# A Distributed Token Passing Protocol for Time Constrained Data Gathering in VANETs
## 2 Related Work
### Token-Based Medium Access Control Protocols for VANET
In this paper, differently from the OTRP scheme, the ring is not formed a priori, but the neighbours' discovery is carried out hop-by-hop _without_ a complete list of nearby devices, such that the resulting ring does not have a specific size, i.e., it is just a _virtual_ ring originated in the initial unit (e.g., the traffic light), connecting the visited vehicles, and reaching again the initial unit in order to gather the collected information within a time deadline. In addition, in the OTPR scheme, only a few devices (about six) can be grouped together to form overlapped rings; as a consequence, this is more suited for dynamic traffic conditions, rather than for an emergency scenario, where it is needed to exchange messages among a high number of vehicles. In the literature, several research papers are mainly focused on _dissemination protocol_ as shown in recent surveys [34; 35]. Blind data flooding is the simplest dissemination way but has limited performance due to a high percentage of redundant data and packet collisions which lead to the broadcast storm problem. Indeed, Geocast data dissemination protocols consist of sending data only to vehicles inside a specific geographical area [36]. Recently, there are in particular novel and efficient proposals identifying a small group of vehicle for re-broadcasting the messages [37]. Data dissemination is more stringent for Vehicle Delay Tolerant Networks (VDTN) with intermittent and opportunistic connections among vehicles than VANET. For example, in [38], the D\({}^{2}\)NFCE algorithm determines the node forwarding capability for data dissemination first by evaluating the effective connection time period then building a predictive traffic model based on wavelet neural network and, finally, fitting the historical and predictive throughputs of vehicles. On the contrary, _data gathering_ schemes have recently been proposed for VANET. Data collection protocol can be organized with a centralized node, e.g., a cluster head (CH) which collects the data of its vehicles and delivers it to the initial unit (e.g., the RSU) using a flooding technique, as in TrafficGatherprotocol [39]. The centralized node can be the eNodeBs of LTE network in an urban environment that creates several clusters of vehicles. The cluster topology is broadcasted to the vehicles by eNodeB. Each cluster head delivers the aggregate data to the eNodeB, which deletes redundant and undesired data, as shown in LTE4V2X [40]. A centralized medium access technique based on space division multiple access (SDMA) is also used in the Clustered Data Gathering Protocol (CDGP) organizing the data collection with the election of a cluster head among the various segments in which the road is divided. The CH assigns a slot to the vehicle and, if data are not available, the entire slot will be lost. The use of a token is considered by the same authors to reduce the loss of slots in the new versions: (i) the token based cluster data gathering protocol (TCDGP) and (ii) its enhanced version Distributed Data Gathering Protocol (DDGP) [17]. Both proposed protocols consider the election of a vehicle as a CH within a specific area of a highway to collect data, while [PERSON] et al. [41] extend this protocol towards an urban environment. Specifically, a sub-set of vehicles is selected to act as relay nodes (RNs), thus creating a temporary backbone network that can be used for data dissemination and collection. In the _discovery_ phase, the selection of vehicles traveling in the region of interest is achieved by broadcasting a request message from the RSU, which is in turn forwarded across the formed network in a multi-hop way. In a different way, according to our proposal, each selected vehicle is responsible for iteratively selecting the best neighbour node (i.e., the one with more data stored on board), thus forming a truly self-organized VANET, without HELLO messages broadcasting. Other protocols are based on a random medium access scheme, such as the Clustered Gathering Protocol (CGP) [42].