Power Over Ethernet, Course 404

End-Point and Mid-Span PSEs

There are two kinds of PSEs. The first is an end-point PSE (Figures 3 and 4) where the power sourcing equipment is within the hub or switch. The Ethernet switch now becomes a Power Sourcing Ethernet switch — adding complexity and expense. However, field cabling is not disturbed. The standard Ethernet switch is now replaced with a PoE Ethernet switch. In terms of data handling, the PoE switch operates identically to that of a non-PSE switch. Additional circuitry and an adequate power supply are necessary to serve the powered Ethernet ports, but not all ports need to be powered and frequently this is done to reduce power supply requirements. A switch-to-switch cascading connection (uplink) would not need powered ports so non-PoE ports would be present in PoE switches.

Figure 3 — An end-point PSE can utilize Alternative A or B. Alternative A is shown above.

Figure 4 — Alternative B uses the spare pairs for carrying power. No special data transformers are required at the PSE.

The second PSE is the mid-span PSE (Figure 5) where the mid-span device can reside anywhere within the 100-meter length of Ethernet cable attaching a conventional hub or switch port to an end station. The advantage of this device is that standard off-the-shelf Ethernet switches can be retained in PoE applications since the power comes from the mid-span device and not the switch. In this application, the Ethernet switch is not disturbed, as is the field cabling. The mid-span device is not a hub or a switch but mostly resembles that of a patch panel with a conventional Ethernet input port and and a corresponding powered output port. The downside is that it doubles the connections and introduces another piece of equipment to mount and energize. Mid-span devices are beneficial for testing the PoE concept or for operating with legacy equipment during a retrofit.

Figure 5 — A mid-span device can only use Alternative B and, therefore, will not operate with Gigabit Ethernet.

Alternatives A or B

As mentioned previously, there are two ways of energizing end devices. Power can be applied to either the data pairs or the spare pairs but not both. An end-point PSE vendor makes the choice. On the other hand, the PD vendor has no choice and must be able to handle either method. Notice in Figure 4 that the PSE vendor decided to use the spare pairs (Alternative B) to power the PD. In this case, no special consideration was given to the transformers on the data lines. Since the PD must accommodate either Alternative A or B, the PD must utilize center-tapped transformers. The PSE vendor could have easily decided to use the data pairs (Alternative A) as shown in Figure 3. The vendor is allowed to support both approaches but not at the same time.

In Figure 5, the mid-span PSE vendor utilized Alternative B. This is the only choice for a mid-span device since the data lines must be passed without disruption. Since spare pairs are being used, a mid-span device cannot be used with Gigabit Ethernet. Only Alternative A systems can support Gigabit Ethernet. In this situation, the end-point PSE and PD would both need four, center-tapped transformers. Regardless of which alternative is being used, there is an electrical isolation issue for the PD. The standard specifies 1500 volt RMS isolation. Not only must the transformers be so rated, the DC-DC converter within the PD must have the same isolation. This eliminates the lower cost converters that have no input transformer. A proper implementation would require transformer isolation on the input to the DC-DC converter. Again, this increases the cost of the PD.