Introduction
One of the bigger changes in the networking world since the release of
the pre- vious Network+ is in wireless networking. Networks of all shapes and
sizes incorporate wireless segments into their networks. Home wireless
networking has also grown significantly in the last few years.
Wireless networking enables users to connect to a network using radio
waves instead of wires.
Network users within
range of a wireless access
point (AP) can move around an office freely without
needing to plug into a wired infrastruc- ture.
The benefits of wireless networking clearly have led to its growth.
Today, wireless local area
networks (WLANs) provide a flexible and secure
data communications system that augments an Ethernet LAN or, in some cases, replaces it. Wireless transmissions send and receive
data using radio frequency
(RF) signals, freeing us from wired solutions.
In a common wireless implementation, a wireless transceiver (transmitter/
receiver), known as an access
point, connects to the wired
network from a fixed
location using standard cabling. The wireless access point receives and then
transmits data between the wireless
LAN and the wired network
infrastructure.
Client systems communicate with a wireless access point using wireless
LAN adapters. Such adapters
are built into or can be added to laptops,
PDAs, or desk- top computers. Wireless LAN adapters
provide the communication point between the client
system and the airwaves via an antenna.
This chapter explores the many facets of wireless networking, starting
with some of the concepts and technologies that make wireless
networking possible.
Wireless Access Points
As discussed in Chapter 3, “Networking Components and Devices,” a
wireless access point (AP) is both a transmitter and receiver (transceiver) device used for wireless LAN (WLAN) radio signals. An
AP typically is a separate network device with a built-in antenna, transmitter,
and adapter. APs use the wireless
infrastructure network mode to provide a connection point between WLANs and a
wired Ethernet LAN. APs also typically have several ports, giving you a way to expand the network to support additional clients.
Depending on the size of the network, one or more APs might be required.
Additional APs are used to allow access to more wireless clients and to expand
the range of the wireless
network. Each AP is limited
by a transmission range—
the distance a client can be from an AP and still get a usable signal. The
actual distance depends on the wireless
standard being used and the obstructions and environmental conditions between the client and the AP. Factors
affecting wire- less transmission ranges are covered
later in this chapter.As mentioned , an AP can be used in an infrastructure wireless net- work
design. Used in the infrastructure mode, the AP receives transmissions from wireless
devices within a specific range and transmits
those signals to the
network beyond. This network might be a private Ethernet network or the
Internet. In infrastructure wireless networking, there might be multiple access
points to cover a large area or only a single access point for a small area, such as
a single home or small building.
Working with APs
When working with wireless APs, you need to understand many terms and
acronyms. This section defines some of the more common wireless acronyms you will see both on the exam and in wireless
networking documentation.
. Service Set Identifier (SSID)—A network
name needed to connect to a
wireless AP. It is like a workgroup name used with Windows networking.
802.11 wireless
networks use the SSID to identify all systems belonging to the same network.
Client stations must be configured with the SSID to be authenticated to the AP. The AP might broadcast the SSID,
allow- ing all wireless clients in the area to see the AP’s SSID. For security rea- sons, APs can be configured not to
broadcast the SSID or to cloak it. This
means that an administrator needs to give client systems the SSID instead of allowing it to be discovered automatically.
. Basic Service Set (BSS)—Refers
to a wireless network that uses a single AP and one or more wireless clients
connecting to the AP. Many home
offices are an example of a BSS design. The BSS is an example of the
infrastructure wireless topology. Wireless topologies and other network
topologies are discussed in Chapter 1.
. Extended Service
Set (ESS)—Refers to two or more
connected BSSs that use multiple
APs. The ESS is used to create WLANs or larger wire- less networks and is a collection of APs and clients. Connecting BSS sys- tems allows
clients to roam between areas
and maintain the wireless con- nection without having to
reconfigure between BSSs.
. Extended Service Set Identifier (ESSID)—Although the terms ESSID
and SSID are used interchangeably, there is a difference between the two. SSID
is the name used with BSS networks. ESSID is the network name used with an ESS
wireless network design. With an ESS, not all APs necessarily use the same name.
