Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 46)

Chapter 2 Campus Infrastructure Considerations

Power to IP Phones

2-12

Cisco IP Telephony Network Design Guide

78-11103-03

Establishing Power to the IP Phone

To establish power to the IP phone, the power-enabled Catalyst switch performs

the following steps:

1. The switch performs phone discovery by sending specific tones down the

wire to the IP phone. In its unpowered state, the IP phone loops these tones

back to the switch.

When the switch receives this tone, it knows that the device connected is a

Cisco IP Phone and it is safe to deliver power to the device. This behavior is

exhibited only by Cisco IP Phones, so that other devices connected to the

switch port are safe from receiving current. This hardware polling is done by

the system at fixed intervals on a port-by-port basis until a LINK signal is

seen or the system has been configured not to apply inline power to that port.

2. When the switch finds an IP phone by using phone discovery, it applies power

to the device. The Cisco IP Phone powers up, energizing the relay and

removing the loopback (normally closed relay becomes open) between

transmit and receive pairs. It also sends a LINK packet to the switch. From

this point, the IP phone functions as a normal 10/100 Ethernet device.

If the LINK packet is received within five seconds, the Catalyst switch

concludes that the attached device is a Cisco IP Phone, and it maintains the

power feed. Otherwise power is removed and the discovery process is

restarted.

3. Once the Cisco IP Phone is powered and responding, the phone discovery

mechanism enters a steady state. If the phone is removed or the link is

interrupted, the discovery mechanism starts again. The port is checked every

five seconds for a LINK packet and, in its absence, the test tone is generated.

The advantage of this mechanism is that power is supplied to the phone by the

switch just as it is in a traditional telephony environment. In some installations, it

is entirely possible that only two pairs have been terminated out of the four

available for the data run between the wiring closet and the desktop location. In

such cases the inline power method can allow customers to deploy IP telephony

by using the existing cable plant without any modification.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 46