The IEEE is working on a MAC standard (802.11p) for VANET based on 802.11e. Although the access function of 802.11e supports QoS by using different backoff counters and inter-frame spaces for different QoS requirements, this reduces to a best effort service and low performance when the number of vehicles increases. Some applications of VANET, particularly safety related, have strict QoS requirements that cannot be guaranteed by 802.11p especially in heavy traffic conditions. To resolve these issues, we propose Space-Orthogonal Frequency-Time medium access control (SOFT MAC) protocol that can support QoS requirements and is compatible with 802.11 standard. The proposed MAC allocates guaranteed transmission slots via reservation and also has a random access period for best effort service. Reservations are allocated in a distributed manner without the need for a basestation or a cluster head. In this paper we analyse and discuss in details the rules and algorithms that govern SOFT MAC protocol and also explain its implementation using 802.11. The analysis of SOFT MAC proves it achieves higher saturation throughput than 802.11.
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