. Basic Service Set Identifier (BSSID)—The MAC address of the BSS AP. The BSSID is not to be confused
with the SSID,
which is the name
of the wireless network.
. Basic Service Area
(BSA)—When troubleshooting or designing wire- less networks, the BSA
is an important consideration. The
BSA refers to the AP’s
coverage area. The BSA for an AP depends on many factors, including the strength of the AP antenna, interference in the area, and
whether an omnidirectional or directional antenna
is being used.
Wireless Antennas
A wireless antenna is an integral part of overall wireless communication.
Antennas come in many different shapes and sizes,
with each one designed for a
specific purpose. Selecting the right antenna
for a particular network implemen- tation is a critical
consideration and one that could ultimately decide how suc- cessful a wireless
network will be. In addition, using the right antenna can save you money on
networking costs, because you need fewer antennas and access points.
Many small home network adapters
and access points
come with a nonupgrad-
able antenna, but higher-grade wireless
devices require you to choose an anten- na. Determining which antenna
to select takes careful planning
and requires an understanding of what range
and speed you need for a network.
The antenna is designed to help wireless
networks do the following:
. Work around obstacles
. Minimize the
effects of interference
. Increase signal strength
. Focus the transmission, which can
increase signal speed
The following sections explore some of the characteristics of wireless antennas.
Antenna Ratings
When a wireless signal is low and is being affected by heavy
interference, it might be possible to upgrade the antenna to create a more
solid wireless con- nection. To determine
an antenna’s strength, we refer to
its gain
value. But how do we determine
the gain value?
Suppose that a huge wireless
tower is emanating
circular waves in all directions. If we could see these waves,
we would see them forming a sphere around the tower.
The signals around the antenna flow equally in all directions, including
up and down. An antenna that does this has a 0dBi gain value and is called an isotropic antenna. The isotropic antenna
rating provides a base point for measur- ing actual antenna strength.
When looking at wireless antennas, remember that a higher gain value
means stronger send and receive signals.
In terms of performance, the rule of thumb is that
every 3dB of gain added
doubles an antenna’s effective power output.
Antenna Coverage
When selecting an antenna for a particular wireless implementation, it is neces- sary to determine the type of
coverage the antenna uses. In a typical configura- tion, a wireless antenna can
be either omnidirectional or directional. Which one you choose
depends on the wireless environment.
An omnidirectional antenna
is designed to provide a 360-degree dispersed
wave pattern. This type of antenna is used when coverage in all
directions from the antenna is required. Omnidirectional antennas are advantageous when a broad- based signal is required. For example,
if you provide an even signal in all direc- tions, clients can access
the antenna and its associated access point from various
locations. Because of the dispersed nature of omnidirectional antennas, the sig- nal is weaker overall and therefore
accommodates shorter signal distances. Omnidirectional antennas are great in an
environment that has a clear line of sight between the senders and receivers. The power is evenly spread
to all
points, making
omnidirectional antennas well suited for home and small office applications.
Wireless Radio Channels
Radio frequency (RF) channels are an important part of wireless
communica- tion. A cftannel
is the band of RF used for the wireless communication. Each IEEE
wireless standard specifies the channels that can be used. The 802.11a standard
specifies radio frequency ranges between 5.15 and 5.875GHz. In con- trast,
802.11b and 802.11g standards operate in the 2.4 to 2.497GHz range. IEEE wireless
standards are discussed
later in this chapter.As far as channels are concerned, 802.11a
has a wider frequency band, allowing
more channels and therefore more data throughput. As a result of the wider band, 802.11a
supports up to eight nonoverlapping channels. 802.11b/g standards
use the smaller band and support
only up to three nonoverlapping channels.
It is recommended that nonoverlapping channels be used for communication. In the U.S., 802.11b/g use 11 channels for data communication,
as mentioned; three of these—channels 1, 6, and 11—are nonoverlapping. Most
manufactur- ers set their default channel to one of the nonoverlapping channels
to avoid transmission conflicts. With wireless devices
you can select which channel
your WLAN operates on to avoid interference from other wireless
devices that oper- ate in the 2.4GHz frequency range.
