Cross Layer Design in Vehicular Ad Hoc Networks

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overview of vehicular ad-hoc networks (VANETs) architecture and its requirements ... (VANETs), routing protocols, layered structure (OSI model), single-layer. 1.
SYSC 5408 CROSS LAYER DESIGN FOR WIRELESS MULTIMEDIA NETWORKS (Prof. F. Richard Yu)

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Cross Layer Design in Vehicular Ad Hoc Networks (VANETs) Electrical and Computer Engineering, University of Ottawa Samaneh Ahmadvand [email protected] Abstract—Vehicular ad-hoc networks (VANETs) play a main role in intelligent transportation systems (ITS) in order to improve security, Quality of Service (QOS) and other main factors in this environment [56]. Routing in vehicular environment in the other words in the VANETs can be considered as a significant challenge since vehicles are continuously moving. Furthermore, routing techniques proposed for Mobile Ad-Hoc Network (MANET) do not provide for the characteristics of VANET [2]. In OSI model as a traditional layered structure model routing is performed only by routing protocols in one layer while important parameters in first three layers in this model are not used for routing since information are not able to be exchanged between layers and also layered structure leads to poor performance. Therefore, to improve the network performance and routing process crosslayered design is used. This paper presents cross layer routing protocols to use information from different layers of network like the physical and medium access control (MAC), single layer routing protocols are covered as well [48]. In section 2 an overview of vehicular ad-hoc networks (VANETs) architecture and its requirements are presented, in section 3 cross-layer design is given, in section 4 some of main challenges of crosslayer design in VANETs with possible solutions to tackle these challenges are presented]. Index Terms—Cross-layer design, vehicular ad-hoc networks (VANETs), routing protocols, layered structure (OSI model), single-layer

1. INTRODUCTION very year many people are killed and injured because of Evehicles accidents on the roads and this matter leads to a big concern all around the world. Furthermore, by increasing the number of vehicles we can see big traffic jams every day almost in all big cities that it means wasting fuels and time [1], [3]. Governments and automotive manufacturers to tackle this issue with considering high speed of wireless communications, are working towards intelligent transportation systems (ITS). Intelligent Transportation Systems is applied to increase safety and efficiency in transportation systems and it supports two types of wireless communications: 1) long-range communication and 2) short-range communication. Long-range communication is based on the existing infrastructure networks. While, short-range communication is based on emerging technologies such as IEEE 802.11 variants [1]. Dedicated Short Range Communication (DSRC) is a technology that used for Vehicular communications in Intelligent Transportation Systems [2]. Moreover, network communication in layered architecture (OSI model) is not so helpful in wireless network unlike wired network and it is

related to the protocols communication. There are two methods for designing protocols. First method, designing protocols based on the OSI model’s rules. In a layered architecture according to protocols only the services at the lower layers are used by higher-layer protocol and the details of how the service is being provided is not concerned. In second method which is called cross-layer design, protocols disobey traditional OSI model that it means protocols are allowed to communicate directly not only with adjacent layers but also with nonadjacent layers and share information between different layers [4]. Cross layer design has layered structure; however, it is more flexible for exchanging information with different layers, it is also able to improve performance in a system in terms of security, mobility and QOS [5]. Vehicular Ad hoc Network (VANET) applications play a main role in safety to the vehicles in the adjacency. VANET can be applied to increase transport efficiency and information/entertainment applications [3], [2]. In VANETs, to make wireless connectivity among vehicles, they are equipped with wireless sensors and on-board units [6]. Mobile ad-hoc networks (MANETs) are considered as a super class for VANETs [7], since they both have some similar features such as infrastructure independence, selforganization and management, low bandwidth and short radio transmission range [1]. In VANETs, however, because of their high mobility and dynamic network topology existing MANET routing protocols do not act properly and they lead to poor performance, low communication throughput and frequent route disruptions [1], [8], [9], [10]. Furthermore, In VANETs the movement of vehicles are based on the road topology and remarkable computing is done by vehicles and vehicles provide continuous transmission power to support these functions, all these features of VANETs have made a considerable difference with MANETs [1], [48], [54]. In layered architecture, different layers interact only with their vicinity layers and each layer makes independent decisions that this matter has brought some challenges for VANETs characteristics and lead to poor network performance in VANET. For example, if a busy channel and interference leads to loss packet, this real reason can have different meanings in each layer. For instance, excessive collisions may be considered as a reason for that lost packet in Medium access control (MAC) layer, network layer may think that the node is out of range and restart finding the node and transport layer may consider this problem because of excessive congestion. To tackle with the challenges of VANET cross-layer design has been proposed [52]. In crosslayer design there is interaction among the layers and different layers can share and exchange information with

