- Sense duration in DCF:
(required)
- A node will
sense the channel for
DIFS
time before sending a
data frame
- A node will sense the channel for SIFS time before sending an ACK frame
This will make sure that ACK frames will always be transmitted before any data frame
- A node will
sense the channel for
DIFS
time before sending a
data frame
- Virtual Carrier Sensing
(Optional)
- The SIFS and
DIFS
mechanism will not
help solve the
hidden node problem
nor the
exposed node problem...
- To solve the hidden node problem, nodes may (optionally) use the Virtual Carrier Sense mechanism to reduce the likelihood of collisions
- The SIFS and
DIFS
mechanism will not
help solve the
hidden node problem
nor the
exposed node problem...
- The Virtual Carrier Sense protocol:
- A node that wants to transmit a
data frame first transmits
a small Request To Send (RTS)
frame that contains:
- Source ID
- Destination ID
- Duration (length) of transmission
- The destination node that
receives the RTS frame
will response with a
small Clear To Send (CTS)
frame that contains:
- Source ID
- Destination ID
- the same duration (length) from the RTS frame
Example:
- Effect of the RTS and CTS frames:
- All stations that receive a RTS or a CTS frame must refrain from transmitting for the during given in the length field of either frame
Example:
- In the example above, node C will not transmit for t sec
- A node that wants to transmit a
data frame first transmits
a small Request To Send (RTS)
frame that contains:
- The Virtual Carrier Sensing
procedure is 802.11
collision avoidance mechanism
This collision avoidance mechanism can effectively solve the hidden node and the exposed node problems !!!
- The Network Allocation Vector
is a map of
reservation periods
maintained by a 802.11 node
- When a node receives a
RTS or
CTS request,
it will mark off
reservation area in its
NAV.
- Example:
- A node will defer (= not access) the transmission medium during the reserved period in the NAV
- Recall the Hidden Node problem:
- Node A is
hidden
(outside the hearing range)
of node C
- Node C's transmission
can collide
with a transmission of node A
without
C's knowledge:
- Node A is
hidden
(outside the hearing range)
of node C
- The collision avoidance protocol
will effective solve
the hidden node problem:
- A sends a
RTS(t) to
B
- B sends back
a CTS(t)
- Node C will hear the RTS(t) message and refrain from transmission for t sec
Graphically:
- A sends a
RTS(t) to
B
- Recall the
Exposed Node problem:
- Node B and
node C
can hear each other
- Their simultaneous transmissions
may not collide
at their intended receivers:
- Node B and
node C
can hear each other
- The RTS from
node B
will shut C off completely:
- We can solve the
exposed node problem
with a modified
collision avoidance scheme:
- The RTS does
not
make reservation in the
NAV
- Only the CTS frame will make reservation in the NAV of a node
- The RTS does
not
make reservation in the
NAV
- Interestingly, the
exposed node problem is
is solved by the
modified
collision avoidance protocol:
- B sends a RTS(t)
frame to A
(This will not prevent C from transmitting)
- A sends a CTS(t) frame back
Notice that node C does not receive this CTS(t) frame
So C is NOT refrained from transmission.
Comment:
- The exposed node is
not a big problem
- The expose node problem
will result in
loss of transmission opportunity
It will not result in collision
- In contrast, the hidden node problem
is much more severe
The hidden node problem will result in collision
Loss of ACK frame is detrimental to system performance (backoffs !!!)
- B sends a RTS(t)
frame to A
- 802.11 Backoff Algorithm:
- 802.11 uses the binary exponential backoff algorithm of 802.3 (Ethernet)
- When to back off:
-
(Unlike Ethernet,
802.11
cannot detect
a collision)
- Backoff is used
when a transmission
is unsuccessful
- A transmission is unsuccessful if the ACK frame is not received within the time out period
- Backoff is used
when a transmission
is unsuccessful
- MAC algorithm:
- Initial attempt:
- Listen for
a IFS period
- If no
channel activity was sensed during
IFS period,
transmit the frame
(If the frame was an ACK frame, you are done - ACK frames are not ACKed !)
If you transmitted a data frame and did not receive the ACK in time, enter the Back off mode
- If channel activity is sensed during
IFS period,
go to Back off mode
(sender tries to randomize its transmission so that collision is minimized)
Note:
- This fact will be important in understaning the analysis of the 802.11 protocol later !!!!
- Listen for
a IFS period
- Subsequent attempts (Back off mode transmissions):
- Select the
(random) back off period
(See: click here )
- Remain inactive (but sensing
the channel all the time) for
the back off period
- The back off count down
can be interrupted
when the node detects
channel activity
The back off count down is stopped when channel activity is detected
- The back off count down
is only resumed
when the channel is idle
for IFS period
- When the back off count down
ends, the node
transmits immediately,
without having to
sense IFS period
If you get an ACK back, then you are done.
Otherwise, restart from the top.... (pick another random number)
- Select the
(random) back off period
(See: click here )
Further notes:
- After each successful
transmission, the
sender will perform a
back off
- This will alleviate
the channel capture phenomenon
(because other senders would have
larger back off periods and
the current sender will keep winning...)
- This is only a band-aid remedy - providing fairness in 802.11 is still active research area...
- Initial attempt:
- There are many problems with the
DCF protocol
And it is a very active research area
- Some research issues:
- DCF does not work completely accurately
(there may still be collisions
caused by hidden nodes):
click here
- DCF can cause congestion: click here
- DCF does not work completely accurately
(there may still be collisions
caused by hidden nodes):
click here
- Providing fairness in the DCF is an active research area...
- 802.11 MAC: click here