When troubleshooting a wireless network, be aware that overlapping
channels can disrupt the wireless communications. For example, in many
environments, APs are inadvertently placed close together—perhaps two access points in sepa- rate offices located next door to each other
or between floors.
Signal disruption
results if channel
overlap exists between
the access points.
The solution is to try to
move the access
point to avoid
the overlap problem,
or to change channels to one of the other nonoverlapping
channels.
Typically you would change
the channel of a wireless
device only if it overlapped with another device. If a channel
must be changed,
it must be changed to anoth-
er, nonoverlapping channel. Table 7.2 shows the channel ranges for
802.11b/g wireless standards. Table 7.3 shows the channel ranges
for 802.11a. 802.11n
has the option of using both channels used by 802.11a
and b/g.
Factors
Affecting Wireless Signals
Because wireless signals
travel through the atmosphere, they are susceptible to different types of interference than
standard wired networks. Interference weak-
ens wireless signals and therefore is an important consideration when working with wireless networking.
Interference Types
Wireless interference is an important consideration when you’re planning
a wireless network. Interference is unfortunately inevitable, but the trick is
to minimize the levels of interference. Wireless LAN communications typically are based on radio frequency signals that require
a clear and unobstructed transmis- sion path.
The following
are some factors
that cause interference:
. Physical objects: Trees, masonry,
buildings, and other physical struc- tures are some of the most common
sources of interference. The density of the materials used in a building’s construction determines the
number of walls the RF signal can pass through and still maintain adequate cov-
erage. Concrete and steel walls are particularly difficult for a signal to pass
through. These structures will weaken or at times completely pre- vent wireless signals.
. Radio frequency interference: Wireless technologies such as
802.11b/g use an RF range of 2.4GHz,
and so do many other devices, such as cord- less phones, microwaves, and
so on. Devices
that share the
channel can cause noise and weaken the signals.
. Electrical interference: Electrical
interference comes from devices such as computers, refrigerators, fans,
lighting fixtures, or any other motor- ized devices. The impact that electrical
interference has on the signal depends on the proximity of the electrical
device to the wireless access point. Advances in wireless technologies and in
electrical devices have reduced the impact
that these types
of devices have on wireless
transmis- sions.
. Environmental
factors: Weather conditions can have a huge impact
on wireless signal integrity. Lightning,
for example, can cause electrical interference, and fog can weaken signals
as they pass through.
Many wireless implementations are found in the office or at home. Even
when outside interference such as weather
is not a problem, every
office has plenty
of wireless obstacles. Table 7.4
highlights a few examples to be aware of when implementing a wireless network
indoors.
 |
|
Wood/wood paneling
|
Low
|
Inside a wall or hollow door
|
|
Drywall
|
Low
|
Inside walls
|
|
Furniture
|
Low
|
Couches
or office partitions
|
|
Clear glass
|
Low
|
Windows
|
|
Tinted glass
|
Medium
|
Windows
|
|
People
|
Medium
|
High-volume traffic areas that have considerable pedestrian traffic
|
|
Ceramic tile
|
Medium
|
Walls
|
|
Obstruction
|
Obstacle Severity
|
Sample Use
|
|
Concrete blocks
|
Medium/high
|
Outer
wall construction
|
|
Mirrors
|
High
|
Mirror or
reflective glass
|
|
Metals
|
High
|
Metal office partitions, doors, metal office furniture
|
|
Water
|
High
|
Aquariums,
rain, fountains
|
Securing Wireless Networks
Many different strategies and protocols are used to secure LAN and WAN transmissions. What about network
transmissions that travel
over the airwaves?
In the last few years, wireless networking has changed the look of modern
net- works, bringing with it an unparalleled level
of mobility and a host of new secu-
rity concerns.
Wireless LANs (WLANs) require new protocols and standards to handle secu-
rity for radio communications. As it stands
today, wireless communications rep- resent a significant security
concern. You should be aware of a few wireless secu- rity standards when working
with wireless, including
Wired Equivalent Privacy (WEP), Wi-Fi Protected Access (WPA), and 802.1X.
