Home > Articles > Cisco

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Power over Ethernet

To simplify deployment and ensure that the Cisco IP phones are powered even in emergencies, power to operate the phones is sourced from the wiring closets by either powered switches or power patch panels running on uninterruptible power supplies (UPSs). Figure 7.3 shows the two methods commonly used to provide power to IP phones.

Figure 7.3

Figure 7.3 Power is supplied from the access switch.

Prestandard power uses a relay in the Cisco IP phone that bridges the transmit pair and the receive pair so that the Fast Link Pulse (FLP) can be detected by the Cisco Catalyst switch. After the switch detects the FLP, the switch applies inline power to the Cisco IP phone to activate the relay so that transmit and receive pairs can power the phone and enable communications.

Cisco prestandard power can be supplied on two pairs of a standard four-pair Ethernet cable. Option 1 is pins 1 and 2 and pins 3 and 6, and option 2 is pins 4 and 5 and pins 7 and 8.

Make sure that all the components in your wiring plant are certified to carry the power and voltage levels required to power Cisco IP phones. Components such as cables and patch panels should be Cat 5e or better.

With the IEEE standard 802.3af and the extended 802.3af plus, the method of detecting a powered device connected to Catalyst switches has been modified to measure resistance by sending low power to the powered device, measuring the result, verifying the result by slightly increasing the power, and remeasuring. After the powered device is verified, full power is applied.

  • + Share This
  • 🔖 Save To Your Account