# A Distributed Token Passing Protocol for Time Constrained Data Gathering in VANETs
## 1 Introduction  
Vehicular Ad Hoc Networks (VANETs) are a Mobile Ad Hoc Networks (MANETs) special case, typically allowing both Vehicle to Vehicle (V2V) or Vehicle to Infrastructure (V2I) wireless communications [1; 2], to support complex Intelligent Transportation System (ITS) applications [3; 4]. The main feature of VANETs is represented by the constrained and correlated node mobility patterns, which in turn implies an extremely time-varying network topology, with network partitioning and merging. Despite this drawback, a VANET is expected to give rise to an intelligent and cooperative eco-system in order to improve the driving experience, with special regard to safety applications [5]. A commonly referenced implementation of the VANETs paradigm is represented by the IEEE 802.11p protocol [6], which is complemented by the IEEE 1609 protocol suite [7; 8]. The set of IEEE 802.11p and IEEE 1609 is usually referred to as Wireless Access in Vehicular Environments (WAVE) in the U.S., while in Europe the corresponding is the cooperative-ITS (C-ITS) based on ITS-G5. In 2017, 3 GPP introduced the Vehicular-to-everything (V2X) communications in the Long Term Evolution (LTE) Release 12 standard, considering a new direct device-to-device (D2D) communication mode as a way to effectively support both public safety services and for Proximity Services (ProSe). D2D, indeed, introduces _direct_ single-hop communications between two devices, with limited or even without any Base Station (BS) support [9; 10]. These direct (or proximity) communications can achieve high data rates with low end-to-end delay, whilst they are time- and energy-consuming,due to the use of beaconing signals and scanning for direct user discovering. Moreover, as D2D communications usually use _unlicensed_ bands like Wi-Fi, they have the drawback that coverage and number of discovered devices are limited due the stochastic nature of interference over these bands. On the other hand, if a cellular network supports D2D communications (the so-called _in band_ D2D communications), it is also in charge of discovering D2D candidates and managing time-frequency resources, which could overload the cellular infrastructure [11; 12]. This paper considers a VANET-aided D2D discovery scenario for data gathering that offloads part of the signaling network traffic to the VANET for allowing low-rate D2D communications as in [13]. In the literature, broadcast approaches are commonly addressed mainly to deliver messages to the desired destinations, additionally via intermediate nodes. However, this approach can increase the traffic volume leading to the well-known _data storm_ effect. As a consequence, an open issue is represented by the design of a protocol, as an alternative to the classic store-and-carry routing paradigm that manages the data gathering and dissemination, together with the on-the-way information processing. To this purpose, in [14], the authors aimed at introducing the Token Ring (TR) approach in an IEEE 802.11 wireless network, which is referred to as Wireless Token Ring Protocol (WTRP). WTRP is a Media Access Control (MAC) designed for Ad Hoc network with dynamic topology and stringent requirements (i.e., band, latency and failure recovery). In particular, it arranges the network topology as a unique _ring_, in which every node has a _predecessor_ and a _successor_, which are a priori known before any transmission occurs. Despite WTRP soon being patented [15], the principle behind it has been investigated with reference to the MANET domain [16] and, in particular, it has been adopted in VANET for information collection and dissemination [17; 18].