+||DC-||TxRx B+||DC-|
|4||Blue||Blue||DC+||TxRx C+||DC+||TxRx C+||TxRx C+||DC+|
|5||White/Blue||White/Blue||DC+||TxRx C-||DC+||TxRx C-||TxRx C-||DC+|
|6||Green||Orange||Tx-||Tx-||DC-||TxRx B-||TxRx B-||DC-||TxRx B-||DC-|
|7||White/Brown||White/Brown||DC-||TxRx D+||DC-||TxRx D+||TxRx D+||DC-|
|8||Brown||Brown||DC-||TxRx D-||DC-||TxRx D-||TxRx D-||DC-|
Figure 3: Power delivery details.
It is tempting to push the distance limitations beyond the IEEE specified maximum of 100m when the only alternative is adding AC power at the PD or an intermediate switch. While not recommended, a network tester can verify the data link and maximum power is still available in these circumstances.
PoE Types and Classes
Over time, PoE standards have evolved to accommodate increasing power demands of Powered Devices (PDs). The original IEEE 802.3af standard written in 2003 provides up to 13W of DC power to each device. The updated IEEE 802.3at standard (2009) is also known as PoE Plus (PoE +) and provides up to 25.5 W of power. The proprietary Cisco UPOE implementation utilized all four pairs to increase the power at the PD to 51W. With the ratification of IEEE 802.3bt there are now 9 possible power classes across 4 classes of PSE. A variety of handshake and negotiation schemes are used between the PSE and PD to recognize each other’s power requirements and capabilities. Figure 4 shows the PoE type, power, pairs and governing standard for each power class.
|Power Class||PoE Type||Power at Source (PSE)||Power at Device (PD)||Number of Pairs||IEEE Standard|
|0||1||15.4 W||13.0 W||2||802.3af|
|1||1||4 W||3.84 W||2||802.3af|
|2||1||7 W||6.49 W||2||802.3af|
|3||1||15.4 W||13 W||2||802.3af|
|4||2||30 W||25.5 W||2||802.3at|
|5||3||45 W||40 W||4||802.3bt|
|6||3||60 W||51 W (4 pairs)||4||802.3bt|
|7||4||75 W||62 W (4 pairs)||4||802.3bt|
|8||4||90 W||71.3 W (4 pairs)||4||802.3bt|
Figure 4: Power levels by Class and Type.
In many PSEs, the maximum chassis power available limits the total number of ports that may be powered. For example, class 4 PDs require 30W at the PSE and a 48-port Type 2 PoE switch should support up to 1440W of power. The addition of 802.3bt and 90W at the PSE per port would require 4320W of power just for the PoE portion of the switch. Many PoE switches support less power and power management becomes necessary. Power management adds complexity during moves, adds and changes, and when troubleshooting. Some PSEs allow different priority levels to be set for each port. When a PD is connected to the PSE, the PSE will check the PD’s class and reserve the power from its available power budget. When the PSE reaches its power limit, the next PD to connect that requests more power than the PSE’s available power budget can still be accepted if the connected port has a higher priority than other ports. The only way to ensure the requested power can be provided on the port is to verify it.
There are many points of failure in delivering PoE especially since many cable infrastructures existed before PoE was deployed or only low power 802.3af was used. Since 802.3af, the power available to devices (PDs) has increased 5-fold by utilizing 2 additional pairs and increasing current to 960ma per pair. This is utilizing the cable infrastructure in a way it was never used before.
There are many points of failure in PoE delivery system depicted above.
- Is the switch (PSE) provisioned correctly to deliver the requested power on the correct ports. If configured correctly, does the switch have any power budget restrictions?
- There are typically 2 patch cables between the PSE and PD. Are the cables the correct cable category, wire gauge and composition?
- Are the RJ-45 connectors making 100% contact on all 8 pins?
- Is the horizontal cabling the proper category, gauge, conductor material and shielding? Are the cable pairs properly terminated at the back of the patch panel and on the wall jack? In high temperature conditions such as dense bundling and hot ceilings the ampacity of the cable may be reduced.
- Is the PD compatible with the PSE? Beside the hardware class negotiation, there are two different protocols (LLDP and CDP) that can be used for negotiating additional power.
Functional verification that the maximum requested power can be achieved at the PD is the best way to ensure existing and future PDs will have all the power they need.
Following these steps will isolate the cause of the problem. The LinkRunner G2 (LRG2) is configurable to any of 9 power classes to emulate any PD. Having a PoE tester that includes active network measurements such as speed/duplex, port discovery, VLAN, helps ensure that you are on the correct switch port from the cable endpoint.
During power negotiation the tester will display the requested class, received class and PSE Type. Once power is negotiated, the LRG2 measures the unloaded voltage, pairs used and polarity. Knowing the pairs and polarity is helpful in identifying and troubleshooting midspan PSEs. In the presence of non-standard PoE, the tester indicates the voltage (typically 12 or 24V), pairs and polarity.
Like a car battery on a cold day, the only way to trust the power source and cabling system is by loading it. The proprietary TruePower™ measurement generates a load, like starting a car. The tester will increase its load to the class maximum level to ensure that full power is available at the PD. Once fully loaded, the LRG2 measures the voltage again to ensure the voltage at the PD is above the minimum allowed. Here we see we were able to draw 71W and the voltage dropped to 49.6V meaning that 5.3V was lost in the cabling. If a longer or lower quality cabling was used the voltage could drop below the specification.
TruePower™ loads the circuit to stress switches, patch and horizontal cabling, and patch panels to verify full power before installing PDs enabling installers and network techs to be confident the eventual PD will work at the required power level.
PoE provides cost savings when network-connected devices need to be deployed in a wide variety and number of locations, particularly when bringing power to the devices is expensive and inconvenient. With the ratification of the 802.3bt that specifies up to 71W available at the PD, the deployment and diversity of PoE devices such as digital lighting, building automation, and signage is forecasted to grow.
Careful attention needs to be paid to the design of the system, selection of the equipment (PSE and PDs) and the integrity and compatibility of the new and existing cable infrastructure to ensure a reliable and interoperable system. The deployment and maintenance phases of the system will benefit from appropriate testing and a well documentation system. Choosing the right tool for the installers and maintenance staffs as well as establishing and following procedure to validate and document the PoE system parameters will increase your chance of success.
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