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Monday, July 8, 2013

Explain Carrier Sense Multiple Access Protocols - CSMA

- 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. 

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