The Need for a System Design Methodology to Address Challenges in Wireless Access in Vehicular Ad Hoc Networks

Transcription

1 The Need for a System Design Methodology to Address Challenges in Wireless Access in Vehicular Ad Hoc Networks Chrysostomos Chrysostomou Department of Computer Science & Engineering Frederick University Nicosia, Cyprus ch.chrysostomou@frederick.ac.cy Lambros Lambrinos, Constantinos Djouvas Department of Communication and Internet Studies Cyprus University of Technology Limassol, Cyprus {lambros.lambrinos, costas.tziouvas}@cut.ac.cy Abstract Vehicular networking is nowadays considered as one of the most important enabling technologies as it allows vehicles to communicate with each other (vehicle-to-vehicle (V2V) communication) and/or with roadside infrastructure (vehicle-toinfrastructure (V2I) communication). Vehicular ad hoc networks (VANETs) are ideal candidates for supporting various types of traffic applications, such as traffic safety, traffic efficiency and management applications, and infotainment type of applications. As a result, there is ongoing research concerning the efficient medium access control and multichannel coordination, separately. In this paper, we review a number of schemes proposed and stimulate the discussion on the need for a generic system design methodology to address efficiently the challenges in the wireless access in vehicular ad hoc networks. Specifically, the aim shall be to design an efficient and robust system architecture that deals with the wireless medium access and multichannel coordination in all possible environments (V2V and V2I in urban, suburban, and highway) and allows differently prioritized types of traffic applications to coexist. Keywords-vehicular ad hoc networks; WAVE standard; wireless medium access; multichannel operation I. INTRODUCTION Vehicular ad hoc networks (VANETs) represent a particularly challenging class of mobile (ad hoc) networks that enable vehicles to communicate with each other (vehicle-tovehicle (V2V) communication) and/or with roadside infrastructure (vehicle-to-infrastructure (V2I) communication). The aim of VANET is to facilitate a plethora of applications, such as traffic safety, traffic efficiency and management, as well as infotainment. The variety of the intended applications, implies that the Quality of Service (QoS) required varies from non-real-time, to soft real-time where a timing failure might compromise service quality, up to hard real-time where a timing failure might lead to a disaster. For this, there is ongoing standardization, notably the Wireless Access for the Vehicular Environment (WAVE) standards [1-4] /10/$ IEEE The IEEE standard body is currently working on a new amendment, IEEE p [5, 8], to address these concerns. The IEEE p WAVE standardization process [9] originates from the allocation of the Dedicated Short Range Communications (DSRC) spectrum band [6, 10] to be used exclusively for V2V and V2I communications. It should be noted that while IEEE p describes how communication takes place over each individual channel of the DSRC spectrum, a complete communication system for WAVE needs to include, among others, support for multi-channel operations. This is addressed by the IEEE trial-use standard [4, 9], which calls for time to be divided into fixed intervals, with a continuous switching between a control channel interval and a service channel interval. This operation needs to be substantially updated in an adaptive manner in order to accommodate the high dynamics of the environment. The IEEE p is based on the carrier sense multiple access with collision avoidance (CSMA/CA) M protocol. It uses the enhanced distributed channel access (EDCA) mechanism originally provided by IEEE e [11] that provides traffic types differentiation through different static M parameters values. An extensive analysis of several CSMA based wireless networking M protocols and tuning mechanisms has already been carried out. However, the performance of IEEE p has not been substantially studied; a number of researchers have given an overview on both the capabilities and the limitations of the technology [12-15]. The goal of this paper is to stimulate discussion on the need for a generic system design methodology to efficiently address the challenges in the wireless access in vehicular ad hoc networks. Specifically, the aim shall be to design an efficient and robust system architecture that deals with the wireless medium access and multichannel coordination in all possible environments (V2V and V2I in urban, suburban, and highway) and with different prioritized types of traffic applications to coexist. Up to date, to the best of our knowledge, no such system architecture has been designed and developed to enhance the appropriate IEEE 1609 protocol suite (namely, IEEE p