SYSC 5408 CROSS LAYER DESIGN FOR WIRELESS MULTIMEDIA NETWORKS (Prof. F. Richard Yu) together and each layer will not make decision independently [2]. The rest of the article is organized as follows. In section 2, a brief overview of vehicular ad-hoc networks (VANETs) architecture which are classified into two types of layered architecture in VANETs and communication architecture and VANET requirements are presented. In section 3 a high level cross-layer design, which is performed into four methods of 1) information flow with new interfaces 2) merging of adjacent layers 3) design coupling without new interfaces and 4) vertical calibration across layers, based on protocols is given. In section 4 some of main challenges of cross-layer design in VANETs, such as Unreliable Broadcasting, Routing, Network Congestion and security, with possible solutions to tackle these challenges are presented. 2.VEHICULAR Ad-hoc NETWORKS ARCHITECTURE AND REQUIREMENTS The standardization of VANETs architecture has made by several companies. One of these architectures which called WAVE (Wireless Access in Vehicular Environments) is introduced by IEEE [11]. ETSI TC ITS and CALM architectures are other standards which are proposed by ETSI [13] and ISO [12] respectively [14], figure 1 illustrates WAVE protocol stack in comparison with TCP model protocol stack [14]. 2.1 Architecture 2.1.1 Layered Architecture in VANETs Wireless access in vehicular environments (WAVE) architecture includes two main protocols, which are called IEEE 802.11p [15] and IEEE 1609.x. the IEEE 802.11 standard [17] approves IEEE 802.11p that incorporates WAVE and functions primarily at the PHY and MAC layers of the stack. To improve public safety applications and increase traffic flow, dedicated Short Range Communications (DSRC) spectrum band is allocated [18] which leads to this standard is illuminated. In WAVE compatible units functions are enabled by IEEE 802.11p in a highly dynamic environment and it allows messages are shared without the need to join a base service set. WAVE interface functions and signaling techniques are controlled and defined by the 802.11 MAC standard [19]. To transport system control and safety messages one control channel (CCH) is reserved which IEEE 802.11p depends on it, and also in order to transmit non-safety data packets 4 to 6 service channels (SCHs) are utilized [20]. To support safety applications in VANETs, a set of standards that function in the middle layers of the protocol stack is introduced by IEEE 1609 which is worked with IEEE 802.11p. This set includes four standards: 1) IEEE 1609.1 [37], 2) 1609.2 [38], 3) 1609.3 [39] and 4) 1609.4 [40], [58]. To create communication between OBUs mounted in vehicles and remote sites through RSUs, applications are enabled by IEEE 1609.1 using a specific WAVE application which is called resource manager. WAVE devices define

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secure message formats and their processing by the IEEE 1609.2 standard. The aim of processing is to make secure application and WAVE management messages. To support main security functions, the executive operations are also defined [51]. Applications utilize WAVE short-message protocol to transfer short messages to all intended parties in time, whereas IPv6 is utilized to decrease applications demanding. To create wireless communication among vehicles and between vehicles and RSUs IEEE 1609.3 is collaborated, by defining services which operate at the NET and transport layers. Multichannel wireless communication between WAVE devices is provided by IEEE 1609.4 in which supporting MAC sublayer services and functions are specified [1]. 2.1.2 Communication Architecture

Vehicular environment in terms of communication can be divided into four types [41]: 1) in-vehicle, 2) V2V, 3) V2I communications and 4) vehicle to broadband cloud communication. • In-vehicle communication between on-board units (OBUs) such as sensors is required inside the vehicle to facilitate various driver and public safety applications by allowing the detection of the vehicle’s performance and especially driver’s fatigue and drowsiness [42], [1]. • V2V is an interaction between vehicles in which data exchange platform can be provided for the drivers to share information and warning messages to driver assistance [1]. • V2I is placed between the vehicle and the roadside unit (RSU) which are used to provide real-time traffic or weather updates for drivers and to enable environmental sensing and monitoring [1]. • Vehicle to broadband cloud communication means to share messages between vehicles and broadband cloud, and can be utilized for active driver assistance and vehicle tracking [1].

Figure 1: WAVE protocol stack in comparison with TCP protocol stack [16].

