CSMA

To control the access to a shared medium, Carrier Sense Multiple Access (CSMA) is the protocol used by several networking standards such as Ethernet and IEEE 802.11. The basic idea in CSMA is that a station listens to the channel before transmitting. CSMA has two variants: CSMA with collision detection (CSMA/CD) used in the wired Ethernet, and CSMA with collision avoidance (CSMA/CA) used in wireless wi-fi cards (IEEE 802.11).

What do we do?

Our research in LCA covers several aspects of CSMA/CA networks:

Wireless network adaptors are becoming more and more configurable without requiring any high skills in programming, neither in hardware manipulations. In DOMINO, we showed how a greedy user of a public hotspot can easily "tune" his wireless card to steal a bigger share of (if not all) the radio channel capacity. We proposed a set of mechanisms to detect cheating techniques in order to re-establish fairness in the network.

In an ad hoc (CSMA/CA) network, the network card configurability enables a single cheater to monopolize the channel capacity if acting in solo. In "On cheating in CSMA/CA networks" we showed that when two or more cheaters coexist, the network heads to collapse, without any payoff to any user. This collapse is an equilibrium state with zero efficiency. We showed how a group of selfish users can move this equilibrium point to a highly efficient and fair point of operation.


To put our undergrad students in touch with our research activities we use the Internet Engineering Workshop (where 60 computers are wirelessly connected) for two practical courses:

The Hands on exercises is a set of practical exercises that introduces IEEE 802.11 wireless networking, their settings, performance evaluation and their security vulnerabilities.

The ns-2 quick tutorial is a 4-hour crash course on wireless ns simlations. It covers ad hoc and infrastructure network simulations basics.

Publications