208 Chapter 9: Discovering Wired Network Requirements
The same rules for deploying QoS in a wired network apply to deploying QoS in a WLAN.
It is imperative that you understand the type of trafﬁc and the requirements of that trafﬁc as
it moves across your network. The protocols, the application’s sensitivity to delay, and
trafﬁc bandwidth all play an important part in using QoS and placing priority on the types
It is also important to investigate the network to verify any limitations imposed by wired
network infrastructure components (switched, routers, or hubs) that could impact the use of
QoS. For example, having advanced QoS on the AP might be of little use if plugged into a
Also keep in mind that QoS does not create additional bandwidth, it just provides additional
control for how the bandwidth is allocated.
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
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 infra-
structure 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:
• Printers and print servers
• Web cameras and security devices
Cabling Requirements 209
• Alarm systems
• Security locks/security door systems
• Any device connected to the LAN
You can provide PoE in three primary ways:
• Placing a standalone device anywhere on the Ethernet cable
• Using a multiport PoE device
• Using an end-span device
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.
Figure 9-11 Single-Port PoE 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.
Figure 9-12 Multiport Mid-Span Device
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