Wired
Equivalent Privacy (WEP)
Wired equivalent privacy
(WEP) was the first attempt
to keep wireless
networks safe. WEP was designed to be easy to configure and implement.
Originally it was hoped that WEP would provide the same level of security to
wireless net- works as was available to wired. For a time it was the best and
only option for securing wireless networks.
WEP is an IEEE standard introduced in 1997, designed to secure 802.11
net- works. With WEP enabled, each data packet transmitted over the wireless
con- nection would be encrypted. Originally,
the data packet was combined with a secret 40-bit number key as it
passed through an encryption algorithm known as RC4. The packet was scrambled
and sent across the airwaves. On the receiv- ing end, the data packet passed
through the RC4 backward, and the host received the data as it was intended.
WEP originally used a 40-bit number key, but
later it specified 128-bit encryption, making WEP that much more robust.
WEP is a protocol designed to provide security by encrypting data from
the sending and receiving
devices. In a short period of time, however, it was discov- ered that WEP encryption was not
nearly as secure as hoped. Part of the prob- lem was that when the 802.11
standards were being
written, security was not the major concern it is today. As a result, WEP security was easy to crack with freely
available hacking tools. From this point, wireless
communication was regarded as a potentially insecure
transmission medium.
The two types of WEP security are static and dynamic. Dynamic and static
WEP differ in that dynamic WEP changes security keys periodically, making it
more secure. Static WEP uses the same security key on an ongoing basis. The
primary security risks
are associated with static WEP, which
uses a shared pass- word to protect communications. Security weaknesses discovered in static WEP mean that WLANs protected by it are
vulnerable to several types of threats. Freely available hacking tools make
breaking into static WEP-protected wire- less networks a trivial task.
Unsecured WLANs are obviously exposed to these same threats as well;
the difference is that less expertise, time,
and resources are required to carry out the attacks.
Wi-Fi Protected
Access (WPA)
Security weaknesses associated with WEP gave administrators a valid reason
to be concerned about wireless security. The need for increased wireless
security was important for wireless networking to reach its potential and to
reassure those who had sensitive data that it was safe to use wireless communications. In response, Wi-Fi Protected Access (WPA) was created. WPA was
designed to improve on the security weaknesses of WEP and to be
backward-compatible with older devices
that used the WEP standard.
WPA addressed two main secu-
rity concerns:
. Enhanced data encryption: WPA uses a temporal key
integrity protocol (TKIP), which scrambles encryption keys using
a hashing algorithm. Then the keys are issued an integrity check
to verify that they have not
been modified or tampered with during transit.
. Authentication: Using
Extensible Authentication Protocol (EAP), WEP regulates access to a wireless
network based on a computer’s hardware-
specific MAC address, which is relatively simple to be sniffed and stolen. EAP
is built on a more secure public-key encryption system to ensure that only authorized network
users can access the network.
802.1X
802.1X is an IEEE standard specifying port-based network access control.
802.1X was not specifically designed for wireless networks; rather, it provides authenticated access
for both wired
and wireless networks. Port-based network
access control uses the physical
characteristics of a switched local area network (LAN) infrastructure to authenticate devices attached to a LAN port and to pre- vent access to that port in cases
where the authentication process fails. The 802.1X framework has three
main components:
. Supplicant: The system or node
requesting access and
authentication to a network
resource.
. Authenticator: A control mechanism that allows or denies traffic
that wants to pass through a port.
. Authentication
server: Validates the credentials of the supplicant that is trying to access the network or resource.
During a port-based network access control interaction, a LAN port adopts
one of two roles: authenticator or supplicant. In the role of autftenticator, a LAN port enforces authentication before it
allows user access to the services that can be accessed through that port. In
the role of supplicant, a LAN port
requests access to the services
that can be accessed through
the authenticator’s port. An authen-
tication server, which can be either
a separate entity or colocated with the authenticator, checks the supplicant’s credentials on behalf of the authenticator. The authentication server
then responds to the authenticator, indicating whether the supplicant is authorized to access the authenticator’s services.
