A Practical Guide to Power over Ethernet (PoE)
1 Introduction
Voice over Internet Protocol (VoIP) is the convergence of traditional voice onto an IP data network to
provide better application integration by using common protocols. This lowers costs by using one network
infrastructure and even melding separate support staffs into one. Other real-time traffic streams, such as
uncompressed video and streaming audio are also converging onto data networks.
Convergence began with basic VoIP and, over time, has expanded traditional features and functions
including powering IP endpoints. The idea to send power through cables to an end device is not new.
Home analog telephones have been powered from the Central Office (CO) for scores of years. PBX type
switches have also provided power to analog and digital phones in business offices for decades. The
evolution to send power over a Cat-5 type cable is then a natural extension of an existing idea applied to a
new platform – the data network.
It is interesting that the IEEE eventually based their standard on the Avaya (Lucent Technologies at that
time) PoE scheme. This means that all Avaya products have been standards compliant even before the
standard was ratified. This also means that proprietary implementations of PoE, like Cisco, had to convert
from their vendor specific methods to the standards-based protocol. Older Cisco chassis-based switch
blades required daughter cards or total blade replacement to provide compliant power to the newer
standard. Adhering to the standard, titled
IEEE 802.3af
, is important because it allows power
interoperability from any vendor’s products. You can have confidence in Avaya’s products knowing that
Avaya led the IEEE standards committee with the ultimate solution for Powering devices over the Ethernet
cable. No one implements Power over Ethernet (PoE) better than Avaya. Avaya can intelligently and
dynamically apply the right amount of power to ANY vendor’s products that are standards compliant.
Please see section 5 of this paper for details.
2 Definitions
Before technologies are discussed, some important definitions must be learned.
End-Span
. An End-Span, or Endpoint PSE is an Ethernet switch that is capable of sending Power over
Ethernet up to 100 meters over copper twisted pair to an endpoint device like an IP telephone.
Mid-Span
. A device that lies between the Ethernet switch (or hub) and injects Power through the Ethernet
(copper) cable to the endpoint device. A Mid Span is not an access device like a switch or a hub. Its only
purpose is to inject power from the middle of the link between an access device (switch) and the endpoint –
hence the name “Mid” span.
Ohm
. A measure of resistance using the Omega symbol -
Ω
PD
– Powered Device. A powered device is an endpoint that requires power. Many devices can
accommodate power in more than one way such as a local transformer or Power over Ethernet. Examples
of devices include IP telephones, Access Points, Web Cameras, magnetic card readers, etc.
PoE
– Power over Ethernet is any scheme, either proprietary or standards-based, that defines how to send
power through an Ethernet cable.
PSE
– Power Sourcing Equipment. A PSE is a device that sends Power over Ethernet to the PD. End-
Span and Mid-Span provide PoE, but there can also be other devices that send power to PDs.
Watt
. A measure of power derived by multiplying current (Amperes) with resistance (Ohms). One Watt
equals one Ampere across one Ohm’s worth of resistance.
MJK
Copyright ©
2006 Avaya Inc. All Rights Reserved.
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