CS485 Sylabus
The collision detection method
Collisions in 802.11
Recall
:
anti
-jamming (collision)
in
802.11
:
When
2 nodes
transmits
at the
same
time
:
The
receivers
can
decode
the
corrupted
data frame
by the use of
Direct Sequence Spread Spectrum
transmission !!!
See:
click here
However:
Collisions
that
cause
frame loss
are
still
possible
!!!
Frame loss
can
happen
when:
More
than 2
nodes
transmit
at the
same
time
The
PN (Pseudo Noise) code
used
colliding nodes
are
similar
How
does a node
detects
a
collision
in 802.11 ??
Not
like
Ethernet
:
Recall
that the
node's own transmission
will
overwhelm
any
transmission
from
other
(remote) nodes
!!!
Because
the
node's receive antenna
is
close
by
the
node's own transmit antenna
!!!
Collision "detection"
in
802.11
:
A
node
that transmitted a
data
frame
will
expect
an
ACK
frame
(within a
time out
period)
A
collision
is
assumed
when:
The
node
does
not
receive an
ACK frame
within the
time out
period