Need assistance with Switch and Phone compatibility

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Mar 6th, 2008
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We are looking to purchase a complete cisco phone system. We are getting

hung up on the compatibility of the phones. Two vendors are saying two different things

and both declare they are correct.


We have nearly 20 WS-C3750G-48PS switches. We would like to purchase the 7965G and 7945G

phones and use PoE for them.


One of our vendors is quoting the 7961G and 7941G. They are stating the the 7965G and the

7945G are class 3 power devices (which is true) compared to the 79X1 devices they are

quoting which are class 2 (also true).


The issue is whether or not the Switches will support the power requirements of the 7945G

and 7965G devices.


We require 1Gig connection to the desktop and know that the 7941G and 61G do not support

it. Only the 7941G-GE and the 7961G-GE meet the Gig requirement, but they too are class 3

power devices.


So, can you help shed any light on this puzzle


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Rob Huffman Thu, 03/06/2008 - 18:03
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Hi Tom,


Great question :) The problem here is that the 48 Port model WS-C3750G-48PS cannot provide 15.4W per port Class 3 power on all 48 ports. The 24 Port model can. It can however provide 7.7W per port Class 2 power on all 48 ports so this is probably why the one company is proposing the 7941's and 7961's.


Have a look under the "key features" in this doc;

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_brochure0900aecd803ca7c6.pdf


Q. How does Power Management work in the Cisco Catalyst 3750 PoE switches?


A. The Cisco Catalyst 3750-24PS switch supports a maximum of 15.4W per port of -48V DC power on all 24 ports over standard Category 3 and 5 unshielded twisted-pair (UTP) cable up to 100 meters, however all 48 10/100 ports in Cisco Catalyst 3750-48PS switch will be able to deliver 7.7W simultaneously and any given port can be configurable to support up to the maximum of 15.4 watts as long as the switch's - 48V power capacity is not exceeded. The Cisco Catalyst 3750 PoE switches monitor and track the requests for power, and only grants power when it is available. If the connected PD draws more power, which results in exceeding the system power budget that is available on the switch, then power will be denied to that port and the PD will not power up. If power is denied, the switch re-checks the power budget periodically (every 15 seconds), and continues to attempt to grant the request for power. If power is granted, the switch updates the power budget.


From this good doc;


http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_qas09186a00801b0971.html


Hope this helps!

Rob

cafetom Fri, 03/07/2008 - 05:44
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Thanks a bunch. I understand enough now.


The chance of us ever using every port on a 48 port switch with a 15.4 phone device is low. 370 watts is the key as you say, just about exactly 24 class 3 devices.


Just curious though, why make a 48 port devices that won't support its maximum load of class 3? I suppose that is in the next versions. :)


Thanks again.

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