CS485 Sylabus

The Non-persistent CSMA protocol

• Non-persistent CSMA:

• The non-persistent CSMA:

 go away if the channel is busy (and come back later.....)

• 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:

• Rationale of the Non-persistent CSMA protocol

• 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

• Strength and weakness of Non-persistent CSMA

• 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:

• 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: