Figure 21.17 DSSS channel without overlap with a channel used by neighbor AP

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21 Wireless Local Area Networks
When a station wishes to access an existing basic service set, it needs to get
synchronization information from the AP. The station can get this information in
one of two ways:
Passive scanning:
In this case the station waits to receive a beacon frame
from the AP. The beacon frame is a frame sent out periodically by the AP
containing synchronization information.
Active scanning:
In this case the station tries to locate
an AP by transmitting
probe request frame
, and waits for
probe response
from the AP.
A method is chosen according to the power consumption/performance trade-
off. Once the station has located an AP, and decides
to join its basic service set, it
goes through the authentication process. This is the interchange of information
between the AP and the station, where each side proves the knowledge of a given
password. This is necessary because WLANs have limited physical security to pre-
vent unauthorized access. The goal of authentication is to provide access control
equal to a wired LAN. The authentication service provides
a mechanism for one
station to identify another station. Without this proof of identity, the station is
not allowed to use the WLAN for data delivery. All 802.11 stations, whether they
are part of an independent basic service set or extended service set (ESS) network,
must use the authentication process prior to communicating with another station.
IEEE 802.11 uses authentication services deﬁ ned in IEEE 802.11i.
Once the station is authenticated, it then starts the association process. It
is used to make a logical connection between a mobile
station and an AP and to
exchange information about the station and basic service set/capabilities, which
allows the distribution system service (DSS) to know about the current position of
the station. This is necessary so that the AP can know where and how to deliver
data to the mobile station. A station is allowed to transmit data frames through
the AP only after the association process is completed.
When a station determines that the existing signal is poor,
it begins scanning
for another AP. This can be done by passively listening or actively probing each
channel and waiting for a response. Once information has been received, the
station selects the most appropriate signal and sends an association request to
the new AP. If the new AP sends an association response, the client connects to the
new AP.
This feature is known as
roaming
and is similar to the cellular handover,
with two main differences:
On a packet-based LAN system, the transition from cell to cell may be per-
formed between packet transmissions as opposed to a cellular system where
the transition may occur during a phone conversation. This makes WLAN
roaming a little easier.
On
a voice system, a temporary disconnection may not affect the conversation,
while in a packet-based data system it signiﬁ cantly reduces performance
because retransmission is performed by the upper layer protocols.
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The 802.11 standard does not deﬁ ne how roaming should be performed,
but deﬁ nes the basic tools including active/passive scanning, and a re-association
process, in which a station roaming from one AP to another becomes associated
with the new AP.
The 802.11 standard also provides a mechanism to
remove a station from the
basic service set. The process is called de-authentication. De-authentication is used
to prevent a previously authenticated station from using the network any further.
Once a station is de-authenticated, it is no longer able to access the WLAN without
performing the authentication process again. De-authentication is a notiﬁ cation and
cannot be refused. When a station wishes to be removed from a basic service set, it
can send a de-authentication management frame to the associated AP. An AP could
also de-authenticate a station by sending a de-authentication frame to the station.