Brief information about all CSMA protocols and types..

1. CSMA Protocols
PREPARED BY : MANAL SHAH
ENROLL. NO. :120280111014

2. CSMA
• Carrier sense multiple
access (CSMA)
• It is a probabilistic media access
control (MAC) protocol in which a
node verifies the absence of
other traffic before transmitting on
a shared transmission medium,
such as an electrical bus, or a band
of the electromagnetic spectrum.

3. Principle: "sense before transmit" or "listen before talk"
• Carrier sense means that a transmitter attempts to determine
whether another transmission is in progress before initiating a
transmission. That is, it tries to detect the presence of a carrier
wave from another node before attempting to transmit. If a carrier
is sensed, the node waits for the transmission in progress to end
before initiating its own transmission.
• Multiple access means that multiple nodes may send and receive on
the medium. Transmissions by one node are generally received by
all other nodes connected to the medium

4. Protocol modifications
• CSMA with collision detection
 CSMA/CD is used to improve CSMA performance by
terminating transmission as soon as a collision is detected, thus
shortening the time required before a retry can be attempted.
• CSMA with collision avoidance
 In CSMA/CA collision avoidance is used to improve the
performance of CSMA. If the transmission medium is sensed busy
before transmission, then the transmission is deferred for a random
interval.

5. Protocol modifications
• Virtual time CSMA
VTCSMA is designed to avoid collision generated by nodes
transmitting signals simultaneously, used mostly in hard real-time
systems. The VTCSMA uses two clocks at every node, a virtual clock
(vc) and a real clock (rc) which tells "real time". When the
transmission medium is sensed to be busy, the vc freezes, when the
transmission medium is free, it is reset. Hence, calculating vc runs
faster than rc when channel is free, and vc is not initiated when the
transmission medium is busy.

6. several types of CSMA protocols
• 1-Persistent CSMA
• Non-Persistent CSMA
• P-Persistent CSMA

7. several types of CSMA protocols
• 1-Persistent CSMA
• Sense the channel
 If busy, keep listening to the channel and transmit
immediately when the channel becomes idle
 If idle, transmit a packet immediately
• If collision occurs
• Wait a random amount of time and start over again
 1-persistent CSMA is used in CSMA/CD systems including Ethernet.

8. several types of CSMA protocols
• Non-Persistent CSMA
• Sense the channel
If busy, wait a random amount of time and sense the channel again
If idle, transmit a packet immediately
• • If collision occurs
wait a random amount of time and start all over again
Merits
 Better channel utilization
 Reduces chances of collision
 Reduces efficiency

9. P-persistent CSMA
• Applied for slotted channels.
• This is an approach between 1-persistent and non-persistent CSMA
access modes.
• When the transmitting node is ready to transmit data, it senses the
transmission medium for idle or busy. If idle, then it transmits
immediately. If busy, then it senses the transmission medium
continuously until it becomes idle, then transmits a frame
with probabilityp. If the node does not transmit (the probability of
this event is 1-p)
• p-persistent CSMA is used in CSMA/CA systems including Wi-Fi and
other packet radio systems.

10. Thank you….