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CHAPTER
7
WIRELESS LOCAL AREA NETWORKS
Wireless
Local Area
Networks
Fundamental Concepts
Network Technologies
Application Layer
LAN WLAN
Backbone
Transport Layer
Network Layer
Data Link Layer
Physical Layer
WAN
Internet
M
k
Network Management
The Three Faces of Networking
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CHAPTER OUTLINE
A
LTHOUGH
T
RADITIONAL
wired Ethernet LANs dominate today’s network
environment, wireless LANs (WLANs) are common. This chapter describes the basic
components of a WLAN and then examines three common wireless technologies: Wi-Fi
(IEEE 802.11), WiMAX (IEEE 802.16), and Bluetooth (IEEE 802.15). The chapter ends
with a discussion of best practice WLAN design, including security, and how to improve
performance.
OBJECTIVES
Understand the major components of WLANs
■
Understand Wi-Fi
■
Be familiar with Wi-Max
■
Be familiar with Bluetooth WLANs
■
Be familiar with how to improve WLAN performance
■
Be familiar with WLAN security
■
Understand the best practice recommendations for WLAN design
■
CHAPTER OUTLINE
INTRODUCTION
WLAN COMPONENTS
Network Interface Cards
Access Points
Radio Frequencies
WI-FI
Topology
Media Access Control
Types of Wi-Fi
Wi-Fi as Public Internet Access
WIMAX
Topology
Media Access Control
Types of WiMAX
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CHAPTER 7
WIRELESS LOCAL AREA NETWORKS
BLUETOOTH
Topology
Media Access Control
THE BEST PRACTICE WLAN DESIGN
Effective Data Rates
Costs
Recommendations
Physical WLAN Design
WLAN Security
IMPROVING WLAN PERFORMANCE
Improving Device Performance
Improving Circuit Capacity
Reducing Network Demand
IMPLICATIONS FOR MANAGEMENT
SUMMARY
INTRODUCTION
The use of
Wireless LANs (WLANs)
is growing rapidly. A recent survey of network man-
agers indicated that 90 percent of companies are using wireless LANs, usually in addition
to traditional wired LANs. Wireless LANs transmit data through the air using radio trans-
mission rather than through twisted-pair cable or fiber-optic cable. This has been one area
of networking that has seen the greatest changes in a short amount of time. From a time
with no widely accepted standards (2000), we have today gone to an alphabet soup of
standards (e.g., 802.11a, 802.11b, 802.11g, 802.11n, 802.15, 802.16d, 802.16e).
WLANs serve the same purpose as LANs: they are used to connect a series of com-
puters in the same small local area to each other and to a backbone network. WLANs are
usually not totally wireless in that they are most commonly used to connect a set of wireless
computers into a wired network. However, WLANs enable you to use the network in places
where it is impractical to put a wired network (either because of cost or access). WLANs
can enable staff to pull up a chair and work on the network from a lunchroom, a corridor, or
an outdoor patio. WLANs also enable mobile staff to work at different locations in the office
building or to move their computers easily from one location to another. WLANs are be-
coming popular in hospitals, for example, because they enable doctors and nurses to use lap-
tops and tablet PCs to access patient records. WLANs are also popular in airports because
they enable business travelers to connect to the Internet from any waiting area.
This chapter examines the basic components of a WLAN and then examines three
commonly used WLAN technologies (Wi-Fi, WiMAX, and Bluetooth). The chapter ends
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235
WLAN COMPONENTS
with a discussion of best practice recommendations for WLAN design and ways to im-
prove WLAN performance.
As with Ethernet in the previous chapters, the three primary WLAN technologies
(Wi-Fi, WiMAX, and Bluetooth) are layer 2 protocols that operate at the data link layer.
They too must have physical hardware at layer 1 that meets their requirements and soft-
ware at layers above them (e.g., TCP/IP) that enables application software to use them.
WLAN COMPONENTS
In the last chapter on LANs, we discussed the three key components of the LAN: the net-
work interface card, the hub/switch, and the cables that connect them. WLANs use the
same basic structure. There is a wireless network interface card that is built into a desktop
or laptop computer (or can be added later). A wireless access point performs the same
functions as a hub or switch. Finally, instead of cable, there is a set of radio frequencies
that are used to transport data (see Figure 7.1).
Network Interface Cards
Each computer has a wireless
network interface card
(
NIC
)
that is used to connect the
computer into the WLAN. The NIC is a radio transceiver in that it sends and receives
radio signals through a short range, usually only about 100 meters or 300 feet. WLAN
NICs are available for laptops as PCMCIA cards and as standard cards for desktop com-
puters, but laptop computers now come with Wi-Fi NICs built-in.
Wireless
Access Point
Ethernet Switch
A wireless access point connected into an Ethernet switch.
FIGURE 7.1
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CHAPTER 7
WIRELESS LOCAL AREA NETWORKS
FIGURE 7.2
A wireless access point.
Access Points
A central wireless
access point
(
AP
)
is a radio transceiver that plays the same role as a
hub or switch in wired Ethernet LANs (Figure 7.2). The AP also connects the WLAN into
wired LANs, typically using 100Base-T.
The AP acts as a repeater to ensure that all computers within range of the AP can
hear the signals of all other computers in the WLAN. All NICs in the WLAN transmit
their packets to the AP and then the AP retransmits the packet over the wireless network
to its destination—or retransmits the packet over the wired network to its destination.
Wireless NICs never communicate with each other directly; they always transmit through
the AP. Therefore, if a message has to be transmitted from one wireless computer to an-
other, it is transmitted twice, once from the sender to the AP and then from the AP to the
destination. At first glance this may seem a bit strange because it doubles the number of
transmissions in the WLAN. However, very few messages are ever sent from client com-
puter to client computer in a WLAN. Most messages are exchanged between client com-
puters and a server of some kind. For this reason, servers should never be placed on a
WLAN. Even if they are intended to serve clients on a WLAN, they should always be
placed on the wired portion of the LAN.
Most WLANs are installed using APs that have
omnidirectional antennas
,which
means that the antenna transmits in all directions simultaneously. One common omnidi-
rectional antenna is the dipole antenna shown in Figure 7.3a (nicknamed the “rubber
duck” because of its flexibility). As Figure 7.3a shows, omnidirectional antennas transmit
in all directions, both horizontally and vertically. The signal goes in all directions, as well
as up and down, although there is often a small dead spot with no signal that is a very
small area directly above the antenna.
The other type of antenna that can be used on APs is the
directional antenna
(Figure
7.3b). As the name suggests, a directional antenna projects a signal only in one direction.
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