Packet Switching in Radio Channels: Part II--The Hidden Terminal Problem in Carrier Sense Multiple-Access and the Busy-Tone Solution

We consider a population of terminals communicating with a central station over a packet-switched multiple-access radio channel. The performance of carrier sense multiple access (CSMA) [1] used as a method for multiplexing these terminals is highly dependent on the ability of each terminal to sense the carrier of any other transmission on the channel. Many situations exist in which some terminals are "hidden" from each other (either because they are out-of-sight or out-of-range). In this paper we show that the existence of hidden terminals significantly degrades the performance of CSMA. Furthermore, we introduce and analyze the busy-tone multiple-access (BTMA) mode as a natural extension of CSMA to eliminate the hidden-terminal problem. Numerical results giving the bandwidth utilization and packet delays are shown, illustrating that BTMA with hidden terminals performs almost as well as CSMA without hidden terminals.