ABSRP - A Service Discovery Approach for Vehicular Ad-Hoc Networks Brijesh Kadri Mohandas,

Amiya Nayak SITE, University of Ottawa,

800 King Edward Ave, Ottawa, ON, Canada, K1N 6N5

Kshirasagar Naik Dept. of Electrical & Computer

Engineering, University of Waterloo,

Waterloo, ON, Canada, N2L 3G

Nishith Goel Cistel Technology Inc., 40-30 Concourse Gate, Ottawa, ON, Canada,

K2E 7V7

Abstract A vehicular ad-hoc network (VANET) is a network of

intelligent vehicles that communicate with other vehicles in the network. The main objective of VANET is to provide comfort and safety for passengers. In addition, various transaction based services, such as information about gas prices, restaurant menu, and discount sale, can be provided to drivers. In order to make these services available, there is a need for an efficient service discovery protocol. In this paper, we propose a new protocol called Address Based Service Resolution Protocol (ABSRP) to discover services in vehicular ad-hoc networks. As most of the transaction based services are provided by roadside units, we exploit their presence to perform service discovery. We utilize the unique address assigned to each service provider in order to discover a route to that service provider. Our technique proactively distributes the service provider’s address along with its servicing capabilities to other roadside units within a particular area. Each roadside unit will then utilize this information to service the request placed by the vehicles. If the service provider (destination node) is not reachable over the vehicular network, we propose to use a backbone network to service requests. Our approach is independent of the network layer routing protocol. We have evaluated the performance of our approach by using the Qualnet simulation tool.

1. Introduction A vehicular ad-hoc network is a kind of mobile ad-hoc

network. Each vehicle in VANET is equipped with a radio interface for communication with other vehicles in the network. The IEEE and ASTM (American Society for Testing and Materials) have designated a block of spectrum in the range of 5.85 to 5.925 GHz to be used for inter-vehicle and vehicle to roadside communication. This radio service is called DSRC (Dedicated Short Range Communication). DSRC supports a data rate of 6 to 54 Mbps with a communication range of 1000 meters. The type of services that can be provided over VANET can be broadly classified into two categories: broadcast application service and transaction application service. A broadcast service is used to inform drivers about an emergency situation. A transaction based service is used to provide location based services, such as information about local gas prices, discounts offered in super markets, etc. The authors of [6] have presented an event dissemination protocol for Vehicular Ad-Hoc Network. One way to request for a service is to broadcast the request throughout the network. The service provider after receiving the message would then

respond with a result. Their method incurs a high message cost. In order to overcome this problem, service discovery protocols are used to resolve service requests. A service discovery protocol enables the application to discover services provided by service providers. The authors of [1] and [2] have proposed a new service discovery protocol, Vehicular Information Transfer Protocol, VITP, for vehicular ad-hoc networks. The drawback of this approach is that as the distance between the service provider and requester increases, the service request dropping rate increases. VITP relies upon the geographic routing protocol to discover services.

Routing in VANET has attracted much attention. The authors of [5] have presented a routing protocol called connectivity aware routing (CAR) for VANET. It is a position based routing scheme capable of finding connected paths between source and destination pairs. Authors of [7] have presented a geographical opportunistic routing for vehicular networks which exploits the topology of VANET and geographical information for routing. It has also been observed that addressed based routing protocol performs better than geographic routing protocol [3] in vehicular ad-hoc networks. In this paper, we have studied a technique to discover services using roadside units. In our approach, we utilize the network layer routing protocols to determine whether or not the service provider is reachable. If a route to the service provider is not available, we propose to utilize a backbone network, such as the Internet, to find a route.

2. Related Work Service discovery plays an important role in providing

location based services. In general, it is necessary to have an efficient service discovery protocol to locate services. Authors of [1] and [2] have proposed a new service discovery protocol called Vehicular Information Transfer Protocol (VITP) for VANETs. VITP is an application layer communication protocol to discover services in VANETs. VITP specifies the syntax and the semantics of the messages exchanged between VITP peers. A VITP peer runs on the centralized on board controller of the vehicle, and can access vehicle sensors to retrieve useful information. VITP relies on geographic routing protocol to route service queries and responses. In this approach, service providers are dynamically chosen based on capabilities and interest to participate in resolving the query.

