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 at any time. 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 at the same time, a collision occurs and the frames then discarded. Then each device waits a random amount of time and retires until successful in getting 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 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 received signal increases by only 5-10% if a collision occurs. It can’t be used by the station to sense collision.

Therefore CSMA/CA has 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 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 sure that 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 the channel is clear. This period of time called the backoff factor and counted down by a backoff counter.

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