Stabilization time comparison of CSMA and Self-Organizing TDMA for different channel loads in VANETS

Traffic safety-related messages have to meet low and predictable delay constraints. For vehicles using IEEE802.11p medium access control (MAC) algorithm the channel access delay increases unpredictably (using random backoff time) every time the channel is sensed busy. In contrast, Self-Organizing Time Division Multiple Access (STDMA) provides an upper bound on channel access delay defined by the selection interval (SI) length. Our contribution studies the performance of both protocols during the start-up phase of the vehicular ad-hoc network (VANET). Results show MAC-to-MAC delay of each correctly decoded packet for lightly-loaded and heavily-loaded scenarios. The cumulative distribution function (CDF) of these measurements for a maximum observable MAC-to-MAC delay value of 100ms for different vehicle densities show that both MAC algorithms perform reliably (above 90%) within 60s simulation. We define stabilization time as the time instant from which on the MAC protocol reaches a reliable performance. For lightly-loaded scenarios with 25 vehicles within range, STDMA and the IEEE802.11p MAC algorithm have a stabilization time of 1s and 20ms, respectively. For heavily-loaded scenarios with 400 vehicles within range, STDMA and the IEEE802.11p MAC algorithm have a stabilization time of 1s and 3.2s. In conclusion, STDMA shows a reliable performance and better predictability, regardless of the number of vehicles accessing the channel, and it also provides lower stabilization time in comparison to IEEE802.11p MAC algorithm for vehicle densities higher than 350 vehicles sending periodic messages every 500ms.