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Appendix D Wireless LANs
P-661HNU-Fx User’s Guide
336
with each other. When Intra-BSS is disabled, wireless client
A
and
B
can still
access the wired network but cannot communicate with each other.
Figure 190
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 191
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
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Appendix D Wireless LANs
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338
wireless gateway, but out-of-range of each other, so they 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 192
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.
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Appendix D Wireless LANs
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339
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.
If the
Fragmentation Threshold
value is smaller than the
RTS/CTS
value (see
previously) you set 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.
Preamble Type
Preamble is used to signal that data is coming to the receiver. Short and long refer
to the length of the synchronization field in a packet.
Short preamble increases performance as less time sending preamble means
more time for sending data. All IEEE 802.11 compliant wireless adapters support
long preamble, but not all support short preamble.
Use long preamble if you are unsure what preamble mode other wireless devices
on the network support, and to provide more reliable communications in busy
wireless networks.
Use short preamble if you are sure all wireless devices on the network support it,
and to provide more efficient communications.
Use the dynamic setting to automatically use short preamble when all wireless
devices on the network support it, otherwise the ZyXEL Device uses long
preamble.
Note: The wireless devices MUST
use the same preamble mode in order to
communicate.
IEEE 802.11g Wireless LAN
IEEE 802.11g is fully compatible with the IEEE 802.11b standard. This means an
IEEE 802.11b adapter can interface directly with an IEEE 802.11g access point
(and vice versa) at 11 Mbps or lower depending on range. IEEE 802.11g has
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340
several intermediate rate steps between the maximum and minimum data rates.
The IEEE 802.11g data rate and modulation are as follows:
Wireless Security Overview
Wireless security is vital to your network to protect wireless communication
between wireless clients, access points and the wired network.
Wireless security methods available on the ZyXEL Device are data encryption,
wireless client authentication, restricting access by device MAC address and hiding
the ZyXEL Device identity.
The following figure shows the relative effectiveness of these wireless security
methods available on your ZyXEL Device.
Note: You must enable the same wireless security settings on the ZyXEL Device and
on all wireless clients that you want to associate with it.
Table 97
IEEE 802.11g
DATA RATE
(MBPS)
MODULATION
1
DBPSK (Differential Binary Phase Shift Keyed)
2
DQPSK (Differential Quadrature Phase Shift Keying)
5.5 / 11
CCK (Complementary Code Keying)
6/9/12/18/24/36/
48/54
OFDM (Orthogonal Frequency Division Multiplexing)
Table 98
Wireless Security Levels
SECURITY
LEVEL
SECURITY TYPE
Least
Secure
Most Secure
Unique SSID (Default)
Unique SSID with Hide SSID Enabled
MAC Address Filtering
WEP Encryption
IEEE802.1x EAP with RADIUS Server
Authentication
Wi-Fi Protected Access (WPA)
WPA2

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