- The algorithm to
re-transmit a
old frame is
as follows:
Explanation:
- The protocol enter
this section when:
- The node has detected a ongoing transmission or
- The node did not receive an ACK frame (assumed collision !!!)
Conclusion:
- The node now knows that there are other node(s) that is trying to transmit
- The node will first
select a
random number x from the
current
back off range [0 .. CW]
(See: click here )
- The node tries to auto-schedule its transmission so the new attempt will not collide with that of other node(s)
- The node will
wait (= count down) for
x×σ sec of
idle channel time
- Idle channel time =
the time when there are
no transmissions on the
wireless network
- σ = slot time (20 μ sec)
More info on the count down procedure:
- The node keeps
sensing the
transmission channel while
counting down
- If the node detect a
transmission while
counting down:
- the node will stop the count down until the channel is idle (clear)
(because the channel is not idle, the back off countdown only counts the idle channel time !!)
- The count down will
only be
re-started:
- After the transmission channel is idle for DIFS sec !!!
(This count down procedure will be explained in more details in the next webpage)
- Idle channel time =
the time when there are
no transmissions on the
wireless network
- At the end
of the back off count down:
- The node transmits the frame immediately --- without having to wait IFS period
Possible outcome of the trasnmission:
- If the node is successful
(= received an ACK frame in time):
- Done !!!
- If the node is
unsuccessful
(= no ACK frame):
- The node
increases the back off range
(unless it reach the maximum range) and
- Select another
random number
- redo the back off step.
- The node
increases the back off range
(unless it reach the maximum range) and
- The protocol enter
this section when: