Daisy Chain PoE Devices Using PoE Power Forwarding
PoE is a point-to-point technology and when PoE Power Forwarding is used, it allows an additional device to be cascaded in series to extend a single Ethernet run.
Power over Ethernet (PoE) has many advantages such as lower installation costs, single wire operation and remote power management of networked devices. A more “hidden” feature is the ability to daisy chain network devices, such as a wireless access point, utilizing a technique called PoE Power Forwarding.
Daisy-chaining allows for extending the Ethernet run over 100m, since each link can be 100m in length. Additionally, some environments may have limited RJ45 Ethernet jacks available (like in an office cubicle or at the top of a pole installation) and daisy-chaining will allow multiple PoE devices to be connected to a single Ethernet jack.
PoE Power Forwarding adds a single power sourcing equipment (PSE) port to a powered device (PD) to power an additional PD such as a wireless access point or an IP camera. Any power the first PD does not need for its application is passed along to the other PD using Power Forwarding.
As an example, consider a PoE powered wireless access point (AP) that consumes 40W of power. The IEEE 802.3bt specifies up to 71W available to a PD attached to a 90W PoE port. In this case, 71W is available to the AP and approximately 30W is available to forward to another PD, such as an IP camera. The camera at the far end of the Ethernet cable will have at least 11W of power available, as shown in figure 1.
See Figure 1. Two Cascaded PDs using PoE Power Forwarding
The IEEE specification allows for 100m of cable between the switch and the AP and another 100m of cable between the AP and the camera thus enabling the camera to be located 200m away from the switch.
PoE Power Forwarding is a feature that adds more flexibility to the installation of networked devices in addition to extending the reach of the network.
The PD69201 single channel PSE device is the recommended device to enable PoE Power Forwarding for daisy-chaining additional PDs. The application note, AN3580, contains the details for designing a PD69201 into a PoE system. For more detailed design information, contact the PoE applications team.
Need to extend the Ethernet range without an AP in between? The Microchip PoE-Extender does just the job. This PoE accessory has both a PD and a PSE chip inside so both the power and the data may be reliably transmitted beyond 100m. The extender acts as a pass-through with data-repeating capabilities.
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