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Appendix C Pop-up Windows, JavaScripts and Java Permissions
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A
PPENDIX
D
Wireless LANs
Wireless LAN Topologies
This section discusses ad-hoc and infrastructure wireless LAN topologies.
Ad-hoc Wireless LAN Configuration
The simplest WLAN configuration is an independent (Ad-hoc) WLAN that connects a set of
computers with wireless adapters (A, B, C). Any time two or more wireless adapters are within
range of each other, they can set up an independent network, which is commonly referred to as an
ad-hoc network or Independent Basic Service Set (IBSS). The following diagram shows an example
of notebook computers using wireless adapters to form an ad-hoc wireless LAN.
Figure 240
Peer-to-Peer Communication in an Ad-hoc Network
BSS
A Basic Service Set (BSS) exists when all communications between wireless clients or between a
wireless client and a wired network client go through one access point (AP).
Intra-BSS traffic is traffic between wireless clients in the BSS. When Intra-BSS is enabled, wireless
client
A
and
B
can access the wired network and communicate with each other. When Intra-BSS is
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disabled, wireless client
A
and
B
can still access the wired network but cannot communicate with
each other.
Figure 241
Basic Service Set
ESS
An Extended Service Set (ESS) consists of a series of overlapping BSSs, each containing an access
point, with each access point connected together by a wired network. This wired connection
between APs is called a Distribution System (DS).
This type of wireless LAN topology is called an Infrastructure WLAN. The Access Points not only
provide communication with the wired network but also mediate wireless network traffic in the
immediate neighborhood.
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An ESSID (ESS IDentification) uniquely identifies each ESS. All access points and their associated
wireless clients within the same ESS must have the same ESSID in order to communicate.
Figure 242
Infrastructure WLAN
Channel
A channel is the radio frequency(ies) used by wireless devices to transmit and receive data.
Channels available depend on your geographical area. You may have a choice of channels (for your
region) so you should use a channel different from an adjacent AP (access point) to reduce
interference. Interference occurs when radio signals from different access points overlap causing
interference and degrading performance.
Adjacent channels partially overlap however. To avoid interference due to overlap, your AP should
be on a channel at least five channels away from a channel that an adjacent AP is using. For
example, if your region has 11 channels and an adjacent AP is using channel 1, then you need to
select a channel between 6 or 11.
RTS/CTS
A hidden node occurs when two stations are within range of the same access point, but are not
within range of each other. The following figure illustrates a hidden node. Both stations (STA) are
within range of the access point (AP) or wireless gateway, but out-of-range of each other, so they
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cannot "hear" each other, that is they do not know if the channel is currently being used. Therefore,
they are considered hidden from each other.
Figure 243
RTS/CTS
When station
A
sends data to the AP, it might not know that the station
B
is already using the
channel. If these two stations send data at the same time, collisions may occur when both sets of
data arrive at the AP at the same time, resulting in a loss of messages for both stations.
RTS/CTS
is designed to prevent collisions due to hidden nodes. An
RTS/CTS
defines the biggest
size data frame you can send before an RTS (Request To Send)/CTS (Clear to Send) handshake is
invoked.
When a data frame exceeds the
RTS/CTS
value you set (between 0 to 2432 bytes), the station
that wants to transmit this frame must first send an RTS (Request To Send) message to the AP for
permission to send it. The AP then responds with a CTS (Clear to Send) message to all other
stations within its range to notify them to defer their transmission. It also reserves and confirms
with the requesting station the time frame for the requested transmission.
Stations can send frames smaller than the specified
RTS/CTS
directly to the AP without the RTS
(Request To Send)/CTS (Clear to Send) handshake.
You should only configure
RTS/CTS
if the possibility of hidden nodes exists on your network and
the "cost" of resending large frames is more than the extra network overhead involved in the RTS
(Request To Send)/CTS (Clear to Send) handshake.
If the
RTS/CTS
value is greater than the
Fragmentation Threshold
value (see next), then the
RTS (Request To Send)/CTS (Clear to Send) handshake will never occur as data frames will be
fragmented before they reach
RTS/CTS
size.
Note: Enabling the RTS Threshold causes redundant network overhead that could
negatively affect the throughput performance instead of providing a remedy.
Fragmentation Threshold
A
Fragmentation Threshold
is the maximum data fragment size (between 256 and 2432 bytes)
that can be sent in the wireless network before the AP will fragment the packet into smaller data
frames.
A large
Fragmentation Threshold
is recommended for networks not prone to interference while
you should set a smaller threshold for busy networks or networks that are prone to interference.

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