SYSC 5408 CROSS LAYER DESIGN FOR WIRELESS MULTIMEDIA NETWORKS (Prof. F. Richard Yu)

2.2. Requirements vehicular networking requirements are classified relies on strategy, system capability and economical terms [43], [58]. Vehicles and static infrastructure alongside the road must be equipped with some basic requirement to provide vehicular networking. For example, a set of wireless transmitter and receiver on-board must be applied that are capable to perform V2V and V2I communications. They should be able to broadcast information in different modes such as unicast, broadcast, multicast and geocast, and also security and privacy must be covered properly. Although navigational tools such as global positioning system (GPS) and street maps do not have to meet these requirements, they are highly encouraged since location information would provide significant assistance in VANETs. Other than the system requirements, vehicular networks also have certain performance requirements such as high packet delivery ratio (PDR) and low delay, especially for safety applications [44], [1]. 3. CROSS LAYER DESIGN IN VANET VANETs are considered as a subclass of Mobile Ad hoc Network (MANET), hence they inherit some features of traditional MANETs such as noise, path loss and interference. Furthermore, because of high speed of vehicles and changing location, VANETs must also deal with high vehicle speeds, and resulting frequent route disruptions. To tackle these issues, information is required to be shared among layers so to achieve better throughput and decrease latency in transmission different layers will be optimized cooperatively by one layer [24]. Therefore, to improve performance, especially in real-time applications Cross-layer design has proposed in wireless networks [24]. There are different definitions of “cross-layer” which are conflicting and nonclear due to many interpretations and these different definitions lead to cross-layer design is proposed in different ways. To increase performance considerably, dependency between protocol layers is exploited which is related to protocol design in cross-layer design [57]. Based on how information is exchanged between layers, protocol design in cross-layer is classified into four approaches [45]. These four approaches are given briefly below and also figure 2 illustrates these approaches [24], [50].

Figure 2: Cross-layer design approaches with respecting protocol design [45]

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3.1 M1 Information Flow With New Interfaces: In layered structure, traditional OSI model, set of variables in each layer are optimized by their own protocols in the same layer. In this class of cross-layer designs, however, the information flow between layers is elevated by specialized interfaces. Information obtained from other layers can make better decision in adjusting parameters, in addition, offers useful information on network status and communication characteristics [24]. Through extra database which is exchanged among layers, or through notification fields inside packets the information flow interface can be performed, [24]. 3.2 M2 Merging of Adjacent Layers: In this approach, the service and functionalities of vicinity layers are combined and turned into a single layer which is called super layer. In order to make a single large uniform protocol, and also to optimize the super layer in this method a common optimization can be done directly on this super layer because super layer is created by combining the layers [24]. In this approach any additional interfaces are not needed obviously. This strategy, however, because of super layer complexity is extreme and uncommon. In addition, maintenance and system stability may be affected by this approach. 3.3 M3 Design Coupling Without New Interfaces: In this method, one layer is designed with respecting another layer functionality that means they are dependent on each other even at the time designing, in other words, there is collaborative way to build multiple layers. The referenced layer is called fixed layer (FL) and the other layer is called designed layer (DL) [24]. Since DL is created based on FL, an explicit interface is not required between them. For instance, if the PHY layer as the FL, is capable of multiple packet reception, so MAC layer as the DL must be capable of multiple packet reception as well since MAC layer should be adjusted by PHY layer. In this strategy any changes in FL should be considered and adjusted in DL [24]. 3.4 M4 Vertical Calibration Across Layers: To cover multiple layers in the stack, the parameters must be adjusted which is done by this method. Application layer performance depends on the parameters on all lower layers, so it is admiring to optimize parameters from all lower layers commonly. This strategy has a better performance in comparison with methods in which each layer parameters are adapted independently. The common optimization can either be static or dynamic, for example, in static, it can be performed at design time and dynamic i.e., performed at run time. In dynamic optimization, to have good accuracy the information is required to be kept update continuously, hence this keeping update information makes dynamic optimization complex. Algorithms that are placed in this

SYSC 5408 CROSS LAYER DESIGN FOR WIRELESS MULTIMEDIA NETWORKS (Prof. F. Richard Yu) category must store the information that is shared among layers by preserving a database or a repository [24], [47]. Base on the classification of different strategies, it is obvious to implement cross-layer in high level with respecting protocols, extra processing or storage capabilities may be required. In vehicular ad hoc network (VANET) Unlike other ad hoc networks, high performance processing units can be carried, large memory space is hosted potentially, and are connected to virtually unlimited power sources by vehicles. A summary can be found in Table 1 [24].

