Wireless Communications 159
complements the Cisco Aironet 1200 Series 802.11a Access Point, providing a solution
that combines performance and mobility with the security and manageability that
enterprises require. It will be possible to achieve data rates of greater than 20 Mbps
in this frequency range. The drawback of the 5-GHz frequency, however, is its limited
range. The typical range for 5 GHz inside is about 50 feet; outside poses a limitation of
approximately 2500 feet.
Spread-Spectrum Technology
Just as the radio in your car has AM and FM bands, other radios use certain bands,
frequencies, and types of modulation. Spread spectrum (SS) is a modulation technique
developed in the 1940s that spreads a transmission signal over a broad band of radio
frequencies. The term spread spectrum describes a modulation technique that sacrifices
bandwidth to gain signal-to-noise performance. This technique is ideal for data com-
munications because it is less susceptible to radio noise and creates little interference.
Spread spectrum, as illustrated in Figure 3-42, is a system in which the transmitted sig-
nal is spread over a frequency much wider than the minimum bandwidth required to
send the signal. The fundamental premise is that in channels with narrowband inter-
ference increasing the transmitted signal bandwidth results in an increased probability
that the received information is correct.
Figure 3-42 Spread-Spectrum Technology
To use the unlicensed radio bands, you have to use spread-spectrum techniques. Fre-
quency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS)
are two ways of doing spread spectrum. These spread-spectrum techniques spread the
RF energy over the available band. The next sections describe FHSS and DSSS in more
detail.
FHSS Versus DSSS
As modulation techniques, both frequency-hopping spread spectrum (FHSS) and
direct-sequence spread spectrum (DSSS) have advantages and limitations.
NOTE
Narrowband interfer-
ence occurs when two

signals are broadcast-
ing at the same fre-
quency in the same
geographic area. The
term band refers to
a grouping of fre-
quencies; narrow-
band would mean a
relatively smaller
range of frequencies.
Narrowband noise
might disrupt certain
channels or spread-
spectrum components.
Narrowband Information
Signal (Before Spreading)
Spread-Spectrum Signal
(After Spreading)
Power
Frequency
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160 Chapter 3: Networking Media
With FHSS technology, transmissions hop from one frequency to another in random pat-
terns. Figure 3-43 illustrates an example of a FHSS. In this example, the transmission
hops from C (2.42 GHz), to A (2.40 GHz), to D (2.43 GHz), then to B (2.41 GHz), and
finally to E (2.44 GHz). This technique enables the transmissions to hop around narrow-
band interference, resulting in a clearer signal and higher reliability of the transmission.
However, FHSS technology is slower, and the receiver must use the same pattern to decode.
Figure 3-43 Frequency-Hopping Spread Spectrum
DSSS technology transmissions, as illustrated in Figure 3-44, are more reliable because

each bit (1 or 0) is represented by a string of 1s and 0s called a chipping sequence. Even
if up to 40 percent of the string is lost, the original transmission can be reconstructed.
DSSS technology also enables high throughput of data and longer-range access.
Figure 3-44 Direct-Sequence Spread Spectrum
Time
5
4
3
2
1
Hopping Pattern: C A D B E
Freq.
(GHz)
2.40
2.41
2.42
2.43
2.44
2.45
B
A
E
D
C
Time
Frequency
(GHz)
1
5
4

3
2
2.40
2.41
2.42
2.43
2.44
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Wireless Networking 161
Limited to a 2-Mbps data-transfer rate, FHSS is recommended for only very specific
applications such as for certain types of watercraft. For all other wireless LAN applica-
tions, DSSS is the better choice. The recently released evolution of the IEEE standard,
802.11b, provides for a full Ethernet-like data rate of 11 Mbps over DSSS. FHSS does
not support data rates greater than 2 Mbps.
Wireless Networking
When the computer was first introduced to the world, it was affordable by only large
corporations, governments, and universities. From the first building-sized devices with
minimal computing power to those that fit in the palm of a person’s hand, huge leaps
in technology have occurred. The same is true on the connectivity side of the industry.
The various types of networking discussed earlier in this chapter have all involved
physical connectivity. The advantages are speed, reliability, and to a certain extent con-
venience. Physical connectivity allows an increase in productivity by allowing the shar-
ing of printers, servers, and software. However, networked systems require that the
workstation remain stationary, permitting moves only within the limits of the media
and office area.
The introduction of wireless technology removes these restraints and brings true port-
ability to the computing world. While the current state of wireless technology does not
provide the high-speed transfers of cabled networks nor the security and uptime reli-
ability, the flexibility justifies the trade off.
When considering the installation of a network in an existing facility, wireless is at the