VITP supports two types of message dissemination, push-based approach and pull-based approach. In push-based approach, a vehicle broadcasts significant information to the neighboring vehicles, it is similar to flooding. In pull-based approach, a vehicle initiates a query for information of its

2008 IEEE Asia-Pacific Services Computing Conference

978-0-7695-3473-2/08 $25.00 © 2008 IEEE

DOI 10.1109/APSCC.2008.44

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2008 IEEE Asia-Pacific Services Computing Conference

978-0-7695-3473-2/08 $25.00 © 2008 IEEE

DOI 10.1109/APSCC.2008.44

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interest. These queries are location sensitive and specify the origin and the destination area. Each location in a VITP message has a road_id and segment_id, where road_id uniquely identifies the road and segment_id identifies a segment of a road. Location-aware service requests originated by vehicles are serviced by the service providers, termed as VITP Ad-Hoc Servers (VAHS). VAHS are a dynamic collection of VITP peers in the target location area which are interested in participating in the resolving the service requests. Each service request generated is uniquely identified using a randomly generated message identifier (msg_id). This is used by the originator of the request to match the service response against the pending requests. VITP peers maintain a cache with recently received message ID’s. A VITP peer will not act upon two identical service requests with same message identifier.

Figure 1 shows an example for service discovery using vehicular information transfer protocol. Driver of vehicle ‘A’ in area ‘X’ in interested to know the price of gas in at least two gas stations located in area ‘Y’. Vehicle ‘A’ constructs a service request, which is routed to the target area over the vehicular ad-hoc network. Geographic routing protocol is used for this purpose. Once the query arrives at the target area it is first received by vehicle B. Vehicle B makes a check to see if it can service the request. As the request for gas price cannot be serviced by vehicle B, it transmits the message to its neighbors. It is then received by Gas station B. Gas station B is capable of servicing the request, it computes the requested information and then checks the return condition provided by the request originator. The request originator has specified that it needs information from at least two gas stations, therefore gas station B will update the message with partial results and will transmit the message (service request) to its neighbors. When this message is received by gas station A, it computes the results and verifies the return condition. As the return condition is met, it constructs a response along with the information sent my gas station B and transmits the same to the originators area (Area X). Upon arrival at are ‘X’, the response message is broadcasted within the originators area, i.e. area ‘X’. If the request originator is still in area ‘X’, then it receives the response.

Figure 1: VITP example

From the simulation results presented in [1], it is observed that as the query distance increases the service request failure rate increases. Query distance is the distance between the originators area and the target area. It is

observed that, for a query distance of 3000 meters more than 50% of the service request failed. The main reason for this is the reliability of Vehicular Network. If the density of vehicles is high, there is a high possibility that there exists a route to the destination or service provider. If the density of the vehicle is low, then a route to destination may not be available. Vehicular information transfer protocol relies on geographic routing protocol to route the query to the target area and thereafter route the response from the service provider back to the service requester. For vehicular ad-hoc networks, it has been observed that performance of addressed based routing protocols like AODV is much better compared to the geographic routing protocol [3], [4]. In [1], it is mentioned that if a VITP request fails, an alternative backbone network could be used to service requests. As the vehicular speed in very high, it is necessary that there is no delay in making such a decision to switch between vehicular network and a backbone network.

In order to increase the efficiency of service discovery, we have proposed a new approach to discover services in the vehicular ad-hoc networks. Our approach relies on the road side units to service requests. Road side units are installed across the network, and they provide different kinds of services. It is possible to interconnect roadside units through internet. This way information provided by roadside units can be exchanged over the backbone network (internet). The service discovery approach presented in this paper is independent of the routing protocol used in the network layer.

