There are two general types of WiFi transmission: DCF (Distributed Coordination Function) and PCF (Point Coordination Function). DCF is ethernet in the air. It employs a very similar packet structure, and many of the same concepts. There are two problems that make wireless different then wired.

• The hidden substation problem.
• High error rate.

These problems demand that a DCF WiFI be a CSMA/CA network (Collision Avoidance) rather than a CSMA/CD network (Collision Detect). The result are the following protocol elements,

• Positive Acknowledgement. Every packet sent is positively acknowledged by the receiver. The next packet is not sent until receiving a positive acknowledgement for the previous packet.
• Channel clearning. A transmission begins with a RTS (Request to Send) and the destination or receiver responds with a CTS (Clear to Send). Then the data packets flow. For the channel is cleared by these two messages. All that hear the CTS squelch. This helps with the hidden substation problem.
• Channel reservation. Each packet has a NAV (Network Allocation Vector) containing a number X. The channel is reserved to the correspondents (the sender and receiver of this packet) for an additonal X milliseconds after this packet. Once you have the channel, you can hold it with the NAV. The last ACK contains NAV zero, to immediately release the channel.

As for PCF, it is a polling, token-ring type communcation system. We shall skip the details. It isn't much used.