While you need to know CSMA/CD for the course, you should be aware it is deprecated – it is not used anymore. It is still available for legacy equipment, but that kind of equipment is no longer available to buy. CSMA/CA (for wireless) is still used.
CSMA/CD and CSMA/CA are protocols to detect and prevent data collisions when two or more devices transmit data simultaneously.
CSMA/CA
For multiple wireless devices to connect to a network they need to communicate with a wireless access point. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) differs from CSMA/CD by avoiding any collisions in the first place.
Before a packet is sent wirelessly the device first checks to see if the medium is clear, if it is it sends the packet. If it is not clear it will set a random time (backoff) until it tries again, first checking to see if the medium is clear.
If there are hundreds or thousands of devices on the same wireless network the time it takes to transmit can grow due to waiting for many collisions to be avoided.
CSMA/CD —- (Deprecated)
When multiple wired devices are connected to a network they need to communicate with each other. Before Switches were commonplace (which do allow simultaneous connections) we had Hubs which meant devices would broadcast to everything, sometimes simultaneously – this resulted in (electrical) signals sent at once over the same wire (medium). Carrier Sense Multiple Access with Collision Detection (CSMA/CD) made sure if data was sent at once it would detect it and deal with the problem.
If a collision is detected it will be detected by all devices trying to communicate. This would trigger a random timer on each device to indicate how long it will wait until it tries sending the packets again. This will continue until all the packets are sent, or it times out.