What About Ability over Ethernet?
An Ethernet about-face can accommodate electrical ability to absorbed stations with the advice of Power
over Ethernet (PoE). Although PoE seems like a baby feature, addition can advance it to
either get chargeless ability or abjure services. To abate the appulse of attacks, you can configure
switches.
Introduction to PoE
Before the IEEE standard, Cisco provided a way1 to ability a accessory through the RJ-45
connector and its associated Category 5 (CAT5) cable. Since 2003, the IEEE 802.3af2
standard specifies the aforementioned affection but in a altered way.
The capital action abaft PoE is to abridge the cabling of Ethernet devices. If the
device’s ability burning is beneath than 15.4 Watts (W), the Ethernet about-face can provide
the electrical power; there’s no charge for the accessory to accept an added power-supply cord
and ability supply.
Although the aboriginal claim was for IP telephony, the branch of PoE now
encompasses wireless admission credibility (AP) and video surveillance. Indeed, the closing devices
are generally placed in a location, such as the top of a bank or ceiling, area putting an electrical
cord is not accessible (especially for a 110–220 Volt [V] cable). These accessories are additionally relatively
small ability consumers.
NOTE A accustomed anthology computer requires added than 400 W, so there’s little achievement of accepting a
notebook get its ability through Ethernet (even if at atomic one British supplier claims to have
built an ultra-low ability burning computer of 15 W). But, absolutely as it has been seen
with accepted consecutive bus (USB)-powered devices, you can apprehend to get baby gizmos, fans,
and others, powered by Ethernet plugs.
136 Chapter 8: What About Ability over Ethernet?
How PoE Works
Both Cisco prestandard and IEEE 802.3af PoE assignment in the aforementioned way:
• Apprehension mechanism. Checks whether the affiliated accessory requires electrical
power
• Powering mechanism. Transmits the electrical ability to the affiliated device
Figure 8-1 represents the archetypal agreement of PoE. Within the Ethernet switch, the
power bartering accessories (PSE), food ability to a powered accessory (PD) that’s located
within the powered end base (PES).
Figure 8-1 PoE Architecture
Detection Mechanism
The Cisco prestandard accomplishing of the apprehension apparatus differs from the IEEE
802.3af:
• Cisco prestandard. Injects an alternating accepted (AC) arresting on one brace of the CAT5
cable and checks whether the PES alternate this accepted on addition pair
• IEEE 802.3af. Applies a absolute accepted (DC) voltage amid two pairs of the CAT5
cable and checks whether some accepted flows
Figure 8-2 shows the Cisco prestandard apprehension mechanism. A fast articulation beating (FLP), such
as a low-frequency, low-intensity AC, is injected on the CAT5 cable’s address pair. If the
station is a powered device, it has a low-pass clarify that allows this FLP to appear aback on the
receive brace to the apprehension mechanism.
A low-pass clarify is required, or abroad the high-frequency arresting of Ethernet frames passes
back and alternating amid the transmitting and accepting pairs. This crosstalk interferes with
normal Ethernet traffic.
Switch PES
Twisted-Pair
Cable
PSE PD
Introduction to PoE 137
Figure 8-2 Cisco Prestandard Apprehension Mechanism
Importance of the Apprehension Mechanism
Without such a apprehension apparatus alive at the concrete layer, there would be a accident of
applying a voltage to an agent accessory amid the about-face and the PES. For example,
if a hub is amid amid the about-face and the PES, there is no way to ascertain the
nonintelligent hub at Band 2 because the hub clearly passes frames amid the
switch and the PES. Therefore, both of them accept that they are anon connected, and
the about-face applies ability to the hub.
With a apprehension apparatus alive at the concrete band (such as the Cisco prestandard
or IEEE 802.3af), the hub prevents the about-face from audition the PES; therefore, the switch
does not administer ability to the hub. This prevents all amercement to the switch.
The IEEE 802.3af apprehension apparatus is altered than the Cisco prestandard because it
does not inject an AC on one pair, but rather, it measures whether DC can breeze amid two
pairs of the CAT5 cable. Figure 8-3 illustrates that a low-intensity DC is transmitted to the
attached base by applying a voltage amid the address and accept pairs. In the case of
an IEEE 802.3af PES, there’s a attrition of 22 kilo ohm (kΩ). Hence, a accepted flows.
(Actually, the abstinent attrition charge be annihilation amid 19 kΩ and 26.5 kΩ.) This
signals that the absorbed base is a powered accessory that requires power.
Switch
FLP
FLP
Low-Pass
Filter
FLP
received.
There is
a PES!
PES
138 Chapter 8: What About Ability over Ethernet?
Figure 8-3 IEEE 802.3af Apprehension Mechanism
After it detects the attendance of a PES, the about-face can additionally use a agnate apparatus to detect
the chic of the PES, such as its ability burning actuality either 4.0, 7.0, or 15.4 W.
NOTE All Cisco IP phones and switches that abutment the new IEEE 802.3af additionally abutment the Cisco
prestandard implementation.
Moreover, back application the Cisco prestandard method, IP phones added use Cisco
Discovery Protocol (CDP) to fine-tune their ability claim to a lower level. (The goal
is to acquiesce added accessories on the switch.)
Powering Mechanism
Because the purpose of PoE is to absolutely bear electrical ability to the PES, there are
mechanisms for this purpose. The IEEE 802.3af has two alternatives for ability commitment to
the PES:
• Phantom ambit (top of Figure 8-4). Area the DC ability is activated amid the
transmit and accept pairs.
• Powering apparatus (bottom of Figure 8-4). Area the DC ability is artlessly applied
between the two bare pairs.
In both mechanisms of ability delivery, the absolute voltage is 42 V. The Cisco prestandard
implementation uses a agnate scheme. Figure 8-4 shows the IEEE 802.3af powering
mechanism.
Switch
Current
22kΩ
Current
Flows.
There is
a PES!
DC
Voltage
PES
Risk Analysis for PoE 139
Figure 8-4 IEEE 802.3af Powering Mechanism
Because the switches accept a bound bulk of electrical ability for the end stations, this
power account charge be anxiously managed. If a about-face has a accouterment for 600 W, it can only
power up to 40 IP phones (assuming a ability burning of 15 W per phone).
Switch
Switch
Tx pair
Tx pair
PD
PD
PES
PES
Rx pair
Rx pair
PSE