an introduction to a gis-based in-road information network

8/13/2019 An Introduction to a GIS-Based in-Road Information Network

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An Introduction to a GIS-Based In-Road Information Network

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An Introduction to a GIS-Based In-Road

Information Network

Karim MohammadiIran University of Science and [email protected]

S. Shahab SahhafiIran University of Science and [email protected]

Abstract

This paper proposes a new in-road information network to significantly improve LocationService and Routing Protocol based on GIS (Geographic Information System) road map,

speed and location of vehicles. Recent surveys have shown that current in-road information

networks are inefficient to find a destination node in long-distant routes (Location Service),

and unable to transfer information between source and destination easily for high-speed

vehicles (Routing Protocol). In order to overcome these difficulties the new in-road information

network suggests using communication map layer and considering each vehicle speed and

ocation, to divide in-road network into two hierarchical levels and makes it similar to a chain of

cells. As a matter of fact, these are the cells which virtually communicate with each other, and

make Location Service and Routing Protocol become easy and possible. The paper just

observes the network layer, and physical layers and interfaces are ignored.

. INTRODUCTION

This paper presents a new method based on Geographical Information system (GIS) maps in

making an efficient location service and routing protocols for in-road information networks.

Creating an in-road information network as an infrastructure for the inter-vehicle

communication has been considered recently. The main goal of this network is to transfer

traffic data and improve the safety of the road transportation. However, there are some

problems which may reduce the reliability and affect the performance of an in-road information

network: it is difficult to find a specific vehicle (node) in this large network. On the other hand,

t is difficult to track a destination vehicle in the network and forward a data packet to it, evenwhen the current position of the destination node is known.

Mobile ad hoc networks which are made by distributed mechanisms and cooperation among

all the nodes are bases of in-road information networks [1] [2].

Problems with physical layer, in which high speed vehicles decrease the capability of current

spread spectrum standards, and network layer, in which location service and routing protocol

are not reliable, affect the in-road performance of mobile ad hoc networks.

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Roads characteristics with fast and unpredictable movements of vehicles drastically increase

rate of the change in an in-road information network topology, so location services and routing

algorithms in current ad hoc networks have many limitations in road communication. This is

because relative positions of vehicles are frequently changed and make creating, control and

maintenance of routes very difficult. Also, it makes an increase in the amounts of control

messages and beacons which are using network band width improperly.

Overcoming this problem, vehicle position data is needed. So that, each vehicle should be

equipped with a GPS receiver to get its own instant geographical coordination. On the otherhand, in a position-based network a source needs to know a destination position before

sending data, so it must flood a request message in all directions of the network. Although this

mechanism can determine the destination position, it is a consuming time and high-cost

procedure. However, by using maps, storing and processing spatial and non-spatial data in

Geographical Information System (GIS), time and cost will be saved.

The objective of this paper is to develop a new method to design network hierarchy structure

based on GIS information and roads maps.

t is hypothesized that all of network nodes (vehicles) are equipped with GPS receivers, maps,GIS and required communication equipment inside.

I. GIS-BASED NETWORK ZONES

Position is an important data in networks using positionbased routings. Large networks use

hybrid routing protocols []. So, the whole network should be divided into many zones with

unique IDs (Zone ID). In fact, Zone IDs are very important because they are a part of the

address field of each packet. Furthermore, each node has a unique ID, too. Making a packet

address, a node has to integrate its ID and its relative zone ID.

n many mobile ad-hoc networks, zones depend on node existence. It means that, somenodes together make a zone, so if these nodes separate from each other, that zone may be

divided into smaller zones. Also, it is possible a zone to be eliminated or merged with other

ones.

According to importance of packet address field, it must be guaranteed that there arent any

similar IDs for different zones. This makes sure that the sending data packet can be received

properly in the destination. To achieve this aim, making vehicle-independent zones for in-road

nformation network is proposed. Nowadays, some cars have GIS maps in their navigation

systems. It is expected that in future, most of the cars will have this system with them, so they

can identify in which road they are moving along.

Using maps and spatial and non-spatial data, makes zone identification so easy and fast. In

this way, Geographical Information System (GIS) as a strong tool to collect, store, process

and display the result of spatial information helps to identify zones and their IDs. Also, it helps

the vehicles to know their relative position against other vehicles. Moreover, if a

communication layer map is added to GIS road maps it can show, in each part of the road,

how much wireless radio range is. This information is used to make unique zones that are the

same for all vehicles.



