Power requirement of 7961G?

[scott_brown]

Hello,

I am trying to calculate the power requirement of a 7961G so I know what is the maximum amount of handsets I can plug into a 3750-48.

The 3750 can handle 24 ports at 15.4watts or 48 ports at 7.7watts. So how do I know what the 7961G's use? Is this power use consistent or does it change depending on the state of the phone (at idle, in use, on a call etc)?

I'd hate to think I just spent tens of thousands$$$ on new 48 port switches but can't use all 48 ports.

[jbarcena]

Scott,

Unfortunately the 7961 is a Class 3 IEEE device, it needs 15.4 watts, so the switch provides the minimum power it needs which is 15.4 Watts.

[scott_brown]

Ok I'm in trouble then :)

So what handsets use 7.7 watts? Or where can I find this info? I'm surprised that this wasn't made clear when we bought the kit .

[thisisshanky]

802.3af standard allocates 15.4 watts per port by default (at startup) and then through CDP the phone tells the switch how much it actually needs, so the remaining power is added back to the pool. So if the phone requires 6.3 or 7 watts, the difference of 15.4 - 6.3 (7) is added back to the pool. The downside of plugging 48 phones into a 3750-48 is that, all phones wont come up right away. Half the phones (24) may come up first and once the negotiation happens and the balance of (15.4 - 6.3 (7)) is added back to the pool, then the remaining phones will get power. If you are using 7961s on all of them, then you will be able to power only half the number of ports. The best bet is to start using two 3750-24s.

HTH

Sankar

PS: please remember to rate posts!

[scott_brown]

So how do I tell if I need 6.3 or 7 or even 15.4 watts? Is this written somewhere? I've looked around and found plenty of references to 48V but no reference to wattage?

[thisisshanky]

Scott,

Here is a link that says how much power the 7960,7940 and 7910 consume.

http://www.cisco.com/en/US/products/hw/switches/ps708/products_installation_guide_chapter09186a008020e088.html

[scott_brown]

Cool, 6.3watts for a 7960, but is this the same for a 7961? The replies here imply that a 7961 uses 15.4w (which seems to be quite a jump). Are the 7961 details listed anywhere officially? (I've looked but find anything).

[thisisshanky]

Scott,

This is what i see from a sh cdp nei detail on a 3750 switch where a 7961 is plugged in. It seems that it draws only 6.3 W for operation. Initially the switch allocates 15.4 watts as its an 802.3 af compatible device. Then it drops down the power to 6.3 after the phone negotiates how much power it reqires. Here is a sample cdp output.

Host#sh cdp nei det

-------------------------

Device ID: SEP0019567E8B0D

Entry address(es):

IP address: 10.50.1.195

Platform: Cisco IP Phone 7961, Capabilities: Host Phone

Interface: FastEthernet1/0/24, Port ID (outgoing port): Port 1

Holdtime : 125 sec

Version :

TERM41.7-0-3-0S

advertisement version: 2

Duplex: full

Power drawn: 6.300 Watts

Power request id: 35597, Power management id: 3

Power request levels are:6300 0 0 0 0

Management address(es):

[f.mottier · Accepted solution]

Hi,

Check out Ciscos PowerCalculator

http://tools.cisco.com/cpc/

Cheers,

Fred

[scott_brown]

Exactly what I was after. Thanks to all that contributed.

[Rob Huffman]

Hi Scott,

We really tossed this question around when we were doing our last purchase. Here are the docs I found;

The 7961G are a class 2 device (7.7 watts) and the 7961G-GE phones are a class 3 device (15.4 watts).These are not configurable settings.

Each Cisco Catalyst 3750-24PS or Cisco Catalyst 3750-48PS switch can optionally detect the PD Power Classification signature and budget the appropriate power. This reduces the maximum power that must be budgeted by the switch and provisioned in the wiring closet. The Cisco Catalyst 3750 PoE switches have **370W**(this is enough for 48 class 2 devices @ 7.7 watts 48x7.7 = 369.6 watts) available for Power over Ethernet, which is shared by all 24/48 ports. The switch allocates the maximum power a PD needs.

Have a look;

Cisco Unified IP Phone 7961G

Power Requirements The phone is interoperable with Cisco Pre-standard PoE and with IEEE 802.3af PoE (the phone is a Class 2 device); 48 VDC is required; it can be supplied locally at the desktop using an optional AC-to-DC power supply (part number CP-PWR-CUBE-3=).

From this doc;

http://www.cisco.com/en/US/products/hw/phones/ps379/products_data_sheet0900aecd802ff020.html

Cisco Unified IP Phone 7961G-GE

Power Requirements The phone supports the IEEE 802.3af PoE (the phone is a Class 3 device) standard; 48 VDC is required; it can be supplied locally at the desktop using an AC-to-DC power supply (part number CP-PWR-CUBE-3=).

The Cisco Unified IP Phone 7961G-GE supports IEEE 803.af Power over Ethernet (PoE). It also can be powered locally with a power supply (part number CP-PWR-CUBE-3). **Note: The Cisco Unified IP Phone 7961G-GE does not support Cisco Pre-standard PoE.**

From this doc;

http://www.cisco.com/en/US/products/hw/phones/ps379/products_data_sheet0900aecd802ff030.html

Hope this helps!

Rob

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