Elsevier

Ad Hoc Networks

Volume 21, October 2014, Pages 42-59
Ad Hoc Networks

Enhanced VANET broadcast throughput capacity via a dynamic backbone architecture

https://doi.org/10.1016/j.adhoc.2014.04.008Get rights and content

Abstract

We propose a high throughput dissemination scheme for Vehicular Ad Hoc Networks (VANET) based on the dynamic formation of a multi-hop backbone network. We describe and analyze such a networking protocol when used to broadcast message flows generated by a Road Side Unit along a linear road, forming a VANET structure that is identified as a Vehicular Backbone Network (VBN). The VBN is based on elected vehicles that reside at preferred locations to act as backbone relay nodes, jointly optimally configured to operate at designated link data rates and spatial reuse factors. We present both an analytical evaluation of the broadcast throughput capacity that can be achieved by using the VBN mechanism and a simulative analysis based on the IEEE 802.11p CSMA/CA MAC protocol, standardized for VANET. The results demonstrate the significant performance enhancement achieved under VBN in attaining enhanced throughput capacity rate and in providing for a wider dissemination coverage span. Admitted packets are assured low end-to-end packet delays through the employment of a flow control scheme.

Introduction

Vehicular networking has significant potential to enable applications associated with road traffic safety, info-mobility, urban sensing, info-tainment and broadcasting of public safety message flows [1], [2]. Wide interest in this area has resulted in the development of Vehicular Ad hoc NETwork (VANET) architectures and standards, including Wireless Access in Vehicular Environment (WAVE) recommendations. The latter makes use of Standards such as IEEE 802.11p and IEEE 1609, aiming to deliver safety and non-safety applications to vehicles on the road [3]. The IEEE 802.11p is designed to operate on a 75 MHz band positioned at 5.850–5.925 GHz. This band is divided into one central Control CHannel (CCH) and six Service CHannels (SCHs). While the IEEE 802.11p defines PHY and MAC layers, the IEEE 1609.4 describes the functioning of the upper part of the MAC layer, including channel switching and per-packet channel routing. ETSI has standardized a VANET protocol based on an IEEE 802.11p profile, called ITS-G5, which uses the Carrier Sense Multiple Access (CSMA) MAC [4]. A description of MAC protocols used in these standards is given in [5].

A key component for providing services by using VANETs involves the use of data broadcasting that allows vehicles to receive information on events that are detected on the road as well as to receive data from the road infrastructure through the use of a Road Side Unit (RSU) [6], [7].

In comparison with the bulk of previously published papers, our focus here involves broadcasting for a class of applications enabling sustained information rates, e.g., info-tainment services, or public safety message flows. The objective is to disseminate packet flows to a wide set of vehicles, over an extended area, much larger than RSU direct-transmission coverage area. Our key performance objective is to maximize the aggregate flow rate that can be accommodated while avoiding network system overloading, and thus assuring admitted packets with low end-to-end dissemination delays and a high probability of reaching all vehicles traveling along the underlying highway. This represents the system’s broadcast throughput capacity rate (also briefly identified here as the broadcast capacity [8]). To this aim, we leverage on a communication structure based on a backbone. We propose a backbone node election and the relevant forwarding protocols to be implemented in a forwarding layer in a manner that is transparent to the operation of the MAC layer; this approach decouples the multiple access technology from the dissemination logic supporting the broadcast VANET applications. Our models can be employed to optimally synthesize the backbone structure of each road segment in an area of operation that consists of a mesh of highways where RSUs or cellular micro-base-stations are placed at strategic points, such as road intersections. Messages broadcasted by such nodes are captured in each road segment by vehicles that are elected to act as Relay Nodes (RNs) and are then distributed in a multi-hop fashion across the optimally synthesized Vehicular Backbone Network (VBN). RNs act as base stations do in a cellular network, except that they are mobile, dynamically elected, and may serve as backbone forwarding nodes for a limited period of time, and/or for a limited number of packets. The use of this concept in the field of the Mobile Ad-Hoc Networks was studied in [9] and the references therein, where such Mobile Backbone Network (MBN) systems are synthesized and studied.

In this paper, we thus focus on the synthesis and analysis of a network that serves the downstream distribution of broadcast flows from the RSU to vehicles traveling along a single highway road.

The main contributions of this work are:

  • We provide motivations for building a backbone network to achieve an efficient data dissemination in a VANET; the novelty of the proposed approach, with respect to schemes that select nodes to forward messages on a packet by packet basis (see Section 5) or to other clustered approaches, is that we elect nodes that act RNs for a period of time based on the proximity of such nodes to targeted optimal positions.

  • We analytically study the broadcast capacity that can be achieved in a linear road by using our backbone structure based on targeted positions; both cases of an idealized setting of network nodes and nodes that are randomly displaced (e.g., due to mobility of the vehicles) are analyzed; we show our analytical models to be highly effective in providing for the setting of the network parameters, leading to optimal synthesis of the network system and to the dynamic configuration of the employed data rates and spatial reuse factors.

  • We describe algorithms and protocols that are used to identify and elect vehicles as RNs, which act as backbone nodes in the formation of a dynamically configured backbone network; we also discuss some implementation issues involving the employment of TDMA and CSMA/CA based medium access control schemes.

