One of the challenges IP Telephony has faced is that traditional
PBX-based digital phones derive power from the PBX through the phone cable.
This allows the phones to operate during times of power outages, provided that
the PBX has a battery, a generator backup or both. First generation IP phones
required a separate power supply for each phone. In order to maintain phone
system availability during a power outage the external power supplies must be
connected to an uninterruptible power source. Cisco has introduced a solution
to this problem by supplying power to the phone via the same Ethernet cable
that carries the data to the phone. This power can be supplied by 10/100
Ethernet blades or modules such as the WS-X6348 that are installed in a chassis
or by a separate device such as the WS-PWR-PANEL.
There are currently two different implementations of in-line powered
Ethernet ports in Cisco products. The first uses the same two pairs of wire
over which the Ethernet signals are carried (pins 1, 2, 3, 6), while the other
uses the two unused Ethernet pairs (pins 4, 5, 7, 8). The IEEE 802.3af
committee has standardized in-line power over Ethernet in June 2003. For more
information with regard to 802.3af, refer to
DTE Power via MDI Task Force
There are no specific prerequisites for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Technical Tips Conventions.
There are currently four Cisco products that have the capacity to
supply in-line power.
The first product is the WS-X6348 48 port 10/100 line module for the
Cisco Catalyst 6000 series switches. The card by itself is only in-line power
capable. To provide in-line power, it must also have the WS-F6K-VPWR daughter
card installed on it. For information on this card, refer to
6500 Series Inline-Power Field-Upgrade Daughter Cards Installation Note.
All 48 ports have the capacity to supply power to a phone, provided there is
sufficient power available on the Cisco Catalyst 6000 switch in which it
The WS-X6348 supplies in-line power via the "used" Ethernet pairs (pins
1, 2, 3, 6).
For more information on the WS-X6348 module, refer to
48 Port IP Phone Ethernet In-Line Power Blade for Catalyst 6500/6000 Series
The second product is the WS-X4148-RJ45V 48 port 10/100 line module for
the Catalyst 4006 switch. In order to provide in-line power using the Catalyst
4006 switch, you must add on several other components. In-line power is only
available on the Cisco Catalyst 4006 and not the Cisco Catalyst 4003 because
only the Cisco Catalyst 4006 chassis has the ability to accept the Power Entry
Module (PEM) and also has traces on the back-plane which allow the DC power to
be supplied to inline-power capable line cards. To enable in-line power on the
Cisco Catalyst 4006, you must have the Cisco Catalyst 4000 Auxiliary DC Power
Shelf and at least two power supplies (WS-P4603-2PSU). The power shelf can
accept up to three power supplies (WS-X4608) for N+1 redundancy. At least two
are required for in-line power to work. Special cables (which are included with
the power supplies) are used to attach each power supply to the PEM
(WS-X4095-PEM). Finally, you must have an in-line power capable line card in
the chassis. The WS-X4148-RJ45V is a 48 port in-line power capable 10/100
Ethernet switching module. The picture below does not show the in-line power
daughter card that is included with the Cisco Catalyst 4148. It is similar to
the daughter-card on the Cisco Catalyst 6000 module. The Cisco Catalyst 4006
switch operates identically to the Cisco Catalyst 6000 switch, in respect to
in-line power detection and delivery.
The WS-X4148-RJ45V supplies in-line power via the "used" Ethernet pairs
(pins 1, 2, 3, 6).
For more information on the WS-X4148-RJ45V module, refer to
Catalyst 4000 Series Inline Power Solution.
The third product is the Cisco Catalyst 3524-PWR-XL (WS-C3524-PWR)
switch, which is based on the Cisco Catalyst 3524XL switch.
The WS-C3524-PWR supplies in-line power via the "used" Ethernet pairs
(pins 1, 2, 3, 6).
For more information on the Cisco Catalyst 3524-PWR-XL, refer to
XL: Catalyst 3524-PWR XL Stackable 10/100 Ethernet Switch.
Note: The Catalyst 3524-PWR-XL has reached end of sale. As an alternative,
use the Catalyst 3550. Refer to
Cisco Catalyst 3550
Finally, there is the standalone in-line power patch panel,
WS-PWR-PANEL, which requires an external switch to provide the Ethernet
connectivity. The in-line power patch panel will supply power "mid-span," that
is it connects between the Ethernet switch and the phone. The in-line power
patch panel is a completely hardware-based solution and has no software or
firmware that can be changed or upgraded in the field.
The WS-PWR-PANEL supplies power using the "unused" pairs (pins 4, 5, 7,
For more information on the WS-PWR-PANEL, refer to the
Inline Power Patch Panel data sheet.
Cisco now offers IEEE 802.3af standard-compliant
over Ethernet (PoE) options for its Intelligent Catalyst switching
portfolio. IEEE 802.3af compliance is delivered in new PoE 10/100/1000 and
10/100 modules on the Cisco Catalyst 6500 Series and Cisco Catalyst 4500 Series
modular chassis; new PoE 10/100 Cisco Catalyst 3750 Series and Catalyst 3560
Series fixed-configuration switches. For more information, refer to
Over Ethernet Solutions.
