- The non-persistent CSMA:
- go away if the
channel is
busy
(and come back later.....)
- go away if the
channel is
busy
- Flow chart:
Explanation:
- If the node senses
that the carrier is idle:
- the node transmits the message in the next slot
After transmitting the message, the CSMA protocol terminates !
(Don't wait for ACK !)
Graphically:
- If the node
senses that
the carrier is busy,
it performs the following
backoff procedure:
- The node picks a
random (integer) number x
- The node then
waits:
x × τ sec
- and repeat the protocol from the start....
Graphically:
- The node picks a
random (integer) number x
- If the node senses
that the carrier is idle:
- Rationele
of the non-persistent CSMA:
- The non-persistent CSMA
is a "pessimistic" protocol:
- When a node
senses that
the transmission channel
is busy,
a pessimist will
think:
- There are other nodes that are also sensing that the channel is busy
Graphically:
- So if the
node transmits after
the current transmission
ends, then:
- There will definitely be a collision !!!
Graphically:
-
Therefore:
- The node will retry after some random amount of (wait) time to avoid the (hypothetical/potential) collision
- When a node
senses that
the transmission channel
is busy,
a pessimist will
think:
- The non-persistent CSMA
is a "pessimistic" protocol:
- Strength of non-persistent CSMA
- Achieves
automatic transmission rescheduling
in heavily loaded systems
Example:
- When many nodes
that have messages to transmit
(= heavily loaded system),
it is likely that
multiple nodes
are waiting:
- Each node will then
pick a (different)
random number:
- The node that
picks the
smaller random number (= B) will
transmits
first:
- The node that
picks the
larger random number (= C) will
back off
again:
- When many nodes
that have messages to transmit
(= heavily loaded system),
it is likely that
multiple nodes
are waiting:
- Achieves
automatic transmission rescheduling
in heavily loaded systems
- Weakness of non-persistent CSMA
- Possible channel wastage
in a lightly loaded system
Example:
- In a light loaded system,
there are not many nodes
waiting to
transmit:
- Suppose the node
pick a large random number:
- Wastage of
transmission time:
- In a light loaded system,
there are not many nodes
waiting to
transmit:
- Possible channel wastage
in a lightly loaded system