- Scenario:
- Multiple nodes are
waiting for
a transmission
(node A) to
finish:
The (multiple) nodes will transmit when the transmission is complete
- Multiple nodes are
waiting for
a transmission
(node A) to
finish:
- Problem description:
- The multiple nodes
are not aware of
each other
- We must make sure that
node B transmits
first
- We must do so without using any messages communicated between the nodes
- The multiple nodes
are not aware of
each other
- Solution:
- When a node want to
transmit a
frame:
- The node must
first wait ("defer")
a predefined amount of time
before the node
can
transmit
its frame
- If the channel is
idle after
waiting the
predefined time:
- The node will
transmit the
frame (immediately)
Schematically:
Otherwise:
- The node must
wait until
the current transmission is
over
- Then go back to step 1: defer the predefined wait time again before transitting !!!
- The node will
transmit the
frame (immediately)
- The node must
first wait ("defer")
a predefined amount of time
before the node
can
transmit
its frame
- Key to
prioritizing
the ACK transmission:
- Different types
of frames will use
different waiting (defer) time !!!
In fact:
- Higher priority frames will use a shorter waiting time !!!
Example:
- Different types
of frames will use
different waiting (defer) time !!!
- When a node want to
transmit a
frame:
-
Interframe Spacing
- Interframe Spacing (IFS) = the period of time that a (transmitting) node must wait (and listen) before the node can start transmitting a frame
- Partial
flow chart of
the 802.11 Medium Access Protocol:
Explanation:
- A (transmitting) node
must listen (monitor)
the transmission medium for
IFS amount of time
- The IFS for different types of frames are different !!!
- If there are no transmissions
for IFS amount of time,
the node will
transmit its
frame
- If the node detect a
transmission
during the
IFS wait time:
- The node will back off and try again later
- A (transmitting) node
must listen (monitor)
the transmission medium for
IFS amount of time
- Types of IFS:
- SIFS =
Short Interframe Spacing
(has the shortest duration)
- SIFS = 28 μsec
Usage:
- SIFS is used as sensing delay for transmitting ACK frames !!!
- PIFS =
Point Coordination Function (PCF)
Interframe Spacing
(has the second
shortest duration)
- PIFS = SIFS + 1 Slot Time = 78 μsec
Usage:
- PIFS is used as sensing delay by a base station (= coordination point) that operates in a special coordination mode
- DIFS =
Distributed Coordination Function
(DCF)
Interframe Spacing
(has the "normal"
duration)
- PIFS = SIFS + 2 × Slot Time = 128 μsec
Usage:
- DIFS is used as sensing delay for transmitting data frames
- EIFS =
Extended
Interframe Spacing
(has the
longest duration)
- EIFS = Ack frame duration + SIFS + DIFS
Usage:
- EIFS is used by a transmitting node that received a corrupted data frame
Explanation:
- Every node in
IEEE 802.11 will
continuously
receive and
CRC check
all frames !!!
- If a received frame
contains an (CRC) error then:
- A node will defer EIFS duration instead of DIFS before transmitting a data frame.
Schematically:
- Reason:
- The corrupt frame may be
a data frame for
another node
(far away -- that's why it was corrupt)
- That (other) node may
have received the
data frame
correctly !
- By waiting
EIFS, the node will
let the other node transmit the
ACK frame
without
collision !!!
- The corrupt frame may be
a data frame for
another node
(far away -- that's why it was corrupt)
- SIFS =
Short Interframe Spacing
(has the shortest duration)