Enhanced Algorithms for the IEEE 802.11p Deployment in Vehicular Ad Hoc Networks

The Enhanced Distributed Channel Access (EDCA) and the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) algorithms are used in the IEEE 802.11p standard to support Quality of Service (QoS) and reliable communication in Vehicular Ad hoc Networks (VANETs). An essential part of the CSMA/CA process is when a packet in an Access Category (AC) waits for an Arbitrary Inter-Frame Space (AIFS) period of time before engaging in the the backoff phase. According to the standard, AIFS values are fixed and deterministic, they do not guarantee strict priority for ACs especially when these values are combined with the Contention Window (CW) size of the backoff phase. On the other hand, the AIFS values are not adjustable, they do not adapt to the current status of the medium. In this paper, we propose two algorithms for AIFS value assignment, the Strict Priority Algorithm (SPA) and the Adaptive AIFS Algorithm (A(3)). With SPA, the AIFS values are fixed, but they are determined according to a mathematical formula that ensures strict priority level among the ACs. With A(3), the AIFS values are adaptively changing depending on the value of the collision probability. AIFS still maintains a strict priority level assignment among the ACs.