3. System Model Although vehicular ad-hoc networks are similar to mobile

ad-hoc networks, there exist some differences. In many cases the mobility pattern of nodes in MANET is random, but in the case of VANET, vehicles follow a strict mobility in the direction towards the destination. Nodes in MANET have a limited or finite power supply, whereas in VANET a continuous power is supplied to the communication device using the battery installed in vehicles. Another important aspect of VANET is the vehicle speed. A vehicle in VANET travels at a much higher speed compared to a node in MANET. Vehicular speed introduces an additional challenge of routing in VANET. Vehicular ad-hoc network is not a reliable network. Reliability depends on the density of vehicles. In VANET, it is not always possible to have a communication path between the service provider and the service requester.

Figure 2 shows the logical network model used in our research. We have considered Roadside Units to be static units providing an external interface to the vehicles. Each roadside unit has two interfaces, one is the wired interface through which it is connected to internet and the other is the wireless interface through which it is connected to the vehicles. The mobile nodes in our model exhibit vehicular mobility, changing speed at different intervals. Each mobile node or vehicle has a single wireless interface through which it communicates with roadside units and other vehicles in the network.

Gas Station A

Gas Station B Originators area

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Target Area or Area Y

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Figure 2: System Model

4. Address Based Service Resolution Protocol (ABSRP) A vehicular ad-hoc network facilitates vehicle-to-vehicle and vehicle-to-roadside communication using the on-board radio device. The type of services that can be provided for drivers over VANET can be broadly classified into broadcast application service and transaction based application service. Broadcast application service involves flooding the network with useful and emergency related information. Location based service is an example for transaction based service. A transaction based service involves two participants, the service provider and the service requester. In vehicular ad-hoc networks the service requesters are the drivers / vehicles, and the service providers are roadside units or other vehicles. Service discovery protocols are used to discover the service provided by the service providers. In this paper, we present a new service discovery approach which exploits the presence of roadside units in vehicular ad-hoc network.

Roadside units are intelligent devices installed across the vehicular ad-hoc network which are capable of providing various services for drivers and passengers. Although the deployment of roadside units in not clear at this time, it can be assumed that roadside units are installed at traffic junctions, major highways, near restaurants, near gas stations and various other hot-spots. These roadside units can have two interfaces: wireless interface and wired interface. Using the wireless interface, the roadside units communicate with the other vehicles in the network, and using the wired interface the roadside units are connected to internet. As a result, two roadside units in a vehicular ad-hoc network can communicate with each other over the backbone network (internet). This ability can be used to distribute the service provided by various service providers over the backbone network. In a vehicular ad-hoc network, it is not necessary for a roadside unit to be aware of the services provided by all the service providers. In this paper, we assume that each service provider is aware of the services provided by other service providers within a fixed area. This can be a small or large area depending on frequency of service requests or the density of vehicles in that area. Within this area, roadside units exchange information related to their service providing capabilities. The kind of information is restricted to the type of service provided and the IP address of the service provider. By doing this, each roadside unit is aware of the IP address of the service provider and the type of the service provided within the designated area. Additional information about the service itself is exchanged during the service

request query resolution. Main functionality of a service discovery protocol is to discover the IP address of the service provider in a chosen area. If the IP address of the service provider can be proactively determined, resolving service requests will be much easier.

Figure 3 shows a simple urban scenario with three roadside units. The dashed connection between the roadside units indicates that the roadside units are interconnected over the backbone network (internet). We assume that roadside units are installed every 2 to 3 kilometers. Roadside units within an area, communicate with each other over the internet to learn about the service provided and the IP address of the service provider. This is done proactively and the frequency of updating this information can be configured based on the requirement.

Over the wireless interface, a roadside unit broadcasts its presence to the vehicular network by periodically transmitting hello_packets. These hello_packets travels MAX_HOPs within the vehicular network. A vehicle after receiving a hello_packet, verifies the hop count in the message, if the hop count is less than MAX_HOPs, it will rebroadcast this message to its neighbors. Each vehicle in the vehicular network associates itself with a nearest roadside unit (leader). If the new hello_packet indicates that the new roadside unit is closer compared to its existing current leader, then the vehicle associates itself to the new roadside unit.

