Monday, July 8, 2013
- 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:
- 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.
- 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.
Less aggressive when compared to the previous one.
It works based according to the assigned transmission order.