2 Figure 1. Channel switching operation in WAVE [4] and IEEE ). In contrast, the proposed solutions targeted specific problems (e.g., enhancing the IEEE p solely, or enhancing the multichannel coordination, separately), and were tested in specific vehicular environments (e.g., V2V or V2I), using a single class of traffic application. The paper is organized as follows. Section II gives the basic features of the WAVE system architecture in concern. In Section III and IV, we study a number of proposed enhancements in IEEE p and in multichannel operation, respectively, and review the significant concepts recently proposed. In Section V, we discuss the need for the design of a system methodology to address the challenges in wireless access in vehicular ad hoc networks. Finally, in Section VI we present our conclusions. II. CURRENT STATE OF THE WAVE SYSTEM The IEEE has developed a system architecture known as WAVE [1-4] to provide wireless access in vehicular environments. Collectively, IEEE p [5] and IEEE 1609.x [1-4] are called WAVE standards because their goal, as a whole, is to facilitate the provision of wireless access in vehicular environments [9]. A WAVE system consists of two main entities: Roadside units (RSUs) include equipment located along highways, at traffic intersections and other type of locations where timely communications with vehicles are needed [7]. Onboard units (OBUs) are processing and communication facilities located in a vehicle, providing an application runtime environment, positioning, security and communication functions and interfaces with other vehicles, as well as human machine interfaces [7]. WAVE units operate independently exchanging information over a fixed radio channel called the control channel (CCH). However, they may also organize themselves in small networks called WAVE basic service sets (WBSSs). WBSSs can consist of OBUs only or a mix of OBUs and RSUs. All the members of a particular WBSS (the provider that initiates the communication and the possible users) exchange information through one of several radio channels known as service channels (SCHs) [9]. In general, the CCH is reserved for system control and safety messages, and the SCHs are used to exchange non-safety data. WAVE units are required to divide their time between the CCH and the SCHs. Therefore, the WAVE protocol stack includes a sublayer dedicated to controlling this multichannel operation. This sublayer is specified in IEEE [4]. IEEE [3] specifies the WAVE Short-Message Protocol (WSMP), which is designed to offer a more efficient alternative to TCP or UDP over IP, for 1-hop messages that require no routing. Therefore, the WAVE system can accommodate high-priority, time-sensitive communications with the aid of WSMP, as well as more traditional exchanges, such as TCP and UDP transactions. IEEE [1] and IEEE [2] specify the resource manager and security services, respectively. IEEE standard specifies the following important features, related to the multichannel operation management [4]: A device monitors the CCH until a WAVE service advertisement (WSA) is received that announces a service operated in a SCH. WAVE devices must also monitor the CCH for additional safety or private service advertisements during specific intervals (CCH intervals). If transactions in progress on an SCH are needed to be suspended, when CCH monitoring is required, the data exchange may be resumed when CCH monitoring is no longer required (i.e., when the device can return to the SCH). In this case, it is recommended to support buffering of data packets while monitoring the CCH, until the device can return to the SCH in order to complete transactions in progress [4]. To accommodate devices that want to exchange data on SCHs, but can not monitor the CCH while doing so, as in the case of single-channel devices, synchronization is necessary. In addition to being synchronized with each other, such devices must also know when it is permissible to finish monitoring the CCH. For this purpose, CCH and SCH intervals are uniquely defined with respect to Coordinated Universal Time (UTC); UTC is commonly provided by Global Positioning Systems (GPS) [4]. A synchronization interval and its CCH interval and SCH interval components are shown in Fig. 1. As the standard [4] states, the CCH and SCH intervals may be adaptable in the future, however, the sum of the CCH and SCH intervals is fixed to 100 ms (equally divided to both intervals). As explained in Section I, the IEEE p is based on the carrier sense multiple access with collision avoidance (CSMA/CA) M protocol. It uses the enhanced distributed channel access (EDCA) mechanism originally provided by IEEE e [11] that provides traffic types differentiation through different static M parameters values. Four applications access categories (s) are defined in the WAVE standards. The differentiation in priority between s for channel access parameters is implemented using the appropriate EDCA parameter set values (see Table I and Table II), which are defined as follows [4]: Arbitration inter-frame space (AIFS): the minimum time interval between the wireless medium becoming idle and the start of transmission of a frame. Contention window (CW): An interval from which a random number is drawn to implement the random backoff mechanism.

