# A Distributed Token Passing Protocol for Time Constrained Data Gathering in VANETs
## 2 Related Work
### Token-Based Medium Access Control Protocols for VANET  
One of the most challenging research topics in VANET is to efficiently design a scheme for accessing the network medium. There has been some effort in the research community to adapt the classical wired-based medium access TR protocols to wireless communications. The reliable neighbor-cast protocol (RNP) [27] uses a token as an acknowledgment message to deliver reliable multicast among nearby vehicles selected by a a voting scheme as a neighbouring group. Wireless TR protocol (WTRP) has been presented in [14] as a novel protocol for a wireless environment designed for an ad hoc network with dynamic topology and stringent requirements (i.e., band, latency and failure recovery). The aim of WTRP was to guarantee the quality of service (QoS) in terms of bounded latency and reserved bandwidth. Additionally, this protocol improved the access efficiency by reducing the number of retransmissions due to collisions. This was done by creating a _virtual_ ring where nodes, upon joining a network, are required to be connected to a predecessor and a successor node. However, WRT is applied to ITS for guaranteeing QoS and recovering from multiple failures, but this protocol does not react to the typical fast topology changes of VANETs. Since the WTRP protocol was proposed, some other improved versions of this protocol have been published in literature. In [28], an enhanced version of the WTRP token passing-based MAC protocol is presented (EWTRP). The improvements in comparison to WTRP include a preemption mechanism, a hibernation mechanism and a contention mechanism. A comparison between EWTRP and WTRP was carried out through analyses. Results show that EWTRP produces higher throughput while it consumes less power and is more suitable to operate in small-scale wireless ad hoc networks. Additionally, the Ripple protocol is presented as a wireless token passing-based protocol especially designed for wireless mesh networks in [29]. This protocol improves existing random-access approaches by proposing a decentralized controlled-access protocol to protect nodes from unintentional packet collisions and to enhance the throughput of the network. In [30], the authors of this study propose a token ring-based protocol for multi-channel routing to improve the performance of mesh networks. This performance is obtained by defining delay-guaranteed rules for the actions of joining a ring and creating new rings. The authors present a state machine to define their protocol. Analytical results on bounded delay are presented and show a good performance when compared to state-of-the-art approaches. Furthermore, [PERSON] et al. propose an automatic adjustment of the token path w.r.t. to the subnet's dynamic topology which is not always arranged in a ring shape [31]. Furthermore, the operation of token path maintenance is simplified and the channel efficiency is increased also in WDTP. In [32], [PERSON] et al. propose a multi-channel TR media access control protocol (MCTRP) for inter-vehicle communications. By means of an adaptive ring coordination and a channel scheduling, vehicles are autonomously managed into different rings, operating on diverse service channels. The authors show that this topology management allows special messages, such as emergency messages, to be disseminated with a limited delay. Additionally, they present a token based data exchange protocol that improves the network throughput for non-safety multimedia applications. In the Overlay Token Ring Protocol (OTRP) [33], vehicles are arranged into _multiple_ overlapped virtual rings, where a unique token performs the control functions. The ring architecture dynamically follows the traffic condition to provide data transmission with QoS and reliable safety messages' exchange.