" An IEEE compliant switch will allocate 7 watts for Class-2 phones and 15.4 watts for Class-3 phones from any manufacturer. Cisco switches utilize the Cisco Discovery Protocol (CDP) V2.0 Power Negotiation Module to identify the actual power required by a Cisco device, e.g. a Cisco IP Phone. For example, a Cisco phone that is classified as Class-3 in the phone hardware actually needs 7.5 watts. A non Cisco switch will budget the full 15.4 watts for this phone and switch port, essentially wasting 7.9 watts of power. With a Cisco phone/switch solution, runnig CDP V2.0, the extra 7.9 watts of power is released back to the switch power budget for use with other switch ports, thus raising the switch power utilization."
Different models of Cisco phones have different power requirements. Have a look;
The 7940G/7941G and 7960G/7961G are class 2 devices (7.7 watts) and the 7941G-GE/7961G-GE phones are class 3 devices (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.
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.**
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