Carrier Sense Multiple Access (CSMA)

Carrier Sense Multiple Access (CSMA) is a type of Multiple Access Control (MAC) protocol used in data networks to control access to the communication channel. In CSMA, each device listens to the communication channel to determine if it is busy or idle before transmitting data.

There are three types of persistence methods used in CSMA:

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  1-persistent CSMA:

In this method, a device continuously senses the channel and starts transmitting as soon as it becomes idle. If the channel is busy, the device waits until it becomes idle before attempting to transmit again. This method is used in wired LANs, where the propagation delay is low, and collisions are rare.

• p-persistent CSMA:

In this method, a device senses the channel and waits for a random amount of time before attempting to transmit. The probability of transmitting data is p, and the probability of deferring is 1-p. If the channel is busy, the device waits for another random time period before attempting to transmit again. This method is used in wireless LANs, where the propagation delay is high, and collisions are common.

• Non-persistent CSMA:

In this method, a device senses the channel and waits for a random amount of time before attempting to transmit. If the channel is busy, the device stops sensing and waits for a random amount of time before sensing the channel again. This method is used in wireless LANs, where the propagation delay is high, and collisions are common.

CSMA works as follows:

1. Before transmitting data, a device listens to the communication channel to determine if it is busy or idle.
2. If the channel is idle, the device begins transmitting data.
3. If the channel is busy, the device waits for a random amount of time and then checks the channel again.
4. If the channel is still busy, the device waits for another random amount of time before checking the channel again. This process continues until the channel is idle, at which point the device can begin transmitting data.

CSMA is a simple and effective MAC protocol that can be used in networks with a small number of devices. However, as the number of devices in the network increases, collisions become more likely, which can reduce the efficiency of the network. To address this issue, CSMA can be enhanced with Collision Detection (CSMA/CD) or Collision Avoidance (CSMA/CA) mechanisms, which are used in Ethernet and Wi-Fi networks, respectively.