top of many an administrator’s lists of options. A simple wireless network can be up
and running in just a few minutes after the workstations are turned on. Connectivity
to the Internet is provided through a wired connection, router, cable modem, or Digital
Subscriber Line (DSL) modem, and a wireless access point that acts as a hub for the
wireless nodes. In a residential or small office environment these devices might be com-
bined into a single unit.
Wireless LAN Organization and Standards
An understanding of the regulations and standards that apply to wireless technology
ensures that deployed networks are interoperable and in compliance. Just as in cabled
networks, IEEE is the prime issuer of standards for wireless networks. The standards
have been created within the framework of the regulations set forth by the FCC.
A key technology contained within the IEEE 802.11 standard is DSSS. DSSS applies to
wireless devices operating within a 1 to 2 Mbps range. A DSSS system can operate at
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162 Chapter 3: Networking Media
up to 11 Mbps but is not considered compliant above 2 Mbps. The next standard
approved was IEEE 802.11b, which increased transmission capabilities to 11 Mbps.
Even though DSSS WLANs are able to interoperate with the FHSS WLANs, problems
developed prompting design changes by the manufacturers. In this case, IEEE’s task
was simply to create a standard that matched the manufacturer’s solution.
IEEE 802.11b, called Wi-Fi or high-speed wireless, refers to DSSS systems that operate
at 1, 2, 5.5, and 11 Mbps. All 802.11b systems are backward-compliant in that they
also support 802.11 for 1- and 2-Mbps data rates for DSSS only. This backward com-
patibility is extremely important because it allows upgrading of the wireless network
without replacing the network interface cards (NICs) or access points.
IEEE 802.11b devices achieve the higher data throughput rate by using a different cod-
ing technique from 802.11, allowing for a greater amount of data to be transferred in
the same time frame. The majority of 802.11b devices still fail to match the 10 Mbps
throughput of wired Ethernet and generally function in the 2–4 Mbps range.
802.11a covers WLAN devices operating in the 5-GHz transmission band. Using the

5-GHz range disallows interoperability of 802.11b devices as they operate within
2.4 GHz. 802.11a is capable of supplying data throughput of 54 Mpbs and with pro-
prietary technology known as rate doubling has achieved 108 Mbps. In production
networks a more standard rating is 20 to 26 Mbps.
802.11g provides the same throughout as 802.11a but with backwards compatibility
for 802.11g devices using Othogonal Frequency Division Multiplexing (OFDM) mod-
ulation technology. Cisco has developed an access point that permits 802.11b and
802.11a devices to coexist on the same WLAN. The access point supplies gateway
services allowing these otherwise incompatible devices to communicate.
Wireless Devices and Topologies
A wireless network can consist of as few as two devices, two nodes with wireless NICs.
Figure 3-45 shows an internal wireless NIC, and Figure 3-46 shows an external USB
wireless NIC. The nodes can be desktop workstations or notebook computers. Equipped
with wireless NICs, an ad hoc network can be established that equates to a peer-to-
peer wired network. Both devices act as servers and clients in this environment, and
although it does provide connectivity, security is at a minimum along with throughput.
Another problem with this type of network is compatibility; oftentimes, NICs from
different manufacturers do not interoperate.
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Wireless Networking 163
Figure 3-45 Internal Wireless NIC
Figure 3-46 External USB Wireless NIC
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164 Chapter 3: Networking Media
More commonly, an access point (AP), as shown in Figure 3-47, is installed acting as
a central hub for the WLAN infrastructure mode. The AP is hard wired to the cabled
LAN to provide Internet access and connectivity to the wired network. APs are equipped
with antennae and provide wireless connectivity over a specified area referred to as a cell.
Figure 3-47 Access Point
Depending on the structural composition of the location in which the AP is installed

and the size and gain of the antennae, the size of the cell can range from a few dozen
feet to 25 miles. More commonly the range is from 300 to 500 feet. To service larger
areas multiple APs can be installed with a degree of overlap, permitting roaming
between cells, as illustrated in Figure 3-48. This roaming is very similar to the services
provided by cellular phone companies. Overlap on multiple AP networks is critical to
allow for movement of devices within the WLAN, and although it is not addressed in
the IEEE standards, a 20–30 percent overlap is desirable. This rate of overlap permits
roaming between cells, allowing for the disconnect/reconnect activity to occur seam-
lessly without service interruption.
When a client is activated within the WLAN, it starts listening for a compatible device
with which to associate. This process is referred to as scanning and can be active or
passive.
Active scanning causes a probe request to be sent from the wireless node seeking to
join the network. The probe request contains the Service Set Identifier (SSID) of the
network it wants to join. When an AP with the same SSID is found, the AP issues a
probe response, and the authentication and association steps are completed.
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Wireless Networking 165
Figure 3-48 Roaming
Passive scanning nodes listen for beacon management frames (beacons), which are
transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node
receives a beacon that contains the SSID of the network it is trying to join, an attempt
is made to join the network. Passive scanning is a continuous process and nodes can
associate or disassociate with APs as signal strength changes.
How Wireless LANs Communicate
After establishing connectivity to the WLAN, a node passes frames similarly to any
other 802 network. WLANs do not use a standard 802.3 frame. Therefore, using the
term wireless Ethernet is misleading. There are three types of frames: control, manage-
ment, and data. The following lists the frames that are included in each type of frame:
■ Management frames

