Carrier Sense Multiple Access (CSMA) is a media access control protocol designed to avoid signal collisions during transmission by allowing nodes to sense the channel before transmitting. The document outlines various CSMA modes, such as 1-persistent, non-persistent, and p-persistent, each with different methods for addressing channel usage and collision risks. CSMA improves channel utilization while reducing the chances of collision, especially in shared channel environments like Wi-Fi.

1. CARRIER SENSE MULTIPLE ACCESS [CSMA]
 PREPARED BY : MUSKAN DHAWAN
 YOUTUBE CHANNEL : IT Hub by Muskan Dhawan
 E-MAIL ID : muskan.dhawan99@gmail.com

2. CSMA ( carrier sense multiple access )
 CSMA is media access control (MAC) Protocol .
 Aim: avoid collision of signals during transmission.
 Working : it sense or listens whether the shared channel for transmission is
busy or not, and transmits if the channel is not busy .
 Here single cable connects multiple nodes .
Working:
when a station has frames to transmit , it attempt to detect presence of the
carrier signal from the other nodes connected to shared channel. If a carrier
signal is detect, it implies that a transmission is in progress .
the transmission waits till the ongoing transmission
executes to completion , and then initiates its own transmission.
Since, the nodes detect for a transmission before sending their own frames,
collision of frame is reduced.
Worst case: if two nodes detect an idle channel at the same time , they may
simultaneously initiate transmission and signals will collide .

3. Example

4.  1 –persistent
 Sense the channel
_ if busy , keep listening to the channel nd transmit immediately
when the channel
becomes idle.
_ if idle , transmit a packet immediately.
 if collision occures
_ wait a random amount of time and start over again.
This is an aggressive transmission algorithm.
Here host transmit with a probability of 1.
That’s why its called 1 persistent.

5.  Non persistent
• Sense the channel
_ if busy , wait a random amount of time and sense the
channel again.
_ if idle , transition a packet immediately.
° if collision occures
Merits
 Better channel utilization
 longer delays
 reduce chance of collision
 Reduce efficiency

6. condition
 If only B becomes ready in the middle of A’s transmission
_ 1- persistent : B succeed as soon as A ends .
_ non persistent : B may have to wait.
 if B and C becomes ready in the middle of A’s transmission
_ 1 – persistent: B and C collide
_ Non-persistent : B and C probably do not collide

8. P-persistent CSMA
Hybrid Approach
 This approach is between 1-persistent and non-persistent CSMA access modes.
 When the transmitting node is ready to transmit data ,it sense the transmission
medium for idle or busy . If idle then it transmits immediately. If busy then it
sense the transmission medium continuously until it becomes idle., then
transmits with probability ( p). If the node doesn’t transmit (the probability of
the event is 1-p) it wait until the next available time slot.
 If the transmission medium is not busy it transmits again with the same
probability [p]. This probabilistic hold-off repeats until the frame is finally
transmitted or when the medium is Found to become busy again ( some other
node has already started transmitting) then whole logic cycle repeated.
 P – persistent CSMA used in wi-fi.

9. THANKYO
U