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`Cisco Unified IP Phones: Conserve Energy with Intelligent
`Power Allocation
`
`When evaluating unified communications options, organizations are increasingly
`considering the environmental effect of these solutions. Communications equipment that
`uses the Power over Ethernet (PoE) standard often allocate more energy than necessary
`for two reasons: First, the power categories defined by IEEE 802.3 PoE are very coarse
`and often result in the overestimation of power requirement; and second, the IEEE 802.3
`PoE does not allow changes to the allocated power levels, so equipment must allocate
`based on maximum power usage rather than actual power requirements. The Cisco®
`Discovery Protocol allows PoE devices such as Cisco Unified IP Phones to intelligently
`collect power-consumption information and communicates that information to the Cisco
`switches supplying the power.
`
`This paper explains how switch power allocation, handled through Cisco Discovery Protocol,
`allows an organization’s systems to efficiently manage power budget for Cisco Unified IP Phones
`across switch ports. You will learn more about power consumption in a Cisco Unified
`Communications environment and how with its market-leading, feature-rich line of IP phones,
`Cisco continues to be a leader in providing solutions for fixed and mobile workspaces that are
`optimized for efficient power consumption.
`
`Power over Ethernet Classifications
`
`In March 2000, well ahead of the Power over Ethernet industry standards, Cisco released the first
`set of switches and phones that used PoE. In June 2003 the IEEE ratified the 802.3af
`specification, which defined the industrywide standard for device detection and delivery of PoE.
`Table 1 gives the classifications of the 802.3af standard.
`
`Table 1.
`
`Standard 802.3af Classifications
`
`Standard 802.3af Class
`
`Maximum Power (W)
`
`Class Description
`
`3
`
`2
`
`1
`
`0
`
`
`
`15.40
`
`7.00
`
`4.00
`
`15.40
`
`Up to full power
`
`Medium power
`
`Low power
`
`Default
`
`The classification process uses a look-up table to assign a class (0–3) based on the direct current
`measurement.
`
`However, the 802.3af classifications are inflexible because peak power usage is based on
`classification instead of negotiation. After initial device power-up, power requirements cannot be
`adjusted. Consider a device that has a Class 3 rating. The power sourcing equipment (PSE) will
`allocate 15.4W of power, even though the device may have a peak consumption of 7.2W of power,
`representing an overbudgeting of twice the amount of power actually needed. This overallocation
`would result in the PSE supporting significantly fewer devices than it has available power capacity
`
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`for. In this example, if the Cisco Unified IP Phone could indicate the peak power usage after the
`802.3af negotiation, a Cisco switch or router would be able to save 50 percent of the power
`allocated for a port when connected to a Cisco Unified IP Phone.
`
`Cisco Discovery Protocol Power Negotiation
`
`When deploying only 30 to 40 IP phones, the overbudgeting of the power requirements based on
`the class of a device may or may not be significant. But when considering a deployment of 10,000
`phones, the standard PoE power classification may reduce the total number of devices that could
`be supported on a switch or other power-sourcing device. To more effectively manage power
`budgeting for Cisco Unified IP Phones when connected to a Cisco switch, the Cisco Discovery
`Protocol is used to communicate a more accurate power-requirement value to the connected
`switch. In fact, select Cisco switches can deliver power to other switches, wireless access points,
`IP cameras, PoE speakers, and PoE digital clocks. The use of Cisco Discovery Protocol does not
`affect the 802.3af discovery process, but instead downwardly adjusts the amount of power
`allocated on the switch port so that additional devices can be supported.
`
`For example, the Cisco Unified IP Phone 7911G is an 802.3af Class 2 device. At their peak power
`level, Class 2 devices may draw 7.0W of power from the PSE. But after the Cisco Unified IP
`Phone 7911G powers up and is classified as a Class 2 device, it exchanges Cisco Discovery
`Protocol with a Cisco switch and the phone reports a maximum peak power of 5.0W. This change
`represents an immediate 29-percent adjustment in the PSE power budget. When measuring the
`actual power consumption of the Cisco Unified IP Phone 7911G after it has registered with a call-
`processing server (using the factory defaults), the phone consumes only 2.3W of power. This
`amount represents a 54-percent difference between the Cisco Discovery Protocol reported value
`and the actual usage, and a 67-percent difference from the 802.3af classification.
`
`Figure 1. Cisco Discovery Protocol for Cisco Unified IP Phones
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`The images in Figure 1 represent the maximum power that these devices theoretically consume,
`with a safety margin added. Cisco took a very conservative approach in deriving the power values
`used in Cisco Discovery Protocol to help ensure maximum reliability. For instance, if the Cisco
`Discovery Protocol power value was set to the calculated or observed peak power level by the
`phone, then any slight deviation could cause a power overdraw on the switch port. When there is a
`power draw over the allocated amount, the port shuts down and therefore removes power to the
`device, resulting in the phones being rendered unavailable for use. In reality, a phone consumes
`significantly less power than that specified in the Cisco Discovery Protocol value (Table 2).
`
`Table 2.
`
`Cisco Unified IP Phone Power Consumption
`
`Cisco Unified IP
`Phone Model
`
`Standard 802.3AF Classification Cisco Discovery
`Protocol (W)
`
`Idle (W)
`
`Maximum Power
`Consumption (W)
`
`7911G
`
`7.00W (Class 2)
`
`7941G and 7961G
`
`7.00W (Class 2)
`
`7942G and 7962G
`
`7.00W (Class 2)
`
`7965G
`
`7970G
`
`15.40W (Class 3)
`
`15.40W (Class 3)
`
`7971G-GE
`
`15.40W (Class 3)
`
`7975G
`
`15.40W (Class 3)
`
`5.00
`
`6.30
`
`6.30
`
`12.00
`
`10.25
`
`14.90
`
`12.00
`
`2.30
`
`3.84
`
`3.84
`
`6.53
`
`6.28
`
`9.17
`
`5.32
`
`3.71
`
`4.95
`
`4.76
`
`9.31
`
`8.11
`
`12.28
`
`8.30
`
`Real-Life Deployments and Cisco IP Phone Power Usage
`
`Precisely how much power a Cisco Unified IP Phone consumes depends on both the phone model
`and individual user settings. Phone model and features and user preferences that can increase
`power consumption on a Cisco Unified IP Phone include the following: screen type (color versus
`grayscale), Gigabit Ethernet line speed, ringer and speakerphone volume, and illuminated keys
`(such as Message Waiting light and line buttons).
`
`The amount of power that a Cisco Unified IP Phone uses daily is reflected predominantly in the
`idle power usage of the phone. Although ringing temporarily increases power usage, it does not
`statistically affect the overall power consumption of the phone over the course of the day. A few of
`the Cisco Unified IP Phones experience a slight power usage increase during an active call, but
`the active call and ringing events add less than 1 percent to the idle power usage, minimally
`affecting any calculations of daily power usage for the phone.
`
`Although users configure and interact with their phones differently, the overall power usage during
`the day remains relatively flat. Even with the usage patterns in Figure 2, the total power draw,
`including ringing and talk time, has less than a 1-percent deviation from idle. This daily usage
`calculation assumes six incoming calls and talk time of 3.5 hours. If calculating the power
`consumption based on the 802.3af standard, then a Cisco Unified IP Phone 7975G consumes 370
`watt-hours per day. If calculating the power consumption based on the Cisco Discovery Protocol
`power allocation, then that same Cisco Unified IP Phone 7975G consumes 288 watt-hours per
`day. But looking at the empirical power consumption in this example, a Cisco Unified IP Phone
`7975G consumes only 127 watt-hours during the day.
`
`Figure 2. Actual Cisco IP Phone Power Usage
`
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`© 2008 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information.
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`Power over Ethernet in the Future
`
`With phone users requiring additional connectivity from their communications device, in the future
`IP phones will probably ship with universal-serial-bus (USB) and Bluetooth capability. Interestingly,
`some of these new connection options will actually increase power requirements of these
`Ethernet-powered devices. But because those add-on devices may not be needed all the time, a
`mechanism is needed to dynamically adjust the power budget at the switch. The IEEE is working
`on a new PoE standard that will allow an Ethernet-powered device to dynamically adjust its power
`requirement. Currently, the IEEE 802.3at Task Force is working on defining this standard, which
`increases the power drawn by a device up to 30W and includes the ability for a device to
`dynamically negotiate power after the initial detection.