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Solution for I2V Communication messages: I2V communication involves transmitting data packets (e.g., service replies) from base stations to individual vehicles. Unlike beacon propagation that relies on broadcasting, I2V is targeted at particular destination vehicles. All vehicles are registered to a nearby base station. Vehicles communicate only with those base stations with which they are currently registered [24]. Here, a location-based paging mechanism is also proposed in which base stations send a paging packet before sending any data packet. This method overcomes the vehicle mobility and improve reliability [46].

4. ISSUES AND POSSIBLE SOLUTIONS While developing a cross layer design protocol, following may be the challenges that a designer would face: firstly, to decide the list of layers that he should include in optimization of cross layers and secondly, to determine the best strategy under the given performance requirements. Let us now discuss the listed issues which need to be concerned in order to achieve safety and security in VANETs while using Cross Layer Design. 4.1 Unreliable Broadcasting When a vehicle recognizes potential risk on the road, emergency alert-message called watchtower message is broadcasted to following vehicles is on the Risk Zone (RZ). In case of emergency alert-message has to be sent out of RZ by using multi-hop broadcasting, this problem will be increased. For increasing efficiency of multi-hop broadcasting, the number of retransmissions need to be minimized in conjunction to fast messages dissemination [21][22][23]. Here, we investigate the issues of supporting data delivery systems on vehicular ad hoc networks, in which vehicles can communicate among each other and with base stations. We primarily address two problems: V2I message transmission from vehicles to base stations; I2V data delivery from base stations to vehicles. [24] Solution for V2I Communication: Vehicles that require to access services like the Internet must communicate with base stations or access points. The request packets are transmitted through the network from individual vehicles to nearby base stations using multi-hop packet relay. Cross Layer Design improves the throughput and delay of these kind of communications by providing leverage to the interactions between MAC and routing layers to provide reliable and efficient communication between vehicles and nearby base stations. MAC collects local packet traffic statistics which are used by the routing layer in finding best available paths [24]. Roadside infrastructure (or base stations) periodically broadcast their service information using beacon messages to all vehicles in the network. Cross Layer Design presents a directional multi-hop broadcasting protocol that helps base stations propagate beacons beyond their immediate service area. Vehicles can potentially receive the same beacon multiple times through different routes, and they can also receive advertisements from multiple different base stations [49], [60].

In order to overcome the limitations of short-range, multihop broadcasting, Hamid at el.in [25] have used long-range cellular networks for emergency message dissemination. Their method is centered upon geographically based unicasting (GeoUnicasting), which uses GPS for finding the location of destination vehicle. When source vehicle has to send a message to destination vehicle (figure 3). Then first it checks that if there is an available route in its routing table. In case, no route is available, then sender vehicle uses cellular services like 3G or LTE to find the position of destination vehicle. Simulation results in [25] show that proposed method is more effective in the case of infotainment applications where clients are ready to pay extra charges for getting superior quality of service (QoS) [60].

Figure 3: Cellular support to Geocasting [25]

4.2 Routing Since the vehicles constantly move across the network, address-based routing suffers from frequent route disconnections and other path maintenance issues. Due to constant vehicle movement, the underlying communication network undergoes constant topological changes. Unlike the nodes in typical MANETs, vehicles move at much higher speeds, resulting in highly dynamic networks. [24] Routing in the presence of such a network is very challenging. Traditional methods that rely on neighborhood maintenance will incur significant overhead, and they are likely to deliver poor performance. It is however important to note that the vehicle mobility is not all that random – vehicle movement is constrained by the fixed road structure and speed limits.

