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Dependability of Decentralized Congestion Control for Varying VANET Density This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TVT.2016.2519598, IEEE Transactions on Vehicular Technology Dependability of Decentralized Congestion Control for varying VANET density Arrate Alonso1 and Christoph F. Mecklenbrauker¨ 2;3 1 Signal Theory and Communications, Computing and Electronics Department, Mondragon Unibertsitatea, Spain 2 Institute of Telecommunications, Technische Universitat¨ Wien, Austria 3 Christian Doppler Laboratory for Wireless Technologies for Sustainable Mobility, Technische Universitat¨ Wien, Austria [email protected] Abstract—IEEE 802.11p is an amendment to the surrounding traffic situation and to detect deviations IEEE 802.11 standard for Wireless Access in Vehicular from safe behaviour. The European Telecommunica- Environments (WAVE). Lower layer wireless transmis- tion Standards Institute (ETSI) has defined two types sion formats and protocols are defined for both vehicle- to-infrastructure and vehicle-to-vehicle communication of messages for safety-related applications, namely links to support cooperative Intelligent Transport Sys- Cooperative Awareness Messages (CAM) [1] and tems (ITS). The Medium Access Control (MAC) layer Decentralized Environmental Notification Messages uses carrier sensing as a means for detecting the (DENM) [2]. CAMs are broadcasted periodically, busy state of the radio channel before transmitting. In with an update rate of 1—10 Hz depending on heavily loaded Vehicular Ad-hoc NETworks (VANETs) the channel access delay when attempting to access the context. CAMs contain the position, speed, and the channel increases unpredictably when it is sensed heading of the vehicle, they are time-triggered and as busy. The European Telecommunications Standards broadcasted continuously. The CAM payload size is Institute (ETSI) defines safety-related messages. These 200 byte, to which some security overhead is added. messages require a dependable behaviour of the com- DENMs, on the other hand, are event-driven and and munication link. In this context, dependable behaviour translates to the reliable delivery of data packets within triggered when a safety-critical situation occurs. The a specified deadline regardless of changes in the traffic Wireless Access in Vehicular Environments (WAVE) density. Unfortunately, the WAVE amendment does not standard which is adopted in the USA does not have fulfil the requirements for such dependable behaviour. a distinct name for event-driven type of messages, This contribution reports on performance evaluations but time-triggered messages are called Basic Safety by simulation studies of the IEEE 802.11p MAC and the Decentralized Congestion Control (DCC) mechanism Messages (BSM). Both message types require pre- specified by ETSI in addition to the existing Enhanced dictability, whereas CAM/BSMs have modest relia- Distributed Channel Access (EDCA). The simulation bility requirements and DENMs have high reliability scenario under study is a two VANET merging highway requirements. By predictability is meant that the situation. The MAC protocol has to adapt to variable MAC layers should have a known maximum delay, traffic densities in this scenario. The performance is evaluated in terms of coverage range and MAC-to-MAC such that a message can be delivered to the receiver delay reliability, and DCC state stability. Furthermore, before a predefined deadline. Therefore the MAC a novel performance indicator a defined, evaluated, layer protocol for scheduling safety-related data traf- and discussed which the authors have named data fic must be predictable, self-organizing and support novelty. Finally, a multistate-active DCC mechanism is both event-driven and time-triggered data traffic. The proposed, which achieves a dependable behaviour. IEEE 802.11p [3]uses Carrier Sense Multiple Access I. INTRODUCTION with Collision Avoidance (CSMA/CA) as Medium UTURE cooperative traffic safety applications Access Control (MAC) with support for Quality of F depend on the timely exchange of data among Service (QoS) through IEEE 802.11e (this is known vehicles and the road infrastructure via wireless com- as Enhanced Distributed Carrier Access, EDCA). In munications. These applications focus on improving Europe a profile standard of IEEE 802.11p has been road safety and aim to increase the drivers’ and approved by ETSI which is called ITS-G5 [4]. As vehicles’ information horizon beyond the line of stated in [5], the two lowest layers of the ETSI sight. The vehicles use messages to monitor their TC ITS protocol stack are almost identical to the WAVE approach with the exception that WAVE has Copyright (c) 2015 IEEE. Personal use of this material is the MAC-sublayer extension 1609.4 while ITS-G5 permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to requires Decentralized Congestion Control (DCC) [email protected]. [6]. 