The authenticator’s port-based network
access control defines
two logical access points to the LAN through one
physical LAN port. The first logical
access point, the uncontrolled port, allows data exchange between
the authenticator and other computers on the LAN,
regardless of the computer’s authorization
state. The second logical
access point, the
controlled port, allows data exchange between an authenticated LAN user and the authenticator.
In a wireless network environment, the supplicant typically is a network
host. The authenticator could be the wireless network switch or AP. The role of authentication server would be played by a Remote Authentication Dial-In
User Service (RADIUS).
RADIUS is a protocol that allows a single server to become responsible
for all remote-access authentication, authorization, and auditing (or accounting) services.
RADIUS functions as a client/server system. The remote user dials in to
the remote access server, which acts
as a RADIUS client, or network access server (NAS), and connects to a RADIUS
server. The RADIUS server performs authentication, authorization, and auditing
(or accounting) functions and returns the information to the RADIUS client
(which is a remote-access server running RADIUS client software). The
connection is either established or rejected
based on the information received.
Temporal Key Integrity Protocol
As mentioned previously, WEP lacked security. Temporal Key Integrity
Protocol (TKIP) was designed to address the shortcomings of the WEP securi- ty
protocol. TKIP is an encryption protocol defined in IEEE 802.11i. TKIP was
designed not only to increase
security but also to use existing hardware, making it easy to upgrade to TKIP encryption.
TKIP is built on the original WEP security standard but enhances it by
“wrap- ping” additional code at both the end and the beginning of the data packet. This code modifies the code for
additional security. Because TKIP is
based on WEP, it too uses the RC4 stream encryption
method. But unlike WEP, TKIP encrypts
each data packet with a stronger encryption key than is available with regular WEP.
TKIP provides increased security for data communications, but it is far from the
final solution. TKIP provides strong encryption for home users and nonsensi-
tive data. However, it may not provide the level of security necessary to protect corporate or more sensitive data while in transmission.
Establishing Communications
Between Wireless Devices
When you work with wireless networks, it is important to have a basic
under- standing of the communication that occurs between wireless devices. If
you’re using an infrastructure wireless
network design, the
network has two key
parts—the
wireless client, also known as the station (STA),
and the AP. The AP acts as a
bridge between the STA and the wired network.
The association process
occurs when a wireless adapter
is turned on. The client adapter immediately begins scanning
the wireless frequencies for wireless APs or, if using ad hoc mode, other wireless
devices. When the wireless client is con- figured to operate in infrastructure mode,
the user can choose a wireless AP with
which to connect. This process may also be automatic, with the AP selection
based on the SSID, signal strength, and frame error rate. Finally, the wireless adapter switches to
the assigned channel of the selected wireless AP and nego- tiates the use of a port.
If at any point the signal between the devices drops below an acceptable
level, or if the signal becomes
unavailable for any reason, the wireless adapter
initiates another scan, looking for an AP with stronger signals. When
the new AP is located, the wireless adapter selects it and associates with it.
This is known as reassociation.
After security requirements are met, you have established IP-level communica-
tion. This means that wireless standard requirements have been met, and
Ethernet networking takes over. There
is basically a switch between 802.11 to
802.3 standards. The wireless standards create the physical
link to the network,
allowing regular networking standards and protocols to use the link. This is how the
physical cable is replaced, but to the networking technologies there is no dif-
ference between regular
cable media and wireless media.
Several components combine to enable wireless communications between
devices. Each of these must be configured on both the client and the AP:
. Service Set Identifier (SSID):
Whether your wireless network is using infrastructure mode or ad hoc mode, an
SSID is required. The SSID is a configurable
client identification that allows clients
to communicate with a particular base station. Only
client systems configured with the same SSID as the AP can communicate with it.
SSIDs provide a simple pass- word arrangement between
base stations and clients in a BSS network.
ESSIDs are used for the ESS
wireless network.