SYSC 5408 CROSS LAYER DESIGN FOR WIRELESS MULTIMEDIA NETWORKS (Prof. F. Richard Yu) We need to develop strategies that exploit cross-layer optimization between routing and MAC layers to adapt to constantly changing network topology [24]. Also, in metropolitan areas, where a large number of vehicles have to connect to each other, there established MANETS based routing protocols are not effective [26], [48]. Possible Solutions: Sofra et al. proposed a cross-layer design that uses a metric known as Link Residual Time (LRT) [27] that is computed based on the received power that is observed at the physical layer. The value of LRT can be used to estimate the longevity of the link, and it denotes the remaining time for which the link can be used for packet transmission. LRT values can be used in higher layers to make better decisions for hand-off, scheduling, and routing packets. Each vehicle monitors and records the arrival time and the received power level for each packet that is received on the link. This time series of values is then used to estimate the value of LRT. While Sofra et al. focused on individual nodes, Singh et al. explored the use of link connectivity information among neighbors to help in addressing the challenges in designing routing protocols for VANET environments. They proposed a cross-layer protocol called Signal Strength Assessment Based Route Selection for OLSR (SBRSOLSR) [28]. In this framework, the link connectivity is based on SNR measurement, and the routing protocol is based on existing Optimized Link State Routing (OLSR). Also, Chen et al. in [29] has proposed an adaptive cross-layer multipath routing protocol called R-S-AOMDV, which is an improvement of AOMDV (ad-hoc on-demand multipath distance vector) protocol. 4.3 Network Congestion In Mobile Ad hoc networks, Congestion avoidance is relatively easy as the devices are mobile but the movement is not that frequent. When it comes to VANETs, due to the fast speed of vehicles, the techniques need to be improved in order to meet the requirements. Also, high mobility of vehicles causes frequent path disconnection. Just reducing the transmission rate when a packet transmission fails can help to suboptimal performance, especially in critical safety applications. In addition to that, it can result in reduced connection throughput, under-utilized bandwidth, and fluctuations in performance. [24] Possible Solutions: The cross-layer design between transport and network layers helps in distinguishing between route interruption and channel congestion. In a traditional layered design, the transport layer is actually responsible for delivering whole data between application layers of host computers. Transport Control Protocol (TCP) is a famous transport layer protocols that provides end-to-end reliable communication among different systems. TCP includes several mechanisms like flow rate control, error recovery, and collision avoidance. In wired networks, the transmission errors or packet losses are assumed to be a result of network channel congestion since the problems due to route disconnection and channel errors are minimal. In such cases,

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the TCP sender mostly reduces the sending rate, and it may also fix the congestion window size to decrease the load on the system. [24], [53]

Figure 4: TCTC Cross Layer Interactions [30]

Liu and Singh have studied the effect of path disconnection and network partition problems that are caused by node mobility including the performance of TCP. They have proposed Ad-hoc TCP ATCP [31], an end-to-end solution to improve the throughput of TCP (figure 4). This cross-layer mechanism tries to deals with a variety of routing issues such as packet loss due to high BER, changes in the route, network partitions, packet reordering, multi-path routing, and network congestion. ATCP allows the network layer to obtain feedback from the intermediate nodes about the route status. To deal with frequent route disconnections, Vehicular Transport Protocol (VTP) maintains in two states - connected or disrupted. To further provide congestion control mechanisms, VTP makes use of information collected from intermediate nodes [24]. Sender uses this information to calculate the product of bandwidth and packet delay. Such a packet-based bandwidth distribution provides fairness among the contending flows without additional maintenance of flow information. 4.4 Security Privacy requirements in VANETs vary from public safety applications to private applications due to their different demands [32]. In VANET, time plays a vital role as a one of them of its factors, since the end device should receive the message with a maximum allowed delay which is less than 100ms. To accomplish this real-time constraint, a very fast cryptographic algorithm is required to be used. In VANET a node can perform malicious activities that has already been validated. This might cause disruption in the network and accident among the nodes. VANET is based on information is too sensitive to even a small error in the algorithm based on probability, since it leads to the huge damage for VANET that is life critical and there is very short time available to perform the actions [48]. VANET security mechanisms are completely based on keys. Every message is encrypted. It is necessary to decrypt the message at the receiving end with the same key or some other

