- CSMA or Carrier Sense Multiple Access is defined as a probabilistic MAC (media access control) protocol.
- In this
protocol the absence of the traffic is verified by the node prior to start
transmitting through a transmission medium that is shared by a number of
channels.
- For example, a band of EM spectrum, electric bus and so on.
- Feedback from the receiver is used by the
transmitter for determining whether the channel is already occupied by some
transmission process or not. - This is termed as the carrier sense.
- The
transmitter before starting a transmission starts detecting whether a carrier
wave is present their transmitted by some another station.
- If a carrier wave is
present, the transmission has no other option other than waiting for the transmission
in progress to end before starting its own transmission.
- To put it simply we
can say that the carrier sense is what the CSMA protocol is based up on.
- By ‘multiple access’, it simply means that a number of stations can
transmit and receive data through the same medium.
- But in this case all the
other stations receive the data transmitted by one node using that same medium.
There are many types of CSMA protocols as mentioned below:
1. CSMA with collision
detection:
- In short, this protocol is known as the CSMA/CD.
- This protocol
is used for improving the performance.
- This it does through the
termination of the transmission once a collision occurs along making
reductions in the probability of occurrence of the other collisions upon
re-transmissions.
2. CSMA with collision
avoidance:
- CSMA/ CA is the abbreviated form of this type of CSMA protocol.
- This protocol avoids collision for improving the performance of the same.
- It tends not to be greedy on the transmitting channel.
- If it happens to
sense the channel as busy prior to transmission then it is said to defer
to a random interval.
- This is a measure for reduction in the probability
of the packet collisions occurring on the channel.
3. Virtual time CSMA:
- In short, known as the VTCSMA.
- This type of CSMA protocol has been designed for
avoiding the collisions that are generated by the nodes.
- These are the
nodes that translate the signals at the same time.
- This CSMA protocol is
employed in the hard real time systems.
- Two clocks are used by each node
namely a real clock (rc) and a virtual clock (vc) for telling ‘real time’.
- If the channel happens to be busy, the virtual clock freezes and if not it
is reset.
- vc is not initialized if the channel is busy
and it runs faster when compared to the real clock.
CSMA
happens to operate in various modes as discussed below:
1-Persistent:
- The sender
stations keeps on sensing the transmission channel until it becomes idle
if previously it was busy.
- When it finally becomes, then it starts
transmitting the data packet i.e., the frame.
- If a collision is detected,
the transmitter has to wait for a random period of time before attempting
to re-transmit the signal.
- It finds use in systems such as Ethernet i.e.,
the systems using CSMA/ CD.
P-Persistent:
- This mode
lies somewhere between the non–persistent and the 1–persistent CSMA
access modes.
- Once the data is ready to be transmitted, it checks for the
status of the medium and starts transmitting the data packets with a
certain probability.
Non-persistent:
Less
aggressive when compared to the previous one.
O-Persistent:
It works based
according to the assigned transmission order.