. Wireless channel: RF channels are an
important part of wireless com- munications. A cftannel is the frequency band used for the wireless com-
munication. Each standard specifies the channels that can be used. The 802.11a
standard specifies radio frequency ranges between 5.15 and 5.875GHz. In
contrast, the 802.11b and 802.11g standards operate in the
2.4 to 2.497GHz
ranges. Fourteen channels are defined in the IEEE 802.11b/g channel
set, 11 of which are available in North America.
. Security features: IEEE 802.11 provides security using two methods, authentication
and encryption. Authentication verifies the client system. In infrastructure
mode, authentication is established between an AP and each station.
Wireless encryption services
must be the same on the client and the AP for communication to occur.
Configuring the
Wireless Connection
Now that we have reviewed key wireless settings, let’s take a look at an actual wireless connection configuration.
As shown in Figure 7.1, the settings for this wireless router are clearly
laid out. For instance, you can see that the wireless connection uses an SSID password of Gigaset602 and wireless channel
11.
. SSID: This name is used for anyone who wants to access the Internet
through this wireless access point. The SSID is a configurable client
identification that allows clients to communicate with a particular base station. In application, only clients
configured with the same SSID can communicate with base stations hav- ing the
same SSID. SSID provides a simple password arrangement between base
stations and clients.
As far as
troubleshooting is concerned, if a client cannot access a base station, make sure
that both are using the same SSID. Incompatible SSIDs are sometimes found when
clients move computers, such as lap- tops, between different wireless networks.
They obtain an SSID from one network. If the system is not rebooted, the old
SSID doesn’t allow communication with a different base station.
. Channel: To access this network, all
systems must use this channel. If needed, you can change the channel using
the drop-down menu.
The menu lists
channels 1 through 11.
. SSID Broadcast: In their default
configuration, wireless access points typically broadcast the SSID name into
the air at regular intervals. This feature is intended to allow clients to
easily discover the network and roam between WLANs. The problem with SSID
broadcasting is that it makes it a little easier to get around security. SSIDs are not encrypted or
protected in any way. Anyone can
snoop and get a look at the SSID and attempt to join the network.
. Authentication: When configuring authentication security for the AP, you
have several options, including WEP-Open, WEP-Shared, and WPA-psk.
WEP-Open is the simplest of the authentications methods because it does not perform
any type of client verification. It is a very
weak form of authentication, because
it requires no proof of identity.
WEP-Shared
requires that a WEP key be configured on both the client system and the access point. This makes authentication with WEP-Shared mandatory, so it is more secure
for wireless transmission. WPA-psk
(Wi-Fi Protected
Access with Pre-Shared Key) is a stronger form of encryption in which keys are automatically changed and authenticated between devices after a
specified period of time, or after a specified number of packets have been transmitted.
. Wireless Mode: To access the network, the client must use
the same wireless mode as the AP. Today most users configure the network for
802.11g for faster speeds or a combination of 802.11b/g because
these wireless standards are compatible.
. DTIM Period (seconds):
Wireless transmissions can broadcast to all systems—that is, they can send
messages to all clients on the wireless network. Multiple broadcast messages
are known as multicast or broad- cast traffic. Delivery Traffic Indication Message (DTIM) is a feature used to ensure
that when the multicast or broadcast traffic is sent, all systems are awake to
hear the message. The DTIM setting
specifies how often the DTIM is sent within the beacon frame. For
example, if the DTIM setting by
default is 1, this means that the DTIM is
sent with every bea- con. If the DTIM is
set to 3, the DTIM is sent every three
beacons as a DTIM wake-up call.
. Maximum Connection Rate: The transfer rate typically
is set to Auto by default.
This allows the maximum connection speed. However,
it is possible to decrease the
speed to increase the distance that
the signal travels and boost signal
strength due to poor environmental conditions.
. Network Type: This is where the network can be set to use the ad hoc
or infrastructure network
design.
Access Point Coverage
Like any other
network medium, APs have a limited transmission distance. This limitation
is an important consideration when you’re deciding where an AP should be placed on the network.
When troubleshooting a wireless network,
pay close attention to how far the client systems are from the AP.
When faced with a problem in which client systems cannot consistently
access the AP, you could try moving
the access point to better cover the area, but then you may disrupt access for
users in other areas. So what can be done to trou- bleshoot AP coverage?