SYSC 5408 CROSS LAYER DESIGN FOR WIRELESS MULTIMEDIA NETWORKS (Prof. F. Richard Yu) key for proper synchronisation. Chances are that various manufacturers will install keys in different start stop managers. Thus the public key infrastructure has to fully depend on Certificate Authority (CA). Thus, one of the primary challenge while designing security protocols for VANETs is how the keys will be distributed among the vehicles. The major focus of the manufacturers is to build applications that are most liked by the customers. There is no driver who will accept a vehicle that will send traffic violation reports automatically. Thus, initiatives from government, manufacturers and consumers are required to overcome challenges in implementing security in VANET. Also, to avoid false messaging problem, regular verification of data is required [59]. Possible Solution: Around 43,000 deaths and more than 1.8 million injuries due to accidents are reported every year in European Union (EU) [34]. For this reason, post-crash carto-car emergency message broadcasting is mandatory as defined in IEEE 802.11p and Wireless access for vehicular standards (WAVE) standards. To address these problems, Code Torrent maintains communication within single-hop neighbors. The file sharing region, however, can be extended through the network of peers using network coding and mobility assisted data propagation. These techniques enable code Torrent to maintain enough connectivity among peers with low overhead, and data is transferred with minimum download delay. Authors showed that such a strategy outperforms another file sharing protocol called Car Torrent [33]. The proposed decision based authentication scheme has a simple, light weight and efficient algorithm compared to other authentication schemes that have been proposed in the recent time. Such schemes as Compared to our algorithm use encryption and decryption or hashing to secure the message and help destination vehicles to authenticate the message they receive. Since authentication is to be employed in vehicles which are fast moving, a simple and light weight scheme like the proposed, would help the vehicles in taking very fast decisions. The algorithm is inspired by human behavior. Thus, the scheme can also handle situations when the driver of the vehicle acts selfishly and propagate wrong information about an incident that has taken place.

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The network suffers from high congestion during the peak hours of the day. Collision also becomes more frequent. It is possible to optimize traffic if signals related to incidents such as accidents and jams are sent to the drivers on time. The driver can save time by choosing an alternative path. Several scheduling techniques have been proposed in the literature for VANETS. As the link capacities are VANETS is not fixed, hence the scheduling is a more complex task and mandates joint optimizations at MAC and physical layers. For addressing these issues, the cross-layer algorithm is proposed in [35] that uses opportunistic cooperation strategy to perform optimal scheduling.

Figure 5: Throughput Comparison in [35]

Possible Solutions: Authors have proposed an Optimal Congestion and Medium Access Control (OC-MAC) algorithm which decomposes a source problem into a flow control problem. figure 5 clearly shows the simulation results, in which proposed OC-MAC algorithm is compared with three approaches namely conventional client-server algorithm which is using IEEE 802.11 at the MAC layer, non-relay cooperation approach and always relay cooperation approach. Another issue is that IEEE 802.11 DCF protocol does not provide support for QoS which results in degraded channel utilization. Solution to this problem was provided by Liu et al. in [36] by using cross-layer based priority scheduling as a result of which timely delivery of priority messages is guaranteed.

4.5 Effective Scheduling As the link capacities are VANETS is not fixed, hence the scheduling is a very complex task and mandates joint optimizations at MAC and physical layers. Furthermore, developing efficient scheduling strategies that enable delayaware transmission of packets with different priorities is also a matter of concern for future VANET applications. As the nodes in a VANET keep on changing their position vary frequently, there is continuous change in the condition of he channel and network topology. The unbounded nature of the network size also poses a challenge. In rural areas the traffic load is comparatively low all time of the day while in urban areas the load is less during the night only [55]. Figure 6: Throughput gain % in AFAE & AEDCF [36]

SYSC 5408 CROSS LAYER DESIGN FOR WIRELESS MULTIMEDIA NETWORKS (Prof. F. Richard Yu) Proposed cross-layer scheme namely AEFE is compared with existing AEDCF technique using simulation as shown in Figure 6 Simulation results clearly show that proposed AEFE cross-layer scheme has higher percentage gain in throughput as compared to AEDCF scheme. 5.CONCLUSION In this article, the performance of traditional layered structure and how it leads to poor performance in wireless networks has been presented. To tackle these issues crosslayer structure has been proposed in Mobile Ad hoc Network (MANET) and Vehicle Ad hoc Network (VANET) to improve the performance of these wireless networks such as, security, QOS, reliability, etc. We described that VANETs are considered as a subclass of Mobile Ad hoc Network (MANET), so they inherit some features of traditional MANETs such as noise, path loss and interference. Furthermore, the challenges are caused by specific features of VANETS, such as highly dynamic network topology, variable node density, and intermittent connectivity have been addressed. The VANET architecture and its requirement have been also given in this article. We have presented the cross-layer design in VANETs and how they are classified in high level based on protocols. Moreover, some of the most common challenges in cross-layer design such as unreliable broadcasting, routing, network congestion, security and effective scheduling with some best possible solutions to tackle with each of these challenges are introduced in order to improve the performance, stability, reliability, delay of the layered structure in the VANETs. Although these five challenges are not all challenges of cross-layer structure in VANETs, there are some other challenges like the co-operation between the modularity and the cross-layer design, route disconnection, security, stability, physical layer and the MAC layer interactions, etc.

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