Solved: What PoE cable pair does Cisco use?

donjohnson911 · ‎06-07-2007

Hello again,

I found yet another reason to be nervous about plugging my 7970 into this 3rd party PoE injector. I think someone mentioned this before, but I found out that PoE has a "mode A" and a "mode B" A using the pairs 1,2 and 3,6 and B using 4,5 and 7,8.

Anyone know which pair the IP phones use? I can see on my injector that the type provided is "mode B" if the phone is "mode A" is there any way to make some kind of custom crossover that will do the trick?

Thanks!

Rob Huffman · ‎06-07-2007

Hey Don,

Better safe than sorry when it comes to expensive phones :)

This does state that the phone will power up in either mode.

Take care,

Rob

View solution in original post

mchandak · ‎06-07-2007

Cabling always remains the same for either of the two Inline Power Mode. The cable you would use would be a regular Straight throught cable with no special considerations. The mode only affects the way the PoE capable device gets power and not the cabling. As long as you have a Straight through cable with all the 8 pins connected at both the ends, you should be good to use the same cable for Mode A or Mode B.

Hope this helps

donjohnson911 · ‎06-07-2007

Right, but what I'm wondering is what power mode the IP phone uses. My 3rd party injector is a mode B if the phone is a mode A does that just make me S.O.L. ?

mchandak · ‎06-07-2007

IP Phones support both modes. Check http://www.cisco.com/en/US/customer/products/hw/phones/ps379/products_qanda_item09186a00801c963a.shtml for more information on power recommendation for 7970 IP Phone.

donjohnson911 · ‎06-07-2007

I think you must be seeing something that I'm not, hah, I looked on that site and it says it supports the local power or Cisco inline power but doesn't seem to mention what inline power mode it accepts.

Did a search for the phrase "inline power" and each of the 5 or so times it hit there wasn't added information about the inline power mode.

Rob Huffman · ‎06-07-2007

Hi Don,

Just to add to Mahesh's great info, I wish I had found this and sent it to you sooner;

The IEEE 802.3af-2003 Power over Ethernet standard defines terminology to describe a port that acts as a power source (PSE) to a powered device (PD), defines how a powered device is detected and also defines two (2) methods of delivering Power over Ethernet to the discovered powered device. IEEE 802.3af power may be delivered using a Power over Ethernet capable Ethernet port, which is referred to as an End-Point PSE or by a mid-span PSE that can be used to deliver Power over Ethernet in the event an existing non-Power over Ethernet capable Ethernet switch is used.

The IEEE 802.3af standard states that power may be delivered by an end-point PSE, using either the active data wires of an Ethernet port or the spare wires, to a powered device. An end-point PSE, such as a Power over Ethernet capable Ethernet switch, may implement either scheme. If a mid-span PSE is used, then the mid-span PSE can only implement power delivery over the spare pairs of the copper cabling and cannot be used to deliver Power over Ethernet over 1000BASE-T connections. It should be noted that even if a device supports both methods of providing power, only one mechanism may be used to deliver power to a powered device.

The first mechanism, which is supported within Cisco Catalyst Ethernet switches, is to use the data pairs (pins 1,2 & 3,6) to transmit power, which is sometimes referred to as "phantom" power. The second power delivery mechanism is to use the unused, from a 10/100BASE-T perspective, pairs (pins 4,5 &7,8) to deliver power that is supported within mid-span power delivery.

From this excellent doc;

http://www.cisco.com/en/US/netsol/ns340/ns394/ns147/ns412/networking_solutions_white_paper09186a008026641c.shtml

Hope this helps!

Rob

donjohnson911 · ‎06-07-2007

Forgive me one last time, but this basically states that the phone will power up under either mode?

Rob Huffman · ‎06-07-2007

Hey Don,

Better safe than sorry when it comes to expensive phones :)

This does state that the phone will power up in either mode.

Take care,

Rob