ALOHAnet, also known as ALOHA, was a pioneering computer networking system developed at the University of Hawaii. It was first deployed in 1970, and while the network itself is no longer used, one of the core concepts in the network is the basis for the widely used Ethernet.http://en.wikipedia.org/wiki/ALOHAnet
Jim Thompson said:802.11a and 802.11g (the non-b part) use OFDM.
802.11b uses CCK modulation (a form of direct spread)
and OFDM is *nothing* like an "inverse multiplexor".
Finally, its not the "two access" that (nearly) halve the throughput, but rather the delay time between them.
Sam Chepkevich said:Sorry for the long winded post. CSMA/CA is essentially the MAC (Medium Access Control) that ALOHAnet used. Slotted Aloha added slot times to minimize collisions. CSMA/CA is used in the IEEE 802.11 MAC. CSMA/CA or just CSMA requires two medium accesses for each transmission, one for the message and one for the acknowledgement. Although CA is for Collision Avoidance, collisions can occur as they do in CSMA/CD Ethernet.
CSMA/CD or Ethernet can detect a collision, but wireless cannot, thus, the CSMA/CA method waits for an acknowledgment from the other end to determine if the packet was transmitted properly. The two medium accesses result in less throughput, by roughly half.
So basically all of the 802.11a/b/g protocols use CSMA/CA of some form in addition to OFDM (Orthagonal Frequency Division Multiplexing) which works similiar to an inverse multiplexer to accomplish transmission.
http://www.rfcafe.com/references/electrical/wireless_comm_specs.htm shows the various technologies used in wireless and cellular communications.