Campus Design

In line power CDP discovery

Cisco Products Currently Available for Supplying In-Line Power

The first product is the WS-X6348 48 port 10/100 line module for the Catalyst 6000 series switches.

The second product is the WS-X4148-RJ45V 48 port 10/100 line module for the Catalyst 4006 switch.

(The Catalyst 4006 switch operates identically to the Catalyst 6000 switch, in respect to in-line power detection and delivery.)

The third product is the Catalyst 3524-PWR-XL (WS-C3524-PWR) switch, which is based on the Catalyst 3524XL switch.

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", meaning that 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-X6348 supplies in-line power via the "used" Ethernet pairs (pins 1, 2, 3, 6). Catalyst 6000 Switch
The WS-X4148-RJ45V supplies in-line power via the "used" Ethernet pairs (pins 1, 2, 3, 6). Catalyst 4006 Switch
The WS-C3524-PWR supplies in-line power via the "used" Ethernet pairs (pins 1, 2, 3, 6). Catalyst 3524-PWR-XL Switch
The WS-PWR-PANEL supplies power using the "unused" pairs (pins 4, 5, 7, 8). In-Line Power Patch Panel (WS-PWR-PANEL)

Detecting that an IP Phone is Connected to a 10/100 Ethernet Port

All the above listed products rely on a phone-discovery algorithm before supplying power to a phone. This algorithm ensures that the switch does not supply power to a device which is not capable of accepting in-line power. The phone-discovery algorithm used by Catalyst switches is different than the algorithm used by the WS-PWR-PANEL.

The following method for detecting that an IP Phone is connected to a 10/100 Ethernet port is used by the Catalyst 6000, Catalyst 4000, and Catalyst 3524-PWR-XL Switches.

1. 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.

2. 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.

3. If a 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 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.

4. Now that the Catalyst switch has determined that it needs to power the port (the special FLP signal was received back from the attached 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 IP Phone will need, it uses the configured default power allocation. Later on it will adjust this allocation based on what the attached IP Phone tells the switch it really needs.

5. The port then provides power to the IP Phone over pairs 1 and 2 as a common mode current.

6. The port is taken out of phone-discovery mode and changed to normal 10/100 Ethernet auto-negotiation mode.

7. The instant that the switch applies power to the port, the relay inside the phone opens and power begins to flow to the IP Phone.

8. At this point a 'wait for link' timer in the switch starts as well. The phone has five seconds to establish link integrity on its Ethernet port. If the switch doesn't 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.

9. If the switch detects a link within the five second window, it will continue to supply power to the IP Phone until it detects a link down event.

10. 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 accordingly.

In-Line Power Modes for Catalyst Switches

auto

Phone-discovery algorithm is operational Catalyst 4006, 6000 and 3500XL

off

Phone-discovery algorithm is disabled Catalyst 4006 and 6000

never

Phone-discovery algorithm is disabled Catalyst 3500XL

In-Line Power Patch Panel

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 79xx IP Phone). The bottom two rows are used to connect to the Switch which will be providing the Ethernet connectivity.

Internally, the IPPP will directly connect the Ethernet pairs from the bottom switch port to the corresponding 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 completely passive.

The phone discovery algorithm for the IPPP is similar to the method used on 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 IP Phones. If an IP Phone is detected, these pins (wire pairs) are also used to provide power.

The following method for detecting that an IP Phone is connected to a 10/100 Ethernet port is used by the The In-line Power Patch Panel (WS-PWR-PANEL).

1. The IPPP starts the phone discovery sequence at port 1.

2. The IPPP sends a 347kHz loopback tone out port 1. The IPPP listens for 50ms to to determine if the loopback tone is being forwarded back by a device that is connected to the port. Only devices that are expecting to receive power on these pins will forward the loopback tone to the sending device (the IPPP in this case). The IPPP must detect 16 transitions within a 50ms period of time to verify that it is sensing the correct loop back tone and not an anomaly.

3. 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.

4. The IPPP continuously cycles through the ports repeating the above steps for each port.

5. Each port that is supplying power is polled for 50ms every 600ms 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 disconnected.

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