Abstract
Vehicular Ad-hoc Networks (VANETs) can improve the road safety by transmitting safety-critical messages such as beacons and emergency messages. IEEE 802.11p VANETs have adopted the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism for the multiple access control. The 802.11p media access control (MAC) protocol, however, can not guarantee the reliability of broadcasting data, since the reception of transmitted messages are not acknowledged. Moreover, the backoff scheme of the 802.11p MAC utilizes a fixed-size contention window for safety message broadcasting, which causes high collision probabilities especially in dense environments. In order to improve such drawbacks, we propose a hybrid access method as follows: Nodes are equipped to reserve time slots for the next round of broadcasting, while unoccupied time slots are preserved for those which have emergency needs. In addition, implicit feedbacks are enabled for detecting collisions incurred during random channel accesses in preserved time slots. We devise a mathematical model which optimally controls the parameters of our scheme while minimizes the cost caused by idle channels and collisions. Extensive simulations show that our mechanism can remarkably improve the performance of VANETs in broadcasting of the safety messages.
Highlights
Vehicular Ad-hoc Networks (VANETs) provide safety services by allowing vehicles to transmit safety-critical messages periodically [1]
We present a hybrid channel access method to enhance the performance of 802.11p VANETs in broadcasting of safety messages, such that the safety critical messages can be transmitted with shorter transmission delays and higher reliabilities
3 A hybrid channel access method we report a hybrid channel access scheme exploiting the strengths of the time division multiple access (TDMA) scheme together with the 802.11p media access control (MAC) protocol: Considering one control channel interval (CCHI) as a sequence of numbered time slots, we propose to enable nodes to reserve time slots for broadcasting of safety messages while maintaining idle time slots available
Summary
Vehicular Ad-hoc Networks (VANETs) provide safety services by allowing vehicles to transmit safety-critical messages periodically [1]. The Wireless Access in Vehicular Environments (WAVE) standard which consists of the IEEE 1609 family and IEEE 802.11p [2–4] specifies most of the key technologies adopted by the VANETs. According to the IEEE 1609 standard, seven channels are allocated to VANETs, including one control channel (CCH) and six service channels (SCHs). The channel access time is divided into consecutive synchronization intervals, called Synchronization Intervals (SIs). Each SI has a fixed length of 100 ms and splits into two alternating 50 ms intervals - control channel interval (CCHI) and service channel interval (SCHI). During the CCHI, vehicles broadcast safety messages periodically for the purpose of advertisement, beacons, cooperative awareness, and etc
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