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PRO 11 SERIES WIRELESS ETHERNET
E.6 Keeping Synchronization
Stations need to keep synchronization, which is necessary for keeping hopping
synchronized, and other functions like Power Saving. On an infrastructure BSS,
this is achieved by all the stations updating their clocks according to the Access
Point’s clock, using the following mechanism:
The Access Point periodically transmits frames called Beacon Frames. These
frames contain the value of the Access Point’s clock at the moment of transmission.
(Note that this is the moment when transmission actually occurs, and not when it is
put in the queue for transmission. Since the Beacon Frame is transmitted using
CSMA rules, transmission may be delayed significantly.)
The receiving stations check the value of their clocks at the moment the signal is
received, and correct it to keep in synchronization with the Access Point’s clock.
This prevents clock drifting, which could cause loss of sync after a few hours of
operation.
E.7 Security
Security is one of the first concerns that people have when deploying a wireless
LAN. The 802.11 committee has addressed the issue by providing what is called
WEP (Wired Equivalent Privacy).
Users are primarily concerned that an intruder should not be able to:
• Access the Network resources by using similar wireless LAN equipment
• Capture wireless LAN traffic (eavesdropping)
E.7.1 P
REVENTING
A
CCESS TO
N
ETWORK
R
ESOURCES
This is done by the use of an Authentication mechanism where a station needs to
prove knowledge of the current key. This is very similar to Wired LAN privacy, in
the sense that an intruder needs to enter the premises (by using a physical key) in
order to connect his workstation to the wired LAN.
E.7.2 E
AVESDROPPING
Eavesdropping is prevented by using the WEP algorithm, which is a Pseudo-
Random Number Generator initialized by a shared secret key. This PRNG outputs
a key sequence of pseudo-random bits equal in length to the largest possible
packet, which is combined with the outgoing/incoming packet, producing the
packet transmitted in the air.
Summary of Contents for LW0050A
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