A Priority based Congestion Prevention Technique for Vehicular Ad-Hoc Networks

Computing For Nation Development, March 10 11, 2011 Bharati Vidyapeeth s Institute of Computer Applications and Management, New Delhi A Priority based Congestion Prevention Technique for Vehicular Ad-Hoc Networks Mohammad Pasha 1 and Mohd Umar Farooq 2 1,2 Muffakham Jah College of Engineering and Technology 1 muhammed.pasha@gmail.com and 2 umarfarooq.mohd@gmail.com ABSTRACT Vehicular ad hoc networks facilitate the communication among the nearby vehicles and between the vehicles and fixed equipment called as Road side unit (RSU). The main purpose of VANETs is to furnish safety and comfort for passengers and even prevent them from accidents. The process of communication comprises of passing of warning or service messages. In this paper we propose priority based congestion prevention technique in VANETs where different types of messages have got different priorities based on their criticality. The lower priority message is denied by a vehicle if any higher priority message arrives. Here accident message has got highest priority. By doing so, firstly we prevent the congestion of vehicles and also this scheme prevents further accidents/ mishap. We evaluate the usefulness and efficiency of proposed mechanism using Ad hoc On-Demand Distance Vector routing protocol (AODV) in our study through Qualnet simulator version 4.5 and its 802.11 radios. The messages that flow throughout the Ad hoc network are prioritized and are serviced based on their priorities and its efficiency is evaluated. KEYWORDS VANET, congestion, AODV, RSU, priority and VEHACOL 1. INTRODUCTION A wireless ad hoc network is a decentralized wireless network [1]. The network is ad hoc because it does not rely on a preexisting infrastructure (such as routers in wired network or access points in infrastructure/managed wireless networks). Instead, each node participates in routing by forwarding message for other nodes, and so the determination of which node forwards the message is made dynamically based on the network connectivity. Vehicular ad hoc network is a type of wireless ad hoc network. Vehicular ad hoc networks (VANETS) are important component of the Intelligent Transportation Systems (ITS) [3]. Vehicles communicate with each other via Inter-Vehicle Communication (IVC) as well as with roadside unit (RSU)/Base Station via Roadside-to-Vehicle Communication (RVC). The optimal goal is that vehicular ad hoc networks will contribute to safer and more efficient roads in the future by providing timely information to drivers and concerned authorities. The main goal of VANET is providing safety and comfort for passengers. A special electronic device will be placed inside each vehicle which will provide Ad-Hoc Network connectivity for the passengers. This special device is called on-board unit (OBU). This network tends to operate without any infrastructure or legacy client and server communication. Each vehicle equipped with VANET device will be a node in the Ad- Hoc network and can receive and relay others messages through the wireless network. Collision warning, road sign alarms and in-place traffic view will give the driver essential tools to decide the best path along the way. In this paper we propose a priority based congestion prevention technique in VANETs, where different types of messages are prioritized based on their criticality. Higher priority message is processed first by the node/vehicle and upon its completion, lower priority messages are accepted and the request or service is processed. This results in the prevention of vehicular congestion and accidents. Here AODV (ad hoc on-demand distance vector) routing protocol is used which is Pure ondemand routing protocol i.e. A node does not perform route discovery or maintenance until it needs a route to another node or it offers its services as an intermediate node and nodes that are not on active paths do not maintain routing information and do not participate in routing table exchanges It uses a broadcast route discovery mechanism and uses hopby-hop routing i.e. routes are based on dynamic table entries maintained at intermediate nodes and similar to Dynamic Source Routing (DSR), but DSR uses source routing. In the rest of this paper, we first include about VANETs in section 2 and brief description of Vehicular congestion and AODV protocol in section 3. We present our idea of preventing congestion and accidents in section 4. Section 5 evaluates the performance of the proposed scheme and includes conclusion part. Last section deals with the future enhancements. 2. ABOUT VANETS Every node/vehicle in VANET is provided with a communication device called as on-board units (OBUs) which permits the vehicles in Ad hoc network to exchange messages and communicate with each other as well as with the roadside units (RSUs) also called as roadside base stations, placed in some critical sections of the road, such as intersections, traffic lights or stop signs; to give the driver essential tool to decide the best path along the way which prevents congestion and accidents thereby providing safety and comfort for passengers. On-board units (OBUs) facilitates the vehicles to communicate with each other and as well as with RSUs. A VANET is an independent network which is not controlled by any centralized authority or organization. It is a self-