Figure 3: Service discovery in VANET

When a vehicle needs a service, it constructs a service request with the type of service and the area where the service is desired and transmits this service request to its current leader. The roadside unit after receiving this message will check if it has proactively learned about the service provider. If the roadside unit is aware of the service provider’s IP address, it will forward the service request to the target service provider. If the roadside unit is not aware of the service provider, it will broadcast the service request destined to the target service provider over the backbone network. The target service provider, after receiving the service request, will construct a service response and will transmit the same to the request originator (vehicle). A roadside unit can transmit a service request to the target service provider over the vehicular network or backbone network. In this paper, we have used the vehicular network to transmit and receive service request and service response. Listed below is the algorithm for our approach.

RSU A RSU B

RSU C RSU – Road Side Unit

A

Wired Network (Internet)

Road Side Units

Wireless Interface

Vehicles

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Protocol for Road Side Unit (RSU): 1: Send Hello Packet every 800 milliseconds. 2: switch (event type) 3: case (service request) 4: if (RSU can service this request) 5: Respond with the service requested. 6: else if (RSU is aware of service provider’s IP address) 7: Forward the request to service provider. 8: else if (service provider’s IP address is not available) 9: Broadcast request in backbone network. 10: end if 11: case (hello packet) 12: if (No. of hops < MAX HOPS) 13: Rebroadcast the hello packet. 14: default: Report Error 15: end switch

Protocol for Vehicle: 1: switch (event type) 2: case (hello packet) 3: Associate with the nearest RSU. 4: case (service desired) 5: Build and send the service request to its current leader. 6: default: Report Error 7: end switch

In Figure 3, vehicle ‘A’ is interested to know the service

provided by roadside unit ‘A’. The current leader of vehicle ‘A’ is roadside unit ‘C’. Vehicle ‘A’, constructs a service request with the type of service and the area where the service is desired and transmits the same to its current leader, roadside unit ‘C’. Roadside unit ‘C’, after receiving this request will check its database to verify if it has already learned about roadside unit ‘A’. As both the roadside units are in same area, roadside unit ‘C’ finds information about roadside unit ‘A’ in its database. It then retrieves the IP address of roadside unit ‘A’ and forwards the service request to roadside unit ‘A’ over the vehicular network. Roadside unit ‘A’ after receiving this request will compute the requested information and will then forward the results to vehicle ‘A’. An advantage with this approach is that if roadside unit ‘C’, does not find a path to roadside unit ‘A’ over the vehicular network, it can obtain the requested information over the backbone network (internet) and can still service the request.

We have used the following metrics to evaluate the performance of ABSRP:

• Success rate (average number of successful service requests)

• Time taken to service request (Round trip time)

5. Simulation Results The performance of service discovery protocol presented

in this paper has been verified using the Qualnet Simulation tool (version 3.8) [8]. We had five road side units deployed in a (5000m * 1000 m) terrain. All the roadside units and the vehicles were equipped with 802.11a compatible radio as the

IEEE 802.11a standard exhibits similarities with DSRC. A group of 50 vehicles entered the network from a fixed point from the left side of the terrain. The inter-arrival time for vehicles is a configurable parameter. We have used an inter-arrival time of 2 to 3 seconds in our simulation. We have made an assumption that the roadside units are already aware of the services provided by its one hop neighbor. As proposed in our approach, this information is gathered by roadside units over the internet. We have not simulated the internet interface in our simulation. A snapshot of the model is shown in Figure 4.

In general, vehicles have a strict mobility pattern, all the vehicle travel in a strict route towards the destination. The existing mobility model like the random waypoint model cannot be used for vehicles. For this reason, we have modified the random waypoint model to make it suitable for vehicular networks. As per the new model, each vehicle randomly chooses a speed between the configured minimum and maximum speed, and travels towards the destination along the Cartesian X terrain. The inter-arrival time between arrivals is a configurable parameter.

Figure 4: Simulation Model

One significant advantage of our approach is that the service discovery is independent of the network layer routing protocol. In this paper, we have tested the performance of our approach using the AODV routing protocol (Ad hoc On Demand Distance Vector). AODV routing protocol has been designed for mobile ad-hoc network. It is a reactive routing protocol wherein the route to the destination node is obtained when needed. Authors of [3] and [4] have verified the performance of AODV in vehicular ad-hoc networks. It has been observed that AODV dominates some the existing MANET routing protocol when used for vehicular ad-hoc networks.