0018-9545 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TVT.2016.2519598, IEEE Transactions on Vehicular Technology The results in [7] showed that for different Ve- line of MAC development, based on time-slotted hicular Ad-hoc NETwork (VANET) densities there is protocols adapted to the VANET environment. Refs. a certain stabilization time required for dependable [10]–[16] stem from Slotted Aloha (S-Aloha) [17]. CSMA/CA performance for safety-related communi- These time-slotted approaches suffer from two major cations. This contribution extends the results in [7] drawbacks: (1) they cannot stably handle scalability and reports on DCC performance in VANETs with in overloaded situations and (2) slot allocation as per- a time-varying traffic density. This occurs whenever ceived by a particular node is not distributed among two VANETs merge. The aim is to evaluate the the neighbouring nodes. In 2009 the work of [18] communication protocol performance in three differ- considered Time Division Multiple Access (TDMA) ent stages of a VANET merging scenario: initially, as potential collision avoidance MAC method for when the two VANETs are created, secondly, when vehicular environments. TDMA [19] is a technique they merge into a single VANET, and, finally, when where the timeline is split into a series of the time they separate again. The questions that arise are: periods, and each period is divided into a set of How does EDCA perform on varying VANET density time slots. Each vehicle is then assigned a slot in situations? Does the DCC enhance the EDCA perfor- which it transmits its messages in every period. As mance? Does the reliable performance hold through- vehicular networks are dynamic in space and time, out the whole simulation time? This contribution the slot assignment must be validated the vehic- evaluates the performance of the IEEE 802.11p MAC ular network evolves in order to keep the MAC and the DCC mechanism recently specified by ETSI. layer protocol mobility-aware. This proposal, known And it proposes multistate-active DCC mechanism as Self-Organizing Time Division Multiple Access that achieves a robust and reliable performance. This (SoTDMA) overcomes the two aforementioned short- work underlines the importance of a suitable param- comings for time-slotted approaches. Subsequently eter setting, namely the Carrier Sensing Threshold in 2010, [20] proposed a different TDMA solution (CST) value selection, so that reliability is achieved for infrastructure-to-vehicle communication. This ap- and sustained regardless of the traffic density. proach is implemented in a novel sub-layer on top of The remainder of the paper is structured as follows. the conventional IEEE 802.11p MAC. The solution is Section II summarizes the background and related likely feasible for infrastructure-to-vehicle communi- work on the MAC layer for vehicular communica- cation scenarios, but much less so in a vehicle-to- tions and congestion control mechanisms. Section vehicle communication context, where the size of the III introduces the basics of EDCA and the DCC coverage area is not as important as a the reliability mechanism, specifying the goals and how they should of the packet transmission at low latency. be implemented. A set of performance indicators for DCC is a cross-layer mechanism that varies the communication protocol evaluation is defined in Sec. parameter setting of the PHY layer based on a IV. Next, the protocol mechanisms are investigated reference measured in the MAC layer. Its aim is to by simulation studies in the scenarios described in provide a fair and harmonized channel access. This Sec. V. The performance indicators for single nodes means limiting the load of a subset of users either as well as the VANET are analyzed and discussed in by reducing the transmit power, packet generation Sec. VI. Finally our conclusions are summarized. rate, or varying some of the threshold values in the communications which affect the channel load. II. BACKGROUND AND RELATED WORK Although, the DCC mechanism is being amended at In the field of vehicular MAC layers, two distinct the access layer, it actually is a cross-layer approach.
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