organized network that enables communications (through messages) between vehicles and RSUs. So as to provide other network applications and services such as internet access to the vehicles, the RSUs can be connected to a backbone network. AODV uses sequence numbers to ensure the freshness of routes. It is loop-free, self-starting, and scales to large numbers of mobile nodes. 3. AODV CONCEPTS Adhoc On Demand Vector protocol is a i) Pure on-demand routing protocol, ii) Uses a broadcast route discovery mechanism and iii) Uses hop-by-hop routing. The next section details its phases. Fig.1. An example of VANETs 2.1. VEHICULAR CONGESTION Vehicular congestion is the primary problem of many developing countries like India. Vehicular congestion occurs when the volume of traffic generates demand for space greater than the available road capacity [5]. Heavy traffic jams or congestions lead to accidents which results in injuries to passengers, cause damage to vehicles and as well as their life. Vehicular congestion has a number of disadvantages or impacts: Wasting time of drivers and passengers, resulting in allocating more time to travel and less time on productive activities. More fuel consumption leads to higher amount of carbon dioxide emission and increases air pollution (which contributes to global warming). Pollution caused by slow moving traffic causes health hazards. These negative effects of congestion can be prevented by controlling the traffic jams. For the prevention of the vehicular congestion we have proposed priority based Congestion prevention technique in VANETs. 2.2. AODV The Ad hoc On Demand Distance Vector (AODV) routing algorithm is a routing protocol designed for ad hoc mobile networks [6, 7]. AODV is capable of both unicast and multicast routing. It is an on demand algorithm, meaning that it builds routes between nodes only as desired by source nodes. It maintains these routes as long as they are needed by the sources. Additionally, AODV forms trees which connect multicast group members. The trees are composed of the group members and the nodes needed to connect the members. 3.1. AODV ROUTE REQUEST PHASE It uses Local HELLO messages are used to determine local connectivity to reduce response time to routing requests and trigger updates when necessary. Sequence numbers are assigned to routes and routing table entries which can supersede stale cached routing entries. Every node maintains two counters i.e. A Node sequence number and a Broadcast ID. the protocol is initiated when a node wants to communicate with another node, but does not have a route to that node then the Source node broadcasts a route request (RREQ) packet to its neighbors. Every neighbor receives the RREQ and either returns a route reply (RREP) packet, or forwards the RREQ to its neighbors. The pair (source_addr, broadcast_id) uniquely identifies the RREQ. Here broadcast_id is incremented for every RREQ packet sent and receivers can identify and discard duplicate RREQ packets. If a node cannot respond to the RREQ then the node increments the hop count and Saves the following information to implement a reverse path set up (AODV assumes symmetrical links) Neighbor that sent this RREQ packet Destination IP address Source IP address Broadcast ID Source node s sequence number Expiration time for reverse path entry (to enable garbage collection) Fig.2: Route Request Packet format 3.2. AODV ROUTE REPLY PHASE If a node receives an RREQ packet and it has a current route to the target destination, then it unicasts a route reply packet (RREP) to the neighbor that sent the RREQ packet. Intermediate nodes propagate the first RREP for the source towards the source using cached reverse route entries. Other RREP packets are discarded unless dest_sequence_# number is higher than the previous, or destination_sequence_# is the

A Priority based Congestion Prevention Technique for Vehicular Ad-Hoc Networks same, but hop_cnt is smaller (i.e., there s a better path). RREP eventually makes it to the source, which can use the neighbor sending the RREP as its next hop for sending to the destination. Cached reverse routes will timeout in nodes not seeing a RREP packet. Fig.3: Route Reply Packet format 3.3. AODV ROUTE MAINTANANCE PHASE Route changes can be detected by failure of periodic HELLO packets, failure or disconnect indication from the link level and failure of transmission of a packet to the next hop (can detect by listening for the retransmission if it is not the final destination). The upstream (toward the source) node detecting a failure propagates a route error (RERR) packet with a new destination sequence number and a hop count of infinity (unreachable). The source (or another node on the path) can rebuild a path by sending a RREQ packet. hence the communication between the subnets takes place. Then the subnet forwards the received messages to all the other nodes in the subnet. The different messages have got different priorities depending on their criticality and importance. Here the highest priority is set for the accident or life saving messages so that when an accident occur the vehicles which met with an accident sends the message to other vehicles with highest priority. Similarly there are other messages which got lesser priorities i.e. internet access message, group communication message etc. When a vehicle receives a message it accepts the message and processes the request. During this process if any message of higher priority arrives, the vehicle discards the request that is being processed and begins processing the newly arrived message (which is of higher priority). The messages (which contain requests) are processed based on the priorities as shown in the figure 4. Fig.5. Message Priorities Here in the scenario of this paper the vehicles which met with an accident send the accident message with high priority, which when received by other vehicles are serviced first and is passed to other vehicles in their vicinity which in turn is passed to vehicles of other subnet. In this way the accident message of one subnet is passed to other subnets and vehicles which were to enter in the affected area diverge or change their paths to avoid congestion of vehicles. By this scheme vehicularcongestion is prevented at first place and further accidents are also avoided. Fig.4: Nodes generating RREP and RREQ Packets-path formation between node A and node J 4. PROPOSED SCHEME The different subnets in the network are allocated different frequencies so that the network traffic is passed on smoothly over different channels with respect to their listenable and listening masks. The different nodes in the subnet pass the information messages to the subnet which in turn passes them to the other nodes in that subnet i.e. vehicles inside a subnet can pass messages to the vehicles in the same subnet only. The messages of one subnet are confined to only that subnet, to pass the messages to other subnets we need to have a RSU. When a vehicle enters into a subnet it registers itself with the RSU [4], the RSU communicates with other subnet s RSU by which messages of one subnet are forwarded to other subnets and 5. PERFORMANCE EVALUATION The previous section describes briefly about the central idea of our proposed scheme. This section evaluates the usefulness and efficiency of proposed mechanism. Here we are using Ad hoc On-Demand Distance Vector routing protocol (AODV) in our study through Qualnet simulation version 4.5 and its 802.11 radios. The messages that flow throughout the Ad hoc network are prioritized and are serviced based on their priorities and its efficiency is evaluated. The vehicles are equipped with wireless interfaces which corresponds to the access points on the road way. Our simulation study has selected the packet-size of 512 bytes. The simulation will run for 30 seconds. 5.1. SIMULATION SETUP The simulation topology consists of ten vehicles each equipped with a wireless interface, three access points and a static internet service provider (ISP). Messages are sent and received among vehicles and between vehicles and RSU using File Transfer Protocol (FTP) generic link. ISP is connected to RSU

