In most wireless ad-hoc multi-hop networks, a node competes for access to shared wireless medium, often resulting in collisions (interference). A node is commonly equipped with a transceiver that possesses mounted half-duplex omnidirectional antenna. Transmission degradation can occur when terminals are hidden from each other by physical structure, such as buildings. Moreover, since half-duplex nodes cannot receive while transmitting, not all packets sent by different terminals are detected by one another. In fact, no channel-access protocol based on the traditional handshake over a single channel can guarantee collision-free transmissions. Problems can arise in multi-hop wireless networks when hidden terminals, exposed transmitters, or exposed receivers are present.
To overcome these challenges, researchers at UC Santa Cruz (UCSC) have developed a new networking approach called Carrier-Sense Multiple Access with Collision Avoidance and Pilots (CSMA/CAP). This new scheme is introduced as a simple extension of the traditional CSMA collision avoidance (CA) approach for ad-hoc networks utilizing single half-duplex radios for each node and by leveraging carrier-detection techniques at the physical layer. Using carrier sensing to access the common channel under the CSMA/CAP framework, a node with a data packet to send transmits a request-to-send (RTS) packet, and if the RTS is sent without interference, the receiver sends a clear-to-send (CTS) packet back followed by a pilot. A sender that receives the CTS back sends its data packet after a delay to let the pilot be heard and then transmits its own pilot to make the total transmission time equal to a maximum data-packet length. Finally, the receiver sends its acknowledgement after receiving the data packet and pilot from the sender. UCSC’s CSMA/CAP ensures that data packets and their acknowledgements are sent without multiple-access interference. While there is overhead with CSMA/CAP in eliminating unwanted interference, the research results show that it is small, and that CSMA/CAP significantly improves performance in standard CSMA when hidden terminals are involved.
Pictured: Eliminating MAI from exposed transmitters using pilots
Collision avoidance, Data packet, Multiple Access Interference, Half-duplex transceivers, Router, Networking, Mesh network, Ad-hoc network