3 TABLE I. EDCA PARAMETER SET USED ON THE CCH [4] Ind ex CW min 1 Background acw min 0 Best effort (acw min + 1)/2-1 2 Video (acw min + 1)/4-1 3 Voice (acw min + 1)/4-1 CW max AIF SN acw max 9 acw min 6 (acw min + 1)/2-1 3 (acw min + 1)/2-1 2 TABLE II. DEFAULT EDCA PARAMETER SET ON THE SCH [4] Ind ex CW min 1 Background acw min 0 Best effort acw min 2 Video (acw min + 1)/2-1 3 Voice (acw min + 1)/4-1 CW max AIF SN acw max 7 acw max 3 acw min 2 (acw min + 1)/2-1 2 acw min and acw max, which are static values (15 and 1023, respectively), are specified in [5]. III. IEEE P ENHANCEMENTS We study a number of enhancements on the IEEE p M protocol, and review the most significant concepts recently proposed. A. Centralized & Distributed Enhancement Algorithms In [16], the authors focus on the M performance of IEEE p. They show that the specified M parameters for this protocol can lead to undesired throughput performance, since the backoff window sizes are not adaptive to dynamics in the number of vehicles attempting to communicate. Two enhanced schemes are suggested in order to overcome this problem; the key feature involves changing the backoff window size depending on the number of communicating vehicles. One solution is a centralized approach where exact information about the number of concurrent transmitting vehicles is used to calculate the optimum window size. The other solution is a distributed approach where vehicles use local observations to adapt the window size. The main challenge addressed in [16] is to enhance the performance of IEEE p in such a way that each transmitting vehicle will be more aggressive when the number of transmitting nodes is small, and less aggressive when the number of contending nodes is large. Specifically, the Centralized Enhancement Algorithm (CEA) models the IEEE p M as slotted p-persistent CSMA. The backoff interval is determined by the transmission probability p, such that a station chooses to transmit with probability p and stays idle with probability 1-p in each subsequent time slot when the medium is sensed busy. The transmission probability p is chosen such that the mean backoff time is equal to the window based backoff mechanism [16]. The CEA scheme, however, assumes that the number of transmitting vehicles is always known by the base station that updates this information to all transmitting vehicles. As mentioned in [16], it is very difficult, if not impossible, to predict/estimate the number of vehicles from an infrastructure node point of view, due to the high variability and dynamics of such an environment; as a result, a centralized M enhancement algorithm might fail if the base station is relaying wrong information to the transmitting vehicles. In the Distributed Enhancement Algorithm (DEA), on the other hand, a node only uses local channel information to change its backoff window size. In particular, a transmitting vehicle measures the channel busy ratio and compares it with the previously obtained one. Based on the amount of change in the busy proportion, a vehicle considers whether the number of transmitting stations has increased or decreased, and changes its backoff window size accordingly, using a linear updating method based on heuristics. We note that both schemes have only paid attention to V2I communication (ignoring the V2V environment). Also, the evaluation of these schemes has taken into consideration only one application access category for each simulated scenario (ignoring the mixture of traffic types that may appear in such an environment), and the only performance metric that has been measured is the throughput (other QoS important metrics, like latency/delay, loss, etc have not been evaluated). B. Fragmentation & Best-fit Schemes Reference [17] points out the bandwidth wastage problem caused by channel switching mandated in the IEEE p/1609 draft standards. Specifically, suppose that the estimated transmission time of a packet exceeds the residual time of the current service frame. The standards recommend that in such a case the transmitting node should prevent sending out this packet, but instead should send it in the next service frame; two schemes are proposed to mitigate this problem. In particular, the Fragmentation scheme partitions a packet, when its estimated transmission time exceeds the residual time of the current service frame, such that the estimated transmission time of the first fragmented packet is equal to the residual time of the current service frame. Following this approach, there is no unused bandwidth in a service frame; however, there is a need for an extra overhead of a M-layer header for the second fragmented packet. This scheme also does not reduce the end-to-end delays experienced by application packets [17]. On the other hand, the Best-fit scheme aims to utilize the residual bandwidth of service frames and decrease the delays experienced by application packets. The main idea behind this scheme is that instead of fragmenting a large packet, a transmitting node can choose a smaller packet in the output queue and send it out, if its transmission time is less than or equal the residual time of the current service frame [17]. Following this approach, however, may result in out of order packets at the receiving end. We note that both schemes have only focused on V2V communication (ignoring the V2I environment). Also, the