The Cisco Catalyst switches that support IEEE 802.3af
standard-compliant PoE also support Cisco's pre-standard PoE implementation and
are backwards compatible with Cisco's existing end devices, such as IP
Telephones and Wireless access points. But the Cisco Catalyst switches that
only support the pre-standard PoE implementation are not able to power up IEEE
All the previously listed products rely on a phone-discovery algorithm
before the power is supplied to a phone. This algorithm ensures that the switch
does not supply power to a device that is not able to accept in-line power. The
phone-discovery algorithm used by Cisco Catalyst switches is different than the
algorithm used by the WS-PWR-PANEL. Both of these algorithms are explained in
Note: It is not possible to provide a detailed explanation of the
phone-discovery algorithms because certain aspects of them are proprietary.
This table explains the parameters available on the three platforms to
enable or disable the supply of power to the ports.
In-Line Power Modes for Catalyst Switches
Phone-discovery algorithm is operational
Cisco Catalyst 4006, 6000, and 3500XL
Phone-discovery algorithm is disabled
Cisco Catalyst 4006 and 6000
Phone-discovery algorithm is disabled
Cisco Catalyst 3500XL
Note: There is no such thing as an 'on' mode on any of these devices. This
should protect customers from accidentally damaging any Ethernet network
interface card (NIC) cards in devices that do not expect to receive power from
The following method for detecting that a Cisco IP Phone is connected
to a 10/100 Ethernet port is used by the Cisco Catalyst 6000, Cisco Catalyst
4000, and Cisco Catalyst 3524-PWR-XL Switches.
The port starts the phone-discovery algorithm by sending a special
Fast Link Pulse (FLP) signal to any device that might be connected to it.
The port waits to see if the special FLP signal is forwarded back by
a connected device. The only devices that are designed to do this are devices
that expect to receive in-line power.
If a Cisco 79xx IP Phone is connected to the 10/100 Ethernet port, it
will forward the special FLP signal back to the 10/100 Ethernet port on the
Cisco Catalyst switch. It is capable of doing this because it has a special
relay that connects its Ethernet receive pair with its Ethernet transmit pair.
This relay is closed when no power is being supplied to the phone. Once power
is supplied, this relay remains in an open state.
Now that the Cisco Catalyst switch has determined that it needs to
power the port (the special FLP signal was received back from the attached
Cisco IP Phone), the Network Management Processor (NMP) is queried to determine
if there is any power available to power the IP phone. Since the NMP does not
know how much power the Cisco IP Phone will need, it uses the configured
default power allocation. Later on it will adjust this allocation based on what
the attached Cisco IP Phone tells the switch it really needs.
The port then provides power to the Cisco IP Phone over pairs 1 and 2
as a common mode current.
The port is taken out of phone-discovery mode and changed to normal
10/100 Ethernet auto-negotiation mode.
The instant that the switch applies power to the port, the relay
inside the phone opens and power begins to flow to the Cisco IP Phone.
At this point a 'wait for link' timer in the switch starts also. The
phone has five seconds to establish link integrity on its Ethernet port. If the
switch does not detect link integrity on the port within five seconds, it will
shut off power to the port and start the phone-discovery process all over
again. The switch has to wait at least five seconds so that the switch has
enough time to detect all devices.
If the switch detects a link within the five second window, it will
continue to supply power to the Cisco IP Phone until it detects a link down
Once the phone has booted up, it will send a CDP message with a Type,
Length, Value object (TLV) that tells the switch how much power it really
needs. The NMP sees this and adjusts the power allocation for the port
Note: Only the Cisco Catalyst 6000 switch keeps track of how much power has
been allocated for each device. The Cisco Catalyst 4006 and the Cisco Catalyst
3500XL switches have enough power available to supply Cisco IP Phones on every
The In-line Power Patch Panel (IPPP) uses the unused Ethernet pairs to
provide in-line power. The IPPP has four rows of RJ-45 connectors each with 24
ports in a row. The top two rows are the powered ports used to connect to the
end device (for example, a Cisco 79xx IP Phone). The bottom two rows are used
to connect to the switch which will be providing the Ethernet connectivity.
Internally, the IPPP directly connects the Ethernet pairs from the
bottom switch port that corresponds to the phone port on the top. The in-line
power patch panel does not interfere with pins 1, 2, 3, and 6 in any way. It
does not monitor link and does not care about speed/duplex, because it is
The phone discovery algorithm for the IPPP is similar to the method
used on Cisco Catalyst switches as explained in the previous section. It relies
on the fact that the phone will loop back a special signal that the IPPP sends
on its ports. In this case, however, the unused pins 4, 5, 7, and 8 are used to
detect Cisco IP Phones. If a Cisco IP Phone is detected, these pins (wire
pairs) are also used to provide power.
This method to detect a Cisco IP Phone is connected to a 10/100
Ethernet port is used by the IPPP (WS-PWR-PANEL):
The IPPP starts the phone discovery sequence at port
The IPPP sends a 347 kHz loopback tone out port 1. The IPPP listens
for 50 ms to determine if the loopback tone is forwarded back by a device that
is connected to the port. Only devices that are expected to receive power on
these pins, forward the loopback tone to the sending device (the IPPP in this
case). The IPPP must detect 16 transitions within a 50 ms-period of time to
verify that it senses the correct loop back tone and not an
If the IPPP verifies that this received signal is the correct one,
power is enabled on the port. If the signal is not correct the IPPP moves on to
the next port and starts the process over again.
The IPPP continuously cycles through the ports repeating the above
steps for each port.
Each port that supplies power is polled for 50 ms every 600 ms to
ensure there is still a device attached. This ensures that power is turned off
at the port if the device that required it has been