  • We present, compare and discuss the performance behavior of such protocols when used for the broadcasting of message flows generated by an RSU and disseminated along a linear highway segment. To attain high throughput rate, packet flows are pipelined along the network, offering high spatial reuse factors. We show that (under both contention-less and contention oriented MAC schemes), when synthesizing the VBN backbone in an optimal manner, we can achieve high throughput rate coupled with low end-to-end delays and a high packet delivery ratio.

The remainder of the paper is organized as follows. Section 2 surveys related work and provides the motivation for the VBN system studied in this paper. Analytical derivation of the broadcast capacity rate attained by VBN, under an adaptive-rate TDMA MAC, is presented in Section 3. Distributed RN election and backbone formation implementation schemes are described in Section 4. In Section 5, we discuss packet-by-packet protocols that are used for our performance comparisons. In Section 6, we exhibit and discuss performance evaluation results attained by the VBN system under different perspectives. Conclusions are drawn in Section 7.

Section snippets

Related work and motivation for the VBN system

In the framework of VANETs and for the class of applications under consideration in this paper (i.e., info-tainment), we identify the following as the main requirements of an “ideal” dissemination protocol:

  • 1.

    mitigate the broadcast storm problem [10];

  • 2.

    extend the coverage area of a RSU;

  • 3.

    achieve low end-to-end delivery delays;

  • 4.

    provide high throughput capacity rate for the efficient dissemination of RSU flows;

  • 5.

    be able to work transparently to the operation of the MAC layer (whether contention-less or

Mathematical analysis and synthesis of a VBN

We define an analytical model that characterizes the dissemination performance behavior of VBN. This model is also used by the system designer, and dynamically by the underlying control mechanism, to optimally configure system parameters. In particular, it is used to determine the optimal locations of RNs by calculating the preferred nominal distance level between elected RNs, denoted as D, which leads to a high throughput capacity rate. We assume a reuse-M TDMA MAC protocol. The model also

Protocol stack for a VBN

The protocol stack architecture includes a physical layer equipped with multiple Modulation and Coding Sets (MCSs), a MAC layer supporting either CSMA/CA scheduling, as used in IEEE 802.11p, or a TDMA scheme. As in the ETSI ITS architecture, additional layers include Access Control, Network and Transport, Facilities and Application [4]. The intelligence conveyed by the algorithm used to elect RNs is embedded in a Forwarding Layer (FL), which can be implemented as a sublayer, or as an

Packet by packet dissemination protocols

In this section, we identify two other VANET protocols that we use for the purpose of performance comparison in assessing the performance behavior of the VBN scheme. In contrast to the dynamic backbone synthesis based operation of the VBN scheme, whereby an elected backbone net remains intact and is employed to forward a large number of packet bursts, other schemes elect vehicles to act as forwarding nodes, on a packet-by-packet basis. The commonality of these protocols is that all of them

Simulation analysis of VBN

We evaluate the VBN performance using a multi-layer simulation tool (see Fig. 7). For this purpose, we have configured the simulation system to employ two main building blocks: a micro-mobility simulator, that models mobility patterns, and the communication and network simulator. Mobility of vehicles along the highway is generated by the micro-mobility simulator (left block of Fig. 7) by using the well known car-following model. This method is used by most current vehicular micro-mobility

Concluding remarks

We introduce and study a multi-hop VANET protocol that is identified as a Vehicular Backbone Network (VBN) mechanism. The objective is to disseminate broadcast packet flows to the bulk of vehicles that travel over an extended area along a highway, at a high throughput rate and low end-to-end packet delays. Under the VBN scheme, an adaptive rate and adaptive grid (based on the use of dynamically adapted inter relay node ranges) based operation is used to dynamically synthesize a temporary

Francesca Cuomo received her “Laurea” degree in Electrical and Electronic Engineering in 1993, magna cum laude, from the University of Rome Sapienza. She earned the Ph.D. degree in Information and Communications Engineering in 1998 (Univ, Rome Sapienza). From 2005 she is Associate Professor at the University of Rome Sapienza and teaches courses in Telecommunication Networks. Prof. Cuomo has advised numerous master students in computer engineering, and has been the advisor of 8 Ph.D. students in

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    Francesca Cuomo received her “Laurea” degree in Electrical and Electronic Engineering in 1993, magna cum laude, from the University of Rome Sapienza. She earned the Ph.D. degree in Information and Communications Engineering in 1998 (Univ, Rome Sapienza). From 2005 she is Associate Professor at the University of Rome Sapienza and teaches courses in Telecommunication Networks. Prof. Cuomo has advised numerous master students in computer engineering, and has been the advisor of 8 Ph.D. students in Networking. Her current research interests focus on: Vehicular networks and Sensor networks, Cognitive Radio Networks, Reconfigurable radio systems, Energy saving in the Internet and in the wireless system. She has participated in several National and European projects on wireless network systems such as the RAMON, VICOM, INSYEME, IST WHYLESS, IST EPERSPACE, IST CRUISE. Francesca Cuomo has authored over 80 peer-reviewed papers published in prominent international journals and conferences. Se his IEEE Senior Member.

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