Success rate of service request has been verified against different velocity gaps. Velocity gap is the difference between the minimum and maximum speed of a vehicle. This is configured for each vehicle in the mobility model. Before the vehicle starts moving, it randomly chooses a speed between the configured minimum and maximum speed and travels towards the destination. For our simulation, we have used different velocity gaps in the range between 15-29 meters per second. As shown in Figure 5, 70-80% of the service requests were successfully resolved.

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Velocity Gap vs. Success Rate

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Figure 5: Velocity gap vs. Success rate

Figure 6 shows the average delay to resolve service requests. It is the average time needed to resolve a service request (round trip time). It is observed from our results that the service requests generated by the vehicles were resolved in less than 270 milliseconds. While using internet as a backbone network, additional delays may impact the round trip time; this is not shown in this figure. Figures 5 and 6 show velocity gap as a single parameter, which is the difference between the configured maximum and minimum speed.

Velocity Gap vs. Delay

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Figure 6: Velocity gap vs. Average delay

Figure 7 shows the success rate of service requests against the vehicular speed (Average Speed). The success rate in Figure 5 and Figure 7 is observed to be around 70 - 80%. The reason for this is that we have used UDP protocol at the transport layer, so there is no recovery of lost or dropped messages. This can be improved by using TCP at the transport layer. But this may introduce additional delay in servicing requests.

Average Speed vs. Success Rate

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Figure 7: Average Speed vs. Success rate

Our results show that using roadside units to service request is a feasible approach. It is observed that 70 to 80% of the requests are serviced with a service time of less than 250 milliseconds.

6. Conclusion In future, vehicular ad-hoc networks will play an

important role in providing comfort and safety for passengers. Various additional services could be provided for drivers and passengers over VANET. In order to discover these services there is a need for an efficient service discovery protocol. In this paper, we have proposed a new service discovery approach. We exploit the presence of roadside units in order to increase the efficiency of service discovery. The results presented in this paper confirm the feasibility of our approach for service discovery in vehicular ad-hoc networks.

7. References [1] Marios D. Dikaiakos, Saif Iqbal, Tamer Nadeem and Liviu

Iftode, “VITP: an information transfer protocol for vehicular computing”, Proc. 2nd ACM Int. Workshop on Vehicular Ad Hoc Networks, VANET 2005.

[2] Marios D. Dikaiakos, Andreas Florides, Tamer Nadeem, and Liviu Iftode. ‘‘Location Aware Services over Vehicular Ad-Hoc Networks using Car-to-Car Communication’’, IEEE Journal on Selected Areas in Communications, Vol. 25, No. 8, October 2007.

[3] Kenya Sato, Yosuke Matsui and Takahiro Koita., “Adaptation of Mobile Ad-Hoc Network Protocols for Sensor Networks to Vehicle Control Applications”. Proc. of SPIE, Vol. 6041, 2005.

[4] Sven Jaap, Marc Bechler and Lars Wolf. “Evaluation of Routing Protocols for Vehicular Ad Hoc Networks in City Traffic Scenarios”, Proc. 5th Int. Conference on Intelligent Transportation Systems Telecommunications, ITST 2005.

[5] Valery Naumov and Thomas R. Gross, ‘‘Connectivity-Aware Routing (CAR) in Vehicular Ad-hoc Networks”, Proc. 26th IEEE Int. Conference on Computer Communications, INFOCOM 2007.

[6] Ilias Leontiadis and Cecilia Mascolo, “Opportunistic spatio-temporal dissemination system for vehicular networks”, Proc. 1st Int. MobiSys Workshop on Mobile Opportunistic Networking, 2007.

[7] Ilias Leontiadis and Cecilia Mascolo, “GeOpps: Geographical Opportunistic Routing for Vehicular Networks”, Proc. IEEE Int. Symposium on World of Wireless, Mobile and Multimedia Networks, WoWMoM 2007.

[8] Qualnet Software. http://www.scalable-networks.com/, 2007.

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