The media access control (MAC) layer of the vehicular ad-hoc network (VANET) is designed to provide fast and reliable vehicles to vehicles (V2V) or vehicles to infrastructures (V2I) access. With the development of autonomous driving and intelligent transportation, many sensor information in large-scale vehicles require efficient and reliable transmission with V2V and V2I technologies. In recent years, the demand for various services in VANETs is growing rapidly, which poses great challenges to the design of the MAC protocol. An adaptive scheme for channel frame structure can enhance channel access flexibility to cope with the changing network topologies in vehicular environments. In this paper, we adopt an adaptive frame scheme on the control channel and the service channels, which mainly improves two aspects of channel access performance: improved time slot utilization of the broadcasting period as much as possible while ensuring the vehicle can access the control channel quickly; and increasing the throughput of the service channels without generating excessive overhead. Analysis and simulation results demonstrate the performance of the proposed mechanism. INDEX TERMS VANET, multi-channel MAC protocol, adaptive frame structure.

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