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Appendix D Wireless LANs
P-663HN-51 User’s Guide
276
keys. This prevent all wireless devices sharing the same encryption keys. (a
weakness of WEP)
User Authentication
WPA and WPA2 apply IEEE 802.1x and Extensible Authentication Protocol (EAP) to
authenticate wireless clients using an external RADIUS database. WPA2 reduces
the number of key exchange messages from six to four (CCMP 4-way handshake)
and shortens the time required to connect to a network. Other WPA2
authentication features that are different from WPA include key caching and pre-
authentication. These two features are optional and may not be supported in all
wireless devices.
Key caching allows a wireless client to store the PMK it derived through a
successful authentication with an AP. The wireless client uses the PMK when it tries
to connect to the same AP and does not need to go with the authentication
process again.
Pre-authentication enables fast roaming by allowing the wireless client (already
connecting to an AP) to perform IEEE 802.1x authentication with another AP
before connecting to it.
Wireless Client WPA Supplicants
A wireless client supplicant is the software that runs on an operating system
instructing the wireless client how to use WPA. At the time of writing, the most
widely available supplicant is the
WPA patch for Windows XP, Funk Software's
Odyssey client.
The Windows XP patch is a free download that adds WPA capability to Windows
XP's built-in "Zero Configuration" wireless client. However, you must run Windows
XP to use it.
WPA(2) with RADIUS Application Example
To set up WPA(2), you need the IP address of the RADIUS server, its port number
(default is 1812), and the RADIUS shared secret. A WPA(2) application example
with an external RADIUS server looks as follows. "A" is the RADIUS server. "DS" is
the distribution system.
1
The AP passes the wireless client's authentication request to the RADIUS server.
2
The RADIUS server then checks the user's identification against its database and
grants or denies network access accordingly.
3
A 256-bit Pairwise Master Key (PMK) is derived from the authentication process by
the RADIUS server and the client.
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Appendix D Wireless LANs
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277
4
The RADIUS server distributes the PMK to the AP. The AP then sets up a key
hierarchy and management system, using the PMK to dynamically generate
unique data encryption keys. The keys are used to encrypt every data packet that
is wirelessly communicated between the AP and the wireless clients.
Figure 164
WPA(2) with RADIUS Application Example
WPA(2)-PSK Application Example
A WPA(2)-PSK application looks as follows.
1
First enter identical passwords into the AP and all wireless clients. The Pre-Shared
Key (PSK) must consist of between 8 and 63 ASCII characters or 64 hexadecimal
characters (including spaces and symbols).
2
The AP checks each wireless client's password and allows it to join the network
only if the password matches.
3
The AP and wireless clients generate a common PMK (Pairwise Master Key). The
key itself is not sent over the network, but is derived from the PSK and the SSID.
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Appendix D Wireless LANs
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278
4
The AP and wireless clients use the TKIP or AES encryption process, the PMK and
information exchanged in a handshake to create temporal encryption keys. They
use these keys to encrypt data exchanged between them.
Figure 165
WPA(2)-PSK Authentication
Security Parameters Summary
Refer to this table to see what other security parameters you should configure for
each authentication method or key management protocol type. MAC address
filters are not dependent on how you configure these security features.
Table 93
Wireless Security Relational Matrix
AUTHENTICATION
METHOD/ KEY
MANAGEMENT
PROTOCOL
ENCRYPTIO
N METHOD
ENTER
MANUAL KEY
IEEE 802.1X
Open
None
No
Disable
Enable without Dynamic WEP
Key
Open
WEP
No
Enable with Dynamic WEP Key
Yes
Enable without Dynamic WEP
Key
Yes
Disable
Shared
WEP
No
Enable with Dynamic WEP Key
Yes
Enable without Dynamic WEP
Key
Yes
Disable
WPA
TKIP/AES
No
Enable
WPA-PSK
TKIP/AES
Yes
Disable
WPA2
TKIP/AES
No
Enable
WPA2-PSK
TKIP/AES
Yes
Disable
Page 279 / 300
Appendix D Wireless LANs
P-663HN-51 User’s Guide
279
Antenna Overview
An antenna couples RF signals onto air. A transmitter within a wireless device
sends an RF signal to the antenna, which propagates the signal through the air.
The antenna also operates in reverse by capturing RF signals from the air.
Positioning the antennas properly increases the range and coverage area of a
wireless LAN.
Antenna Characteristics
Frequency
An antenna in the frequency of 2.4GHz (IEEE 802.11b and IEEE 802.11g) or 5GHz
(IEEE 802.11a) is needed to communicate efficiently in a wireless LAN
Radiation Pattern
A radiation pattern is a diagram that allows you to visualize the shape of the
antenna’s coverage area.
Antenna Gain
Antenna gain, measured in dB (decibel), is the increase in coverage within the RF
beam width. Higher antenna gain improves the range of the signal for better
communications.
For an indoor site, each 1 dB increase in antenna gain results in a range increase
of approximately 2.5%. For an unobstructed outdoor site, each 1dB increase in
gain results in a range increase of approximately 5%. Actual results may vary
depending on the network environment.
Antenna gain is sometimes specified in dBi, which is how much the antenna
increases the signal power compared to using an isotropic antenna. An isotropic
antenna is a theoretical perfect antenna that sends out radio signals equally well
in all directions. dBi represents the true gain that the antenna provides.
Types of Antennas for WLAN
There are two types of antennas used for wireless LAN applications.
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Appendix D Wireless LANs
P-663HN-51 User’s Guide
280
•
Omni-directional antennas send the RF signal out in all directions on a horizontal
plane. The coverage area is torus-shaped (like a donut) which makes these
antennas ideal for a room environment. With a wide coverage area, it is possible
to make circular overlapping coverage areas with multiple access points.
•
Directional antennas concentrate the RF signal in a beam, like a flashlight does
with the light from its bulb. The angle of the beam determines the width of the
coverage pattern. Angles typically range from 20 degrees (very directional) to
120 degrees (less directional). Directional antennas are ideal for hallways and
outdoor point-to-point applications.
Positioning Antennas
In general, antennas should be mounted as high as practically possible and free of
obstructions. In point-to–point application, position both antennas at the same
height and in a direct line of sight to each other to attain the best performance.
For omni-directional antennas mounted on a table, desk, and so on, point the
antenna up. For omni-directional antennas mounted on a wall or ceiling, point the
antenna down. For a single AP application, place omni-directional antennas as
close to the center of the coverage area as possible.
For directional antennas, point the antenna in the direction of the desired
coverage area.

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