`
`But the arrival of the 802.3at standard is still a few years away, and most PoE manufacturers and
`customers are looking for a more immediate solution to power budgeting.
`
`One way to accomplish power savings is to use a management application to deny power to a
`connected Ethernet port. In environments where there is no phone use for extended periods of
`time (for example, from 8 p.m. until 7 a.m. and weekends), the use of Cisco switches and the
`Cisco IOS® Embedded Event Management (EEM) system will allow ports to deny power to Cisco
`Unified IP Phones (the use of EEM to deny power will render IP phones unusable). The EEM
`scripts are activated at a predefined time, typically after all users have left for the day. At that point
`the script is executed, and it identifies all inline-powered line cards and shuts down inline power for
`all ports on that module. Similarly, at a later predefined time, typically before users return to the
`office, the script reactivates inline power for all of those ports. This script is designed to offer those
`sites that do not have 24-hour high-availability requirements a means to conserve power during
`those times when use of inline powered devices is not required. The use of the script in this
`manner can lead to significant power savings.
`
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`The other method for reducing power consumption is to have the device enter a sleep or deep-
`sleep mode. The 802.3af specification requires the device to draw a minimum amount of power to
`keep the connection active. So if a device has been idle for a certain period of time, the device
`may enter a sleep mode and reduce power consumption to a minimal level to merely keep the
`network interface active while still providing complete services as needed by the user. However,
`this solution also would be practical only if the IP phone had “instant-on” capability so it could be
`used within 250 milliseconds of being awakened from the sleep mode. This example of network-
`based intelligence being used to facilitate changes in switch sleep modes and control networkwide
`IP phone state exemplifies the benefits of a Cisco on Cisco solution where Cisco Unified
`Communications interacts with Cisco infrastructure.
`
`Summary
`
`As corporations start looking at reducing power in their enterprise, Cisco Unified Communications
`Solutions, including Cisco Unified IP Phones, are an optimal choice for this environmental and
`business transformation. As a participating member of the IEEE 802.3af and 802.3at Task Forces,
`Cisco has been helping define the power negotiation standard for the PoE industry. At the same
`time, Cisco is working on reducing the power usage of its best-in-class IP phones. The newer
`versions in the Cisco Unified IP Phone portfolio include devices that offer more features while
`reducing overall power consumption by 41 percent (when comparing a Cisco Unified IP Phone
`7971G-GE to the Cisco Unified IP Phone 7975G factory default power-consumption values).
`
`One important point to consider when comparing environmental friendliness of unified
`communications solutions and IP phones is that the published numbers for Cisco Discovery
`Protocol are very conservative and intended to represent the maximum possible power draw with
`an added buffer to prevent overcurrent that may shut the power on the port off. Other factors
`including line loss have also already been calculated into the Cisco Discovery Protocol value.
`
`Cisco Unified IP Phones are part of a network-centric unified communications architecture that is
`open, inclusive, and extensible. Cisco Unified Communications Solutions enable collaboration
`every time, everywhere, and with everyone included.
`
`Additional References
`
`● IEEE 802.3af standard: http://www.ieee802.org/3/af/
`
`● Wikipedia Power over Ethernet: http://en.wikipedia.org/wiki/Power_over_ethernet
`
`● Cisco Integrated Services Router: Reduce Power Consumption Through Integrated
`Services Delivery:
`http://www.cisco.com/en/US/solutions/collateral/ns340/ns517/ns477/net_implementation_w
`hite_paper0900aecd80716af6_ns296_Networking_Solutions_White_Paper.html
`
`● Cisco Power over Ethernet solutions:
`http://www.cisco.com/en/US/netsol/ns340/ns394/ns147/ns412/networking_solutions_packa
`ge.html
`
`● Cisco IOS Embedded Event Management Port Power Script
`http://forums.cisco.com/eforum/servlet/EEM?page=eem&fn=script&scriptId=1041
`
`
`
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`Printed in USA
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`© 2008 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information.
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`C11-481292-00 06/08
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