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Figure 1 shows a hypothetical map including zone configuration in a road.

Fig. 1. Road topography effect on the length of zones.

As radio signals can not be broadcasted in large distances in mountain regions, zones aresmaller than smooth areas.

Having road communication GIS map, locations of zones and their geographical regions are

determined. By loading them in all vehicles navigation system, in advance, all of the cars use

this information (zones geographical attribute and their unique IDs) and identify the road in

which they are moving.

t is possible one thinks that if there isnt such a map in a particular road, what will happen for

the network. The answer is, the network is just used for roads with specific characteristics

such as high density of vehicles (crowded roads), and so it is not necessary to have this kindof map for all roads. Since there arent enough cars in some roads, making such a network is

not always possible, because cars can not often connect together by using short range

wireless communication. Also, cars can get road division information in any ways (e.g.

cooperation among cars); the important fact is geographical based division.

After all, as in position-based addressing methods, each node is equipped with a GPS

receiver; each node will know its instant geographical position. Therefore, if a node wants to

obtain position of another node, it must search whole network i.e. it floods request messages.

Since there aren't any predetermined information stations in the network, the only node which

can reply properly is destination node. However, it is a time consuming and high costprocedure. To overcome these limitations, a virtual access point (VAP) in each zone will be

ntroduced in the next part.

II. VIRTUAL ACCESS POINT (VAP)

After zones determination, a leader must be identified for each zone, similar to an access

point in hierarchical ad-hoc networks [1]. In fact the leader (access point) is one of the cars

existing in the zone. Since this access point is varied by time, it is called a Virtual Access

Point (VAP). VAP controls the zone during a specific time and when it leaves the zone, gives

ts duty to another car.



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Suppose all vehicles are aware about zones in a specific road. When a vehicle enters a new

zone or starts its activity, it sends a beacon and waits to receive a response from the

environment. In case, there is no response, it starts sending a new beacon once again. This

process may be repeated so many times (depending on the network design).

After all, when the vehicle receives no response at all, the car will conclude that the only car

existing in the zone is it. So this new car will be the zone VAP. On the other hand, if another

car exists in the zone when a new car sends beacons, the zone VAP will reply it and after aittle time the VAP will take charge of the new car.

Fig. 2. A typical road zone scheme.

Figure 2 shows a typical road zone. Here VAPs take charge of their zones and vehicles. Since

the network have two levels, like other hierarchical ad-hoc networks, if a typical node (vehicle)wants to transfer a data packet to another node outside of the zone, it must be sent through

VAPs. It is clear that to have a reliable and unconnected link, the length (L) of a zone must be

up to half of the maximum radio range (R) in that part of the road.

When a new vehicle took charge of a zone, it starts to send beacons and control messages.

According to these messages, all nodes configure themselves and consequently the network.

Messages contents and their sending rate affect the performance of the network significantly.

High sending rate occupy bandwidth and reduce efficiency. In GIS-based network, since

zones are independent and not time varying, control messages and beacons will be reduced.

For example, each car needs to send its information twice during its presence in a zone; whent enters and exits, but VAP have to send its messages periodically because it must announce

ts presence to other nodes all the time. For example, if VAP is damaged, other nodes can

understand it, because they wont receive any messages (beacons) from their VAP.

Each VAP has a table including some information such as unique ID, speed, entrance and

estimated exit time of vehicles which are inside the zone. By looking up this table, a VAP can

dentify which nodes are inside its zone, quickly. Also VAP uses this information to determine

ts substitute VAP (SVAP). It is important to keep this selection up to date periodically. If for a

reason, VAP is damaged, after a little time the SVAP will take charge of the zone so other



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nodes will send their information to it. This work is done to save time and prevent any

disorders in network. In case there is no SVAP while VAP is broken, nodes have to cooperate

and determine a new VAP. This is a time consuming process.

Neighbor VAPs exchange their network information and cooperate with each other. When a

node leaves a zone, its VAP announces neighbor VAP and gives it the control and information

of the leaving node. This procedure is a little complicated for a VAP when it wants to leave its

zone. As it is mentioned, a VAP knows its neighbor zones, so when it is going to leave the old

zone, it knows whether there is a VAP in neighbor or not. If there is no VAP in the next zone, itwill be the next zone new VAP, but if a VAP exists; it will change to an ordinary node and

accept the next zone control. On the other hand when a VAP leaves its zone, it gives its table

nformation to SVAP