4 evaluation of these schemes has taken into consideration only one application access category (ignoring the mixture of prioritized traffic types that may appear in such an environment), and the only performance metric that has been measured is the throughput (other QoS important metrics, like latency/delay, loss, etc have not been assessed). C. Self-organizing Time Division Multiple Access A totally different approach is followed in [18], where a number of concerns have been stated regarding the suitability of the IEEE p for traffic safety (real time) applications. The authors suggest the use of a decentralized self-organizing time division multiple access (STDMA) scheme, where the transmitting nodes are themselves responsible for sharing the communication channel, and the synchronization among the nodes is done through a global navigation satellite system. All nodes in the network regularly send messages containing information about their own position. The STDMA scheme uses this position information when choosing slots in the transmitting frame. The proposed solution has its merits but the arguably limited capabilities of TDMA M schemes will be a serious impediment in deploying a wide range of applications in VANETs. Further, the evaluation of the proposed scheme has only considered V2V communication, and all vehicles have the same data traffic type to send, thus all data packets have the same priority. IV. MULTICHANNEL OPERATION We study a number of proposals to enhance the channel switching operation, as defined in IEEE standard, and review the most significant concepts proposed. A. A Methodology of Channel Intervals Derivation In [19] a methodology is proposed to derive the channel intervals in the IEEE multichannel operation scheme. Through simulation scenarios, the reliability performance of safety applications is evaluated with different channel intervals and traffic densities. Based on the simulation results, the share of nonsafety applications is analyzed as the function of safety performance requirements and traffic density. Interesting challenges needed to be addressed is the use of packet generation rate optimization in order for the nonsafety applications to benefit from any improvements to the reliability of safety applications. Further, the dynamic adjustment of the control channel interval, as opposed to a static configuration, needs to be explored to support effective coexistence of safety and nonsafety applications in VANETs. B. Modified Channel Access Scheme In [20] a modified channel access scheme is proposed to allow a node to stay on a service channel as long as it requires before returning to the control channel. Even though the simulation results show that the modified scheme improves the service channel utilization, the main drawback of this scheme is the deactivation of the control channel for a significant amount of time; this deactivation results in the loss of a number of safety messages. C. CRaSCH Cooperative Reservation of SCH A channel reservation scheme is proposed in [21]. It aims to enhance the SCH selection procedure in order to improve the delivery of nonsafety applications without affecting safety messages distribution on CCH. The CRaSCH scheme allows WAVE providers to self-coordinate in order to minimize contention; cooperation relies solely on exchange of information about the immediate neighborhood of every provider. V. THE NEED FOR A SYSTEM DESIGN METHODOLOGY As discussed in the previous sections, there is ongoing research concerning the efficient medium access control and multichannel coordination. It is well accepted in the literature that the proposed standards for the IEEE p and the multichannel operation defined in IEEE need enhancements to accommodate the highly dynamic requirements of the environment. The proposed schemes, discussed in Sections III & IV, have the potential to be incorporated as part of the WAVE system architecture to facilitate the efficient operation of vehicular ad hoc networks. However, the proposed solutions targeted specific problems (e.g., enhancing the IEEE p solely, or enhancing the multichannel coordination, separately), and were only tested in specific vehicular environments (e.g., V2V or V2I), with no mixture of prioritized traffic applications. Therefore, in our opinion, it is a necessity to start thinking about how the WAVE system architecture, as a whole - concerning the wireless medium access and multichannel operation- can be enhanced. The design of an efficient and robust system architecture to deal with the wireless medium access and multichannel coordination in all possible environments (V2V and V2I in urban, suburban, and highway) and with different prioritized types of traffic applications to coexist, is realistic and can provide substantial benefits when applied in a real vehicular environment. Up to date, to the best of our knowledge, no such system architecture has been designed and developed to enhance the IEEE 1609 protocol suite in concern (namely, IEEE p and IEEE ). The goal of this paper is to stimulate the discussion on the need for a generic system design methodology to address efficiently the challenges in the wireless access in vehicular ad hoc networks. A number of important challenges and factors need to be addressed in order to design an efficient and robust system architecture. These include: The appropriate design of the underlying vehicular communication protocols for V2V and V2I networking. The rapid network topology changes and fast time-varying channel conditions that are the result of high vehicle mobility. The need of vehicular applications for the acquisition of critical messages quickly (low communication latency) and accurately (high reliability).

