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Appendix E Wireless LANs
VMG8924-B10A and VMG8924-B30A Series User’s Guide
376
disabled, wireless client
A
and
B
can still access the wired network but cannot communicate with
each other.
Figure 240
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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Appendix E Wireless LANs
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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 241
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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Appendix E Wireless LANs
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378
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 242
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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Appendix E Wireless LANs
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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.
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 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 Device are data encryption, wireless client
authentication, restricting access by device MAC address and hiding the Device identity.
The following figure shows the relative effectiveness of these wireless security methods available on
your Device.
Note: You must enable the same wireless security settings on the Device and on all
wireless clients that you want to associate with it.
Table 164
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 165
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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Appendix E Wireless LANs
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380
IEEE 802.1x
In June 2001, the IEEE 802.1x standard was designed to extend the features of IEEE 802.11 to
support extended authentication as well as providing additional accounting and control features. It
is supported by Windows XP and a number of network devices. Some advantages of IEEE 802.1x
are:
•
User based identification that allows for roaming.
•
Support for RADIUS (Remote Authentication Dial In User Service, RFC 2138, 2139) for
centralized user profile and accounting management on a network RADIUS server.
•
Support for EAP (Extensible Authentication Protocol, RFC 2486) that allows additional
authentication methods to be deployed with no changes to the access point or the wireless
clients.
RADIUS
RADIUS is based on a client-server model that supports authentication, authorization and
accounting. The access point is the client and the server is the RADIUS server. The RADIUS server
handles the following tasks:
• Authentication
Determines the identity of the users.
• Authorization
Determines the network services available to authenticated users once they are connected to the
network.
• Accounting
Keeps track of the client’s network activity.
RADIUS is a simple package exchange in which your AP acts as a message relay between the
wireless client and the network RADIUS server.
Types of RADIUS Messages
The following types of RADIUS messages are exchanged between the access point and the RADIUS
server for user authentication:
• Access-Request
Sent by an access point requesting authentication.
• Access-Reject
Sent by a RADIUS server rejecting access.
• Access-Accept
Sent by a RADIUS server allowing access.
• Access-Challenge
Sent by a RADIUS server requesting more information in order to allow access. The access point
sends a proper response from the user and then sends another Access-Request message.
The following types of RADIUS messages are exchanged between the access point and the RADIUS
server for user accounting:

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