— Association request frame
— Association response frame
— Probe request frame
— Probe response frame
— Beacon frame
— Authentication frame
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166 Chapter 3: Networking Media
■ Control frames
— Request to send (RTS)
— Clear to send (CTS)
— Acknowledgment
■ Data frames
Only the data frame type is similar to 802.3 frames. However, the payload of wireless
and 802.3 frames is 1500 bytes, and an Ethernet frame cannot exceed 1518 bytes. On
the other hand, a wireless frame can be as large as 2346 bytes. Usually the WLAN frame
size is limited to 1518 bytes because it is most commonly connected to a wired Ether-
net network.
Because RF is a shared medium, collisions can occur just as they do on wired shared
medium. The significant difference is that there is no method by which the source node
is able to detect that a collision has occurred. In view of this, WLANs use carrier sense
multiple access with collision avoidance (CSMA/CA). This feature is somewhat like
Ethernet carrier sense multiple access collision detect (CSMA/CD). Chapter 5, “Ether-
net Fundamentals,” discusses CSMA/CD in greater detail.
When a source node sends a frame, the receiving node returns a positive acknowledg-
ment (ACK); which can consequently cause consumption of 50 percent of the available
bandwidth. This overhead, when combined with the collision avoidance protocol
overhead, reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on
an IEEE 802.11b wireless LAN rated at 11 Mbps.
Performance of the network will also be affected by signal strength and degradation in

signal quality due to distance or interference. As the signal becomes weaker, Adaptive
Rate Selection (ARS) can be invoked, and the transmitting unit drops the data rate from
11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps, or 2 Mbps to 1 Mbps, as illustrated
in Figure 3-49.
Authentication and Association
WLAN authentication occurs at Layer 2 and is the process of authenticating the device,
not the user. This point is a critical one to remember when considering WLAN secu-
rity, troubleshooting, and overall management.
Authentication might be a null process, as in the case of a new AP and NIC with default
configurations in place. The client sends an authentication request frame to the AP,
and the frame is accepted or rejected by the AP. The client is notified of either course of
action via an authentication response frame. The AP might also be configured to hand
off the authentication task to an authentication server, which performs a more thorough
credentialing process.
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Wireless Networking 167
Figure 3-49 Adaptive Rate Selection
Association, performed after authentication, is the state that permits a client to use the
AP’s services to transfer data.
Authentication and Association Types
The authentication and association types are as follows:
■ Unauthenticated and unassociated—The node is disconnected from the network
and not associated to an access point.
■ Authenticated and unassociated—The node has been authenticated on the net-
work but has not yet associated with the access point.
■ Authenticated and associated—The node is connected to the network and able
to transmit and receive data through the access point.
Methods of Authentication
IEEE 802.11 lists two types of authentication processes:
■ Open system—This process is an open connectivity standard in which only the

SSID must match. It can be used in a secure or non-secure environment, although
the ability of low-level network sniffers to ascertain the SSID of the WLAN is
fairly high.
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168 Chapter 3: Networking Media
■ Shared key—This process requires the use of Wired Equivalent Privacy (WEP)
encryption. WEP is a fairly simple algorithm using 64- and 128-bit keys. The AP
is configured with an encrypted key, and nodes attempting to access the network
through the AP must have a matching key. Statically assigned WEP keys provide
a higher level of security than the open system but are definitely not hack proof.
The susceptibility to unauthorized entry into WLANs is being addressed by a number
of emerging security solution technologies.
The Radio Wave/Microwave Spectrum
Computers send data signals electronically. Radio transmitters convert these electrical
signals to radio waves. The radio waves are generated by changing electric currents in
a transmitter’s antenna. These radio waves radiate out in straight lines from the antenna.
However, radio waves weaken (attenuate) as they move out from the transmitting
antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet)
from the transmitting antenna is only 1/100th of its original strength. Like light, radio
waves can be absorbed by some materials and reflected by others. When passing from
one material like air into another material like a plaster wall, radio waves are refracted
(bent). Radio waves are also scattered and absorbed by water droplets in the air.
These qualities of radio waves are important to remember when a WLAN is being
planned for a building or for a campus. The process of evaluating a location for the
installation of a WLAN is called making a site survey.
Because radio signals weaken as they travel away from the transmitter, the receiver
must also be equipped with an antenna. When radio waves hit a receiver’s antenna,
weak electric currents are generated in that antenna. These electric currents, caused by
the received radio waves, are equal to the currents that originally generated the radio
waves in the transmitter’s antenna. The receiver amplifies the strength of these weak

electrical signals.
In a transmitter, the electrical (data) signals from a computer or a LAN are not sent
directly into the transmitter’s antenna. Rather these data signals are used to alter a
second, strong signal called the carrier signal.
A receiver demodulates the carrier signal that arrives from its antenna. The receiver
interprets the phase changes of the carrier signal and reconstructs from it the original
electrical data signal.
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