Fairness-Based Measures for Safety-Critical Vehicular Ad-Hoc Networks

Transmission timing and delay between communicating vehicles are two important performance measures for safety-critical applications of Vehicular Ad-Hoc Networks (VANETS). Safety-Critical VANETS are composed of multiple nodes each requiring access to a shared communication medium. This access is controlled by the Medium Access Control (MAC) protocol. Numerous MAC protocols have been proposed for VANETS, each with varying degrees of reliability and fairness. A general measure for fairness is required in order to compare different protocols using the same criteria, however, finding a quantifiable measure for fairness is not a simple task and any developed quantifiable measure is application-specific. In this paper, we investigate different measures of fairness, particularly the Jain Index and the Gini Coefficient. We apply these measures to experimental environments in ns-3 to contrast the different fairnesses of three MAC protocols: CSMA/CA, TDMA, and STDMA. It became evident, after experimentation, that the Gini Coefficients obtained from these experiments would vary between iterations and the limits of these variances were uncertain. Therefore we derive a formula which allows a theoretical limit to be placed on the unfairness of a system based on its upper and lower bounds of delay. This formula is not only relevant to VANETs but to any network. The work in this paper is applicable to any discipline wishing to measure the theoretical worst case fairness of a population.