using point-to-point link and Internet service is provided to vehicles through Constant Bit Rate (CBR) link. 5.2. SCENARIO FOR VEHICULAR COLLISION The mobility of vehicles is set such that the performance of proposed mechanism is best evaluated. In this scenario each subnet has one access point (RSU). Two vehicles (V1 and V2) in subnet1 follow a straight path and after some time they meet with an accident. The occurrence of this accident is notified to RSU and other vehicles with in the vicinity of collided vehicles by sending an accident message with the highest priority. Vehicles (V3 and V4), which were following the same path, changes their track when they are notified about the accident that happened ahead. All the three subnets and the vehicles in their region are informed about the accident. In another subnet (subnet2), two vehicles (V5 and V6) collide and they send accident message as done in the first accident. Here one more vehicle (V7) from subnet1 which moves into the region of subnet2, receives the accident message and changes its path. This accident message and an internet access message are sent to the RSU of subnet2 at the same time. But as the accident message has got the highest priority the RSU drops the internet access message and broadcasts the accident message with in its vicinity. Other vehicles (V8, V9 and V10) access the internet or communicate with each other. Fig.7: CBR throughput for internet connectivity for vehicles Fig.6: FTP throughput between Vehicles and RSU 6. RESULTS OF THE SIMULATION In the graph (Figure 6 and 7) dark lines indicates the sending of messages (packets) and light lines indicates the receiving of messages (packets). The graph (Figure 8) shows the messages (packets) dropped at various nodes. This drop is due to the message s priority. The lower priority messages are discarded when higher priority messages arrives. Fig.8: Packet drop at different nodes. 7. CONCLUSION In this paper we propose a priority based congestion prevention technique in VANETs for the safety and comfort of passengers. Here the different types of messages are assigned different priorities based on their importance and criticality. Whenever

A Priority based Congestion Prevention Technique for Vehicular Ad-Hoc Networks the collision between two vehicles occurs, all the vehicles in the accident region receive the accident message (which has got higher priority). They make decisions of changing path as soon as they receive the message so as to prevent congestion and further accidents. And when a vehicle receives low priority and high priority messages at the same time, then the vehicle ignores the lower priority message and accepts the higher priority message. 8. FURTHER ENHANCEMENT This scheme can be made more efficient by using VEHACOL mechanism [7]. If two accident messages arrive at a node at the same time, the vehicle decides which message is to be accepted first using VEHACOL. The vehicle has to check the angle and the direction of the source vehicle and make appropriate decision. REFERENCES [1]. Ad hoc networks http://en.wikipedia.org/wiki/wireless_ad_hoc_networ. [2]. Ad Hoc wireless network architecture and protocols by c.sricaram murthy and B.S manoj, pearson education 068, June 2002. [3]. Vehicular ad hoc network http://en.wikipedia.org/wiki/vehicular_ad-hoc_network. [4]..Yong Hao Yu Cheng Kui Ren Dept. of Elec. & Comp. Eng., Illinois Inst. of Technol., Chicago, IL. Global Telecommunications Conference, 2008. IEEE GLOBECOM 2008. IEEE. [5]. Vehicular Congestion http://en.wikipedia.org/wiki/traffic_congestion. [6]. C.E.Perkins, E.M Belding-Royer and S.Das, Ad-hoc On- Demand Distance Vector (AODV) Routing, RFC 3561, July 2003. [7]. C.E. Perkins and E.M. Royer, The Ad hoc On-Demand Distance Vector Protocol. In C.E. Perkins, editor, Ad hoc Networking. Addison-Wesley, 2000. [8]. Ashwin Gumaste and Anirudha Sahoo Vehacol: Vehicular Anti- Collision Mechanism using a combination of periodic information exchange and power measurements, Indian Institute of India, Bombay, INDIA.