check whether it remains free or another transmission begins. At the end of its
contention window, if the medium is still free the station can send its frame. If
during the contention window another station begins transmitting data, the back-
off counter is frozen and counting down starts again when the channel returns to
the idle state.
There is a problem related to the CW dimension. With a small CW, if many
stations attempt to transmit data at the same time it is very possible that some
of them may have the same backoff interval. This means that there will continu-
ously be collisions, with serious effects on the network performance. On the other
hand, with a large CW, if few stations wish to transmit data they will likely have
long backoff delays resulting in the degradation of the network performance. The
solution is to use an exponentially growing CW size. It starts from a small value
(
CW
min
31) and doubles after each collision, until it reaches the maximum
value
CW
max
(
CW
max
1023). In 802.11 the backoff algorithm must be executed
in three cases:
When the station senses the medium is busy before the fi rst transmission of
a packet
After each retransmission
After a successful transmission
This is necessary to avoid a single host wanting to transmit a large quantity
of data, occupying the channel for too long a period, and denying access to all
other stations. The backoff mechanism is not used when the station decides to
transmit a new packet after an idle period and the medium has been free for more
than the DIFS (see Figure 21.14).
The transmission time for a data frame
PLCP
D
R
s
where:
•
•
•
Busy Medium
DIFS
PIFS
SIFS
Contention Window
Slot-time
Next Frame
Defer Access
Select Slot and decrement backoff as
long as medium is idle
DIFS
Backoff Window
