Implementing Correct and Efficient Collision Avoidance in Multi-Hop Ad-Hoc Networks

CSMA/CAP (Carrier-Sense Multiple Access with Collision Avoidance and Pilots) is introduced for ad-hoc networks in which each node has a single half-duplex radio. Using carrier sensing to access the common channel, a node with a data packet to send transmits a request-to-send (RTS) packet. If the RTS is sent without interference, the receiver sends a clear-to-send (CTS) packet followed by a pilot. A sender that receives a CTS for itself sends its data packet after a delay to let the pilot be heard and and then transmits its own pilot to make the total transmission time equal to a maximum data-packet length. The receiver sends its ACK after receiving the data packet and pilot from the sender. It is shown that no channel-access protocol based on the traditional RTS-CTS handshake over a single channel can guarantee collision-free transmissions of variable-length data packets and ACK's, and that CSMA/CAP does ensure that data packets and their ACK's are sent without multiple-access interference. The throughput of CSMA/CAP is compared with the throughput of CSMA in networks with and without hidden terminals, and the results show that the overhead of CSMA/CAP to eliminate collisions is small and that CSMA/CAP performs far better than CSMA when hidden terminals are present.