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• Ad hoc network: IEEE 802.11
stations can dynamically form network without AP
• Applications:
– “laptop” meeting in conference
room, car
– interconnection of “personal”
devices
– battlefield
Frame structure
There are three types of frames
Management frame-used for station
association and dissociation with the AP
timing and synchronization and
authentication and deauthentication
Control frame-used for
handshaking and for positive ack
Data frame-for transmission of
data
MAC header provides information
on frame control, duration, addressing and
sequence control
MAC sublayer is responsible for
channel access procedures, pdu addressing
,formatting, fragmentation and
reassembly of MSDUs
supports security services
through authentication and privacy mechanisms
management services support
roaming within and ESS and assist stations in
power management.
The figure on the
next figure shows the MAC architecture
It is defined using the
coordination functions
DCF offers the contention service
where the stations have to contend to use the channel.
Uses CSMA/CA
The basic operation is
as follows
• 802.11 CSMA: sender
• - if sense channel idle for DISF
sec.
• then transmit entire frame (no
collision detection)
• -if sense channel busy then
binary backoff
• 802.11 CSMA receiver:
• if received OK
• return ACK after SIFS
– receiver: reply with short CTS:
clear to send
• CTS reserves channel for
sender, notifying (possibly hidden) stations
• avoid hidden station collisions
• RTS and CTS short:
– collisions less likely, of
shorter duration
– end result similar to collision
detection
• IEEE 802.11 alows:
– CSMA
– CSMA/CA: reservations
– polling from AP
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