Cabling Requirements
Typically, APs are connected to the network via some type of
Ethernet cabling. This could be Category (Cat) 6
cable, Cat 5 cable, fiber-optic cable, or (in some of the older WLAN products)
coax (although not many products support this today). In almost all cases, the
same limitations apply to data cable that feeds an AP as would apply to feeding
a wired client device. Limitations include distance, number of connections,
plenum ratings, and so on.
Network engineers usually have a good understanding of Ethernet
cabling, but an AP has one other requirement: power. Because APs are typically
placed in the ceilings or other areas where the availability of AC power might
not be easily accessible, vendors sought a way to use the same Ethernet cable to
provide power to the AP.
Power over Ethernet
Most network devices require some form of power, whether
central LAN infrastructure devices such as switches and routers, or peripheral
components such as IP telephones and APs. Power over
Ethernet (PoE) combines both data and power onto existing LAN
infrastructure cabling, which is typically Cat 5 or Cat 6 cable.
PoE had a very high adoption rate for VoIP applications, and
its usage in WLANs has recently been increasing (primarily because it can reduce
installation cost by removing the need for local AC power at the AP and
eliminates the need for extra cabling to feed power to the AP). In many cases,
the cost of adding an AC power source (by a licensed electrician) for every AP
location exceeds the overall cost of the AP itself.
PoE is used for many different applications. VoIP was one of
the initial PoE applications, but now many other network devices benefit from
this feature, including the following:
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Printers and print servers
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Web cameras and security devices
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Alarm systems
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Security locks/security door systems
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Any device connected to the LAN
You can provide PoE in three primary ways:
Placing a standalone device anywhere on the Ethernet cable
provides a single Ethernet input and a single Ethernet with PoE output for
connecting to a single device. Figure
9-11 shows examples of single-port power injectors.
The second method for supplying PoE is through the use of a
multiport PoE device. These come in several configurations, supporting as few as
6 or 8 devices (as the one shown in Figure
9-12) or up to 24 or more devices. Basically, these devices provide one
Ethernet input and one Ethernet output with PoE for every device
supported.
For both the single-port power injector and the multiport power
injector, the Ethernet input for the injector comes from the wired network,
usually the local Ethernet switch. The device is typically located in the same
location as the Ethernet switch, where power is readily available. The output of
the injector is connected to the Ethernet cable that feeds the device (such as
the AP). Both of these types of devices are known as mid-span devices because power is injected in the
middle of the Ethernet run. It is important to remember that this does not
increase the length of the Ethernet run, and the limitation of the cabling
applies to the total length of the cable between the switch or network
connection and the end device. Figure
9-13 shows a typical connection for both a single-port power injector and a
multiport power injector.
The third type of PoE-supporting device is the end-span device.
This type of device is both the data source and the power injector. An Ethernet
switch that provides power over the Ethernet without any external devices is
such a product.
802.3af
The initial PoE implementations were proprietary and unique
to certain vendors' products. The Cisco PoE format was the most widespread, with
many vendors supporting it, and in some cases also supporting their own
implementations. With the rapid acceptance of PoE, the IEEE determined that
there should be a standard format for everyone to follow. Therefore, as part of
the 802.3 Group, the 802.1af
specification was developed. Although many companies were involved in the
development of this standard, PowerDsine was a major driving force behind
completing the standard.
The standard defines the following key elements:
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The physical
method for power distribution Defines the basic requirements for power
delivery.
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The voltage,
power, and other parameters Defines the maximum current capacity, the
minimum and maximum voltage, and the overall maximum power availability.
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A standard
method for end-device power request (signature recognition) Defines a
specification to determine whether the end device requires PoE. This provides
protection of non-802.3af devices. By the use of a physical "signature" on the
end device, the power source will know whether power should be applied to the
cable, protecting the end device from improper voltage on Ethernet
cables.
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Devices such as wireless switches that provide PoE power on
either data pairs 1,2 (+) and 3,6 () or spare pairs 4,5 (+ve) and 7,8 (ve).
Mid-span devices such as in-line power injectors provide power
on spare pairs 4,5 (+) and 7,8 ().
End devices (AP, phone, or camera, for example) using PoE must
have the capability to accept power on both the data and spare pairs.
The maximum power of any PoE device is 12.95 watts.
Devices providing PoE must have the capability to supply a
minimum current of 350 mA continuous, with a voltage between 44V DC and 57V DC.
Average power capability must be, at a minimum, 14.4 watts continuous.
A signature recognition must be achieved by an impedance of 20k
ohms to 30k ohms in the device requiring PoE.
A voltage between 2.8V DC and 10V DC will be used by the device
providing PoE as the probe to discover end devices with the PoE
signature. |
Proprietary PoE Methods
As previously mentioned, it was only recently that 802.3af
was ratified, even though PoE has been around for several years. Because many
hardware vendors took it upon themselves to develop their own PoE solution, it
is important to understand the basic differences between implementations to
avoid integration and installation problems.
The Cisco PoE implementation was originally for supplying power
to the IP Cisco IP phone products. The power requirements for the early IP
phones were very minimal. Therefore, the Cisco PoE solution provided
approximately 7 watts per port. When PoE became a requirement for Cisco APs, the
same Cisco PoE scheme was used. At that time, the power consumption of the AP
fell within the capabilities of most Cisco PoE-supplying switches. However, as
new APs were developed, power requirements increased, requiring more power be
made available. As a result, newer APs require either in-line power injectors or
the use of newer PoE-capable switches.
When designing and installing PoE for APs, you must verify that
the intended PoE switch can support the power required by the AP model that will
be used (today, and in the future with upgrades). If not, you might have to find
an alternative method such as the single-port power injectors.
Cisco Aironet power injectors provide an alternative powering
option to local power or using a PoE switch. The single-port Ethernet power
injector provides the required 48V DC power and carries it over the same cable
as the data signal, by utilizing the unused pairs of the cable.
Cisco took the idea of power injectors one step further. Many
networks today use fiber-optic connections, especially for long runs in places
such as manufacturing facilities and warehouses. However, providing power over
fiber is not possible (at least today). It is possible, however, to provide
power from a location near the factory floor or warehouse, but again that is not
where the AP is likely to be mounted.
The Cisco Media Converter converts fiber media to Cat 5 media
and combines the resulting data signal with power for delivery to the AP or
bridge. The power-injector media converter accepts 48V DC power from either the
barrel connector of the local power supply or an alternative 48V DC power
source. When powered by an alternative 48V DC power source connected using the
provided power-supply pigtail, the power injector media converter is UL2043
certified and suitable for installation in plenum areas.
This device enables you to run the fiber to a location in the
factory where AC is available, and from that point you can run a Cat 5 cable to
the AP location (see Figures 9-14 and 9-15).
There are other possible PoE schemes, and you need to be
aware that not all PoE schemes are created equal. Cisco PoE uses 48V DC over
unused pairs of the Cat 5 cable, whereas the PoE scheme offered by 3Com provides
24V DC power over the spare pins in standard Cat 5 cable.
Because the 3Com devices use 24V rather than 48V, and the Cisco
devices use a different pin-out for the power than an 802.1af system uses, both
APs require a converter to receive power from an 802.3af source. PowerDsine and
3Com offer converters for this requirement.
If you are dealing with an existing network for which it would
be financially unacceptable to swap or install new LAN switches to add APs, you
might find that using a mid-span device is a more feasible and cost-effective
option. When purchasing equipment for either mid-span application or for powered
switches, ensure that there is an easy and practical upgrade path.
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