5 Requirements Analysis Architecture Specification Adaptive M Protocol Subsystem Multichannel Coordination Subsystem Figure 2. Generic system architecture of the M with channel coordination [4] The new application ideas that will come forward in the future which may broaden the communication requirements [7]. The adaptation of medium access control protocols so that they are scalable in terms of performance and adaptable to environment changes. The need for stringent quality of service support. The development of simulation methodologies to evaluate and validate vehicular networking protocols under realistic assumptions. A generic system architecture of the M with channel coordination is shown in Fig. 2 [4]. The internal contention algorithm, as indicated in Fig. 2, calculates the backoff independently for each based on access parameters. The with the smallest backoff wins the internal contention, and the winning then contends externally for the wireless medium [4]. Based on the above, a proposed system design methodology can consist of the following steps (see Fig. 3): Requirements Analysis: A thorough identification and analysis of the communication performance requirements imposed by different VANET applications (traffic safety, traffic efficiency and management, and infotainment type of applications) should be conducted. The proposed system is expected to meet these requirements to provide Quality of Service, with respect to throughput/channel utilization, bounded channel access delay/latency, minimal losses, and other significant performance metrics. A number of important system performance requirements need to be specified, such as the type of communication (V2V, V2I), transmission mode (event-driven or periodic), allowable latency, content/data to be transmitted and/or received, maximum required range of communication. Also, the proposed system needs to take into account the different treatment among messages due to service differentiation and/or the maximum allowable delay a message can suffer. Further, the design requirements for a multi-channel operation should be clearly specified; the efficient coordination of the available channels is a must Implementation Performance Evaluation Performance Metrics Identification & Reference Scenarios Figure 3. Proposed Generic System Design Methodology considering the type/severity of the content to be transmitted and/or received. Architecture Specification: The proposed system s architecture should be specified. It is of major importance to ensure the design of an efficient architecture that all components, that constitute it, will be based on the requirements identified. Two key components will be designed that constitute the proposed QoS-aware congestion control system: a robust and efficient M subsystem that can be adapted to environment dynamics, and an intelligent multi-channel coordination subsystem that offers tight control of the radio spectrum to reduce/control congestion. The underlying system architecture lies in the data link layer of the protocol stack; the key services offered from/to the lower/upper layers of the protocol stack will remain unchanged, in order to keep the interoperability of the whole protocol stack intact. Enhanced IEEE p M Protocol Specification: An enhanced, adaptive M protocol should be designed, based on the IEEE p, which is still under standardization for VANETs. The proposed mechanism is needed to offer QoS-aware wireless access in vehicular environments (both V2V and V2I). Specifically, the new protocol must take into account the differentiation of the various traffic types, which are categorized into different priorities with different basic M parameters. One possible solution is to design a simple, effective and efficient nonlinear control law, based on fuzzy logic control principles, in order to offer inherent robustness with effective control of the system. The new scheme can adapt the backoff window size (transmission probability) for each transmitting vehicle based on the channel traffic occupancy and type of applications, in order to adapt to the high environment dynamics. The perspective achievement is the QoS provision with respect to important performance metrics, such as throughput/channel utilization, bounded channel access delay/latency, minimal losses and high percentage of successful transmissions (i.e., high reliability, even when the number of vehicles is high). Also, it can offer adequate and effective differentiation among traffic types that belong to different access categories; especially to offer bounded/low delay (significantly lower than the tolerated delays in intelligent vehicular communications) to event-driven traffic safety applications.

6 Multichannel Coordination Subsystem Design: An intelligent multi-channel coordination subsystem that should offer tight control of the radio spectrum to reduce/control congestion should be designed. An efficient multi-channel communication mechanism is required so that sufficient safety messages on the control channel are received to maintain adequate and timely safety awareness, while non-safety communication on service channels is supported in the most efficient manner feasible. The channel coordination service will coordinate the channel intervals dynamically, using an intelligent scheduler, in such a way that no safety critical messages are lost. Further, messages could be assigned to the various channels according to their current benefit/characteristics and thus realize traffic differentiation as well. Identification of Performance Metrics and Reference Scenarios: The related performance metrics should be specified, such as bounded channel delay, channel utilization, minimal losses, and high percentage of successful transmissions that are needed to evaluate each proposed subsystem, and the proposed system as a whole. Also, the reference scenarios needed to evaluate the proposed system should be specified. These scenarios need to cover widely differing operating conditions. That is, we need to take into account the different traffic types of applications, the type of communications (V2V and/or V2I), the city-urban and highway traffic network conditions, and both ends of the density spectrum (scarce and dense). Further, we need to consider scenarios that at best imitate realistic traffic conditions. This can be fulfilled by incorporating real data/information of various traffic networks. Implementation Performance Evaluation: an extensive simulative evaluation should be conducted, based on the scenarios proposed in the previous step, to show the validity of the proposed system. VI. CONCLUSIONS Vehicular ad hoc networks have enormous potential benefits and represent a particularly challenging class of mobile networks. In this paper we reviewed the recent research concerning the efficient medium access control and multichannel coordination, and we have identified possible advantages and drawbacks. A common observation that can be taken from the proposed solutions is that they only targeted specific problems (e.g., enhancing the IEEE p solely, or enhancing the multichannel coordination, separately), and were only tested in specific vehicular environments (e.g., V2V or V2I), with no mixture of prioritized traffic applications. Thus, we attempt to give a generic system design methodology to deal with the need to address important challenges in wireless access in vehicular ad hoc networks. Of course, additional research is needed to assess the feasibility of the proposed methodology. 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