Depending on the network environment, the quick solution may be to throw
money at the problem and purchase another access point, cabling, and other
hardware to expand the transmission area. However,
you can try a few things before installing another
wireless access point.
The following list starts with the
least expensive solution and progresses to the most
expensive:
. Increase transmission power:
Some access points have a setting to adjust the transmission power output. By
default, most of these settings are set to the maximum output; however, this is worth verifying just in
case. Also note that you can decrease the transmission power if you’re trying to reduce the dispersion of radio waves beyond the immediate net- work. Increasing the power
gives clients stronger
data signals and greater
transmission
distances.
. Relocate the AP: When wireless client
systems suffer from connectivity problems, the solution may be as simple as
relocating the AP. You could relocate it across the room, a
few feet away, or across the hall.
Finding the right location will likely take a little trial and error.
. Adjust or replace antennas: If the
access point distance is insufficient for some network clients, it might be
necessary to replace the default antenna used with both the AP and the client
with higher-end antennas. Upgrading an antenna can make a big difference in
terms of transmission range. Unfortunately, not all APs have replaceable antennas.
. Signal amplification: RF amplifiers
add significant distance to wireless signals. An RF amplifier increases the
strength and readability of the data transmission. The amplifier improves both
the received and transmitted signals, resulting in an increase
in wireless network
performance.
. Use a repeater: Before installing a
new AP,
you might want to think
about a wireless repeater. When
set to the same channel as the AP, the
repeater takes the transmission and repeats it. So, the AP transmission gets to
the repeater, and then the repeater
duplicates the signal and pass- es it on. This is an effective strategy to increase
wireless transmission dis- tances.
Wireless Signal Quality
Because wireless signals
travel through the atmosphere, they are subjected to all sorts of environmental and external factors.
This includes storms
and the num- ber of walls, ceilings, and so on
that the signal must pass through. Just how weakened the signal becomes depends
on the building material used and the level of RF interference. All these
elements decrease the power of the wireless signal.
If you are troubleshooting a wireless connection that has a particularly weak sig-
nal, you can do a few things to help increase the signal’s power:
. Antenna: Perhaps the first
and most obvious thing to do is to make sure that the antenna on the AP is
positioned for best reception. It often takes a little trial and error to get
the placement right. Today’s wireless
access cards commonly ship with diagnostic software that displays signal
strength and makes it easy to find the correct
position.
. Device placement: One factor that can
degrade wireless signals is RF interference. Because of this, it is important
to try to keep wireless devices away from appliances that output RF noise. This
includes microwaves, electrical devices, and certain cordless devices using the
same frequency, such as phones.
. Network location: Although there may
be limited choice, as much as possible it is important to try to reduce the
number of obstructions that the signal must pass through. Every obstacle strips
a little more power from the signal. The type of material a signal must pass
through also can have a significant impact on signal integrity.
. Boost the signal: If all else fails,
you can purchase devices, such as wire- less repeaters, that can amplify the
wireless signal. The device takes the signal and amplifies it to make it stronger. This also increases the dis-
tance that the client system can be placed from the AP.
To successfully manage
wireless signals, you need to know which wireless stan- dard you are using. The standards used today specify
range distances, RF ranges,
and speeds. It may be that the wireless standard
cannot do what you need it to.
Wireless Troubleshooting Checklist
Poor
communication between wireless devices has many different potential causes. The following is a review checklist of wireless troubleshooting:
. Auto transfer rate: By
default, wireless devices are configured to use the strongest, fastest signal.
If you’re experiencing connectivity problems between wireless devices, try using the lower transfer
rate in a fixed mode to
achieve a more stable connection. For example, you can manu- ally choose the
wireless transfer rate. Also, instead of using 11Mbps, the highest rate for
802.11b, try 5.5Mbps, 2Mbps, or 1Mbps. The higher the transfer rate, the shorter
the connection distance.
. Router placement: If signal
strength is low, try moving the access point to
a new location. Moving it just a few feet can make a difference.
