Carrier Sense Multiple Access (CSMA)

CSMA is the abbreviation for Carrier Sense Multiple Access. This networking protocol listens to network signals on the carrier/medium before transmitting data on the media. CSMA is implemented in Ethernet networks with more than one computer or network device attached to it. It is part of the Media Access Control (MAC) protocol. The protocol decreases the chances of collisions when two or more stations start sending their signals over the data-link layer. It requires that each station first check the state of the medium before sending.

Carrier Sense Multiple Access/Collision Detection (CSMA/CD)

Carrier Sense Multiple Access/Collision Detection (CSMA/CD); is the most widely used transmission method in half-duplex Ethernet networks.  On Ethernet, any device can attempt to send a frame anytime. Each device senses whether the line is idle and available for use. If it is, the device begins to send its first frame. If another device has tried to send simultaneously, a collision occurs, and the frames are discarded. Then, each device waits a random amount of time and retires until it successfully gets its transmission again.

Carrier Sense Multiple Access with Collision Avoidance

CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is a method for carrier transmission in 802.11 networks. If a collision has occurred in the wired network, then the energy of the received signal almost doubles, and the station can sense the possibility of collision. In the case of wireless networks, most of the energy is used for transmission and the energy of the received signal increases by only 5-10% if a collision occurs. The station can’t use it to sense collision.

Therefore, CSMA/CA has been specially designed for wireless networks. It uses a method similar to CSMA/CD to detect if the media is clear. CMSA/CA also uses additional techniques. This method does not detect collisions but attempts to prevent collisions before they happen. Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know how long the medium will be unavailable.

In CSMA/CA, when a node receives traffic for sending, it checks to ensure the channel is clear (no other node is transmitting at the time). The packet is sent when the channel is clear. If the channel is not clear, the node waits for a randomly chosen period of time and then checks again to see if it is clear. This period of time is called the backoff factor and is counted down by a backoff counter.

The node transmits the packet if the channel is clear when the backoff counter reaches zero. If the channel is not clear and the backoff counter reaches zero, the backoff factor resets again, and the process repeats. Carrier sense multiple access with collision avoidance (CSMA/CA) is the least popular of the access methods. The WLANs now use this access method.