TABLE 2: PoE types, standards, and classes with maximum power delivered by the power sourcing equipment (PSE) and maximum allowed power at the powered device (PD). (Source: Fluke Corporation)
Ethernet Alliance-approved devices provide a simple way to ensure adequate power is supplied. Power Sourcing Devices (PSEs) and Powered Devices each feature a simple label indicating the amount of power they can supply or require. Checking that the switch's power class is greater than the AP's ensures adequate power will be available. This certification also ensures that the devices comply with Ethernet standards. However, while today's advanced PoE switches can determine power requirements of connected devices via power-negotiation messages and allocate power appropriately, problems can arise if that communication malfunctions, PoE has been manually (and incorrectly) allocated per port, or the overall power budget of the switch is oversubscribed. Testers leveraging LLDP and CDP for network testing can also use these protocols to determine the advertised PoE power on a switch port. Testers that perform PoE load testing go a step further by indicating the specific negotiated power class (i.e., 0 through 8) and actual power provided by the switch port. Because power negotiation between a switch and a device happens both at the hardware and software levels, a PoE load tester should ideally display both hardware and software negotiated power information. Negotiated power for a device greater than what a switch port can deliver, whether due to improper allocation, or an oversubscribed power budget, will prevent a PoE- enabled Wi-Fi AP from functioning properly.
To fully understand what's happening, a PoE load tester can indicate which cable pairs carry power. While a single-signature device maintains the same power signature across Pairs 1,2 and 3,6 and Pairs 4,5 and 7,8, a dual-signature device can have independent power signatures on each pair set. Knowing which pairs carry power is helpful, because IEEE 802.3bt PoE (Type 3 and Type 4) supports both topologies. If a switch only supports single-signature devices, it may need to be updated to support dual-signature devices. Such testers can also measure the amount of power under load to confirm that the sourced power is reaching the AP and not lost due to poor-quality cable or a cable link that is too long. STEP 4: CHECK AP CONFIGURATION Recent advancements in Wi-Fi technology have introduced more operating frequencies, channels, channel widths, and security protocols. This increased complexity can lead to configuration issues unrelated to cabling, networking, or PoE operation. The Federal Communications Commission (FCC) and the International Telecommunication Union (ITU) allocate the 2.4 GHz, 5 GHz, and 6 GHz frequency bands for Wi-Fi. Within these bands, specific 20 MHz-wide channels are designated for Wi-Fi. As shown in Figure 3, the 2.4 GHz band has 14 channels, the 5 GHz band has 29, and the 6 GHz band has 59.
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