. Antenna: The default antenna shipped
with wireless devices
may not be powerful enough for a
particular client system. Better-quality antennas can be purchased for some APs, which
will boost the distance the signal
can go.
. Building obstructions: Wireless RF communications are weakened if they have to travel
through obstructions such as metal
and concrete.
. Conflicting devices: Any device
that uses the
same frequency range
as the wireless device can cause interference. For example,
2.4GHz phones can cause interference with devices using
the 802.11g standard.
. Wireless channels: If connections are inconsistent, try changing the channel to another,
nonoverlapping channel.
. Protocol issues: If an IP address
is not assigned to the wireless client,
an incorrect SSID or incorrect WEP settings can prevent a system from obtaining
IP information.
. SSID: The SSID number used on the client system must match the one
used on the AP. Typically,
the default SSID assigned is sufficient, but you
might need to change it if you’re switching a laptop between different WLANs.
. WEP: If WEP is enabled, the
encryption type must
match what is set in the
AP.
Infrared Wireless Networking
Infrared has been around for a long time; perhaps our first experience
with it was the TV remote. The commands entered
onto the remote control travel over
an infrared light wave to the receiver on the TV.
Infrared technology has pro- gressed, and today infrared development in
networking is managed by the Infrared Data Association (IrDA).
Infrared wireless networking uses infrared beams to send data transmissions
between devices. Infrared
wireless networking offers higher transmission rates, reaching 10Mbps to 16Mbps.
As expected, infrared
light beams cannot penetrate objects;
therefore, the signal is disrupted when something blocks
the light. Infrared can be either a directed (line-of-sight) or diffuse
technology. A directed infrared system provides a lim- ited range of
approximately 3 feet and typically is used for personal area net- works.
Diffused infrared can travel farther and is more difficult to block with a
signal object. Diffused infrared wireless LAN systems do not require line of
sight, but usable distance is limited to room distances.
Infrared provides a secure, low-cost, convenient cable-replacement technology. It is well suited for many
specific applications and environments. Some key infrared points
are as follows:
. It provides adequate
speeds—up to 16Mbps.
. Infrared devices
use less power
and therefore don’t drain
batteries as much.
. Infrared is a secure
medium. Infrared signals
typically are a direct-line
implementation in a short range
and therefore do not travel
far outside the
immediate connection. This eliminates the problem of eavesdropping or signal tampering.
. Infrared
is a proven technology. Infrared devices have been available for some time and as such are a proven,
nonproprietary technology with an
established user and
support base.
. It has no radio frequency interference issues or signal conflicts.
. It replaces cables for many
devices, such as keyboards, mice,
and other peripherals.
. It uses a dispersed mode or a direct line-of-sight transmission.
. Transmissions travel over short
distances.
Bluetooth
Bluetooth is a wireless standard used for many purposes, including
connecting peripheral devices to a system.
Bluetooth uses a low-cost, short-range radio link that replaces
many of the cords used to connect
devices.
Bluetooth is an easily configured technology. When Bluetooth-enabled
devices are within 10 or so meters of each other,
they can establish a connection. Bluetooth establishes the link using an
RF-based link and therefore does not require a direct line-of-sight connection.
The Bluetooth Standard defines a short RF link that is capable of voice or data
transmission up to a maximum capacity of 720Kbps per channel.
Bluetooth operates at 2.4 to 2.48GHz and uses an FHSS technology.
The sig- nal can hop
between 79 frequencies at 1MHz intervals to give a high degree of interference immunity.
As an established technology, Bluetooth
has many advantages, but the speed of 720Kbps is limiting. The newest version
of Bluetooth, Bluetooth 2.0, increases overall
speed to a data rate of 3Mbps.
This speed may still be significantly slow- er
than 802.11b or 802.11g, but for an easily configured cable replacement tech- nology,
it is an attractive option. Table 7.7
highlights the advantages of the Bluetooth standard.
Table 7.7 Bluetooth Characteristics
Specification Bluetooth
 |
Topology Ad hoc
Spread spectrum FHSS
|
Medium 2.4GHz RF
Speed 720Kbps
|
Range 10 meters in optimal conditions
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