POE IP Phones

[Sivanesan Rathinam]

Hi All,

     I have few clarifications on POE, Actually cisco switches will send discovery signal to detected whether powered devices is connected or not.

     1. Discover signal refers FLP here?

      2. Incase of Inline CDP use to determine actual power requ of phone, Incase of 802.3 af it uses power classification at the detection stage?

      can i know the mechanism where 802.3af used to classify the power according to Powered device.

SIVANESAN R       

[Leo Laohoo]

I have few clarifications on POE, Actually cisco switches will send discovery signal to detected whether powered devices is connected or not.

Ok, here's how I see it:

Let's say you are a PoE switch.  You plug a device into the port.  Straight up.  How will the PoE switch know if the end-device requires PoE?  By default, the switch will provide 15.4w power on every port.  This allows the end-device to power up.   Whilst the end-device is powering up, the Cisco switch sends out CDP at regular intervals to determine if the end-user talks CDP or not.  CDP is important because it will enable the switch to regulate the power provided.

If the end-device can't talk CDP then Cisco switch will push the default of 15.4w of power whether or not the end-device requires that much or not. 

[Sivanesan Rathinam]

No, How come switch knows connected device require POE or not.

802.3 af it uses power classification at the detection stage? can i know how/what mechanism used for power classification?

SIVANESAN R

[Leo Laohoo]

How come switch knows connected device require POE or not.

The switch doesn't know.  By default, the switch powers up the port.

[Aman Soi · Accepted solution]

Hi Siva,

You can refer this URL for 802.3af working.it works on DC power detetction method in which DC current is applied and detects the presence of a powered device by measuring the  load applied by powered device.

http://www.cisco.com/warp/public/cc/so/neso/bbssp/poeie_wp.pdf

regds,

aman

[Sivanesan Rathinam]

Hi Aman  Here i have another question Cisco prestandard will send 15.4 W by default right?

Suppose let say i have configured the port max power of 10W? How abt now? It eill send 15.4 or 10 W initially before determine actual power from CDP?

SIVANESAN R

[Leo Laohoo · Accepted solution]

Here's another document:  Cisco Prestandard Power over Ethernet Implementation

[Sivanesan Rathinam]

1. If the PD and PSE supoort both cisco pre-std and 802.3AF. At this time it is possible to find wherheter PD is powered up using Pre-std or 802.3AF?

2. Low Pass Filter used by Cisco IP Phones in Pre-std mentod to loop back FLP Signal and incase of 802.3 AF cisco uses Assert Load current to classify the power? Suppose lets say CISCO IP Phones which supports both std will have Low pass filter and Assert Load current?

SIVANESAN R

[Leo Laohoo · Accepted solution]

At this time it is possible to find wherheter PD is powered up using Pre-std or 802.3AF?

The output to the command "sh power inline detail" will.  See below:

Switch#sh power in Gi1/0/9 det

Interface: Gi1/0/9

Inline Power Mode: auto

Operational status: on

Device Detected: no

Device Type: Cisco IP Phone 7911

IEEE Class: 2

Discovery mechanism used/configured: Unknown

Police: off

Power Allocated

Admin Value: 30.0

Power drawn from the source: 5.0

Power available to the device: 5.0

Actual consumption

Measured at the port: 2.7

Maximum Power drawn by the device since powered on: 2.9

Absent Counter: 0

Over Current Counter: 0

Short Current Counter: 0

Invalid Signature Counter: 0

Power Denied Counter: 0

Power Negotiation Used: CDP

LLDP Power Negotiation --Sent to PD--      --Rcvd from PD--

Power Type:          -                    -

Power Source:        -                    -

Power Priority:      -                    -

Requested Power(W):  -                    -

Allocated Power(W):  -                    -

Switch#sh power in Gi1/0/30 det

Interface: Gi1/0/30

Inline Power Mode: auto

Operational status: on

Device Detected: no

Device Type: Cisco IP Phone 7937

IEEE Class: 3

Discovery mechanism used/configured: Unknown

Police: off

Power Allocated

Admin Value: 30.0

Power drawn from the source: 10.5

Power available to the device: 10.5

Actual consumption

Measured at the port: 4.2

Maximum Power drawn by the device since powered on: 5.6

[Sivanesan Rathinam]

Thanks Leo, I read few documents where it says below statements, I will check out in my lab today and update you.

If a PD supports both IEEE 802.3af and Cisco pre-standard, the PD is detected as an IEEE device.

Low Pass Filter used by Cisco IP Phones in Pre-std mentod to loop back FLP Signal and incase of 802.3 AF cisco uses Assert Load current to classify the power? Suppose lets say CISCO IP Phones which supports both std will have Low pass filter and Assert Load current?

SIVANESAN R

[Leo Laohoo]

If a PD supports both IEEE 802.3af and Cisco pre-standard, the PD is detected as an IEEE device.

Firstly, let me be honest to admit that I'm not a "PoE expert".  I don't understand electrical engineering.  Let's just say I had a traumatic experience when I was a child.

Let me say that the statement you've posted is not complete.

When a PD is initially plugged in, how does a Cisco know if it supports 802.11af, pre-standard or not?  The Cisco switch doesn't.  So, by using the concept of "benefit of the doubt", by default all PoE ports pushes power.  The philosophy is to get the PD to boot up and interrogate what language it speaks.

During this stage, the bootup of the PD, Cisco switches will detect the device as "IEEE".  Only when the device starts to talk CDP or LLDP does the state changes and the PD is categorizes to the different PoE "classes".

Have you heard of the new technology called Universal PoE (uPoE)?