When does CSMA become ALOHA?

Tien V. Nguyen (1) , François Baccelli (2, 3, 1) , Kai Zhu (4) , Sundar Subramanian (5) , Xinzhou Wu (5)

1 TREC - Theory of networks and communications
2 University of Texas at Austin [Austin]
3 LINCS - Laboratory of Information, Network and Communication Sciences
4 ASU - Arizona State University [Tempe]
5 Qualcomm Technologies, Inc.

The broadcast of periodic messages is a key functionality in vehicular ad hoc networks. In the emerging vehicular networks, IEEE 802.11p is the standard of choice to support the PHY and MAC layer functionalities. The broadcast process in IEEE 802.11p is based on the CSMA mechanism where a device transmitting a packet senses the channel for on-going transmissions and performs a random back-off before accessing the channel. In spite of the absence of RTS/CTS mechanisms, carrier sensing is expected to provide a protection region around the transmitter where no other transmitters are allowed. However, at increasing densities, the CSMA behavior breaks down to an ALOHA-like transmission pattern indicating a lack of protection around transmitters. In this paper, we model the CSMA mechanism as a slotted system and analytically characterize the critical node/packet arrival density where the CSMA mechanism approaches an ALOHA-like behavior. Further, when the node density exceeds the critical density and ALOHA-type of behavior dominates, we can use tools from stochastic geometry to establish closed form expressions for the key performance metrics of the broadcast mechanism.

Domaines: Réseaux et télécommunications [cs.NI]

Tien V. Nguyen, François Baccelli, Kai Zhu, Sundar Subramanian, Xinzhou Wu. When does CSMA become ALOHA?. Allerton 2012 - 50th Annual Allerton Conference on Communication, Control, and Computing, Oct 2012, Monticello, IL, United States. pp.1634-1636, ⟨10.1109/Allerton.2012.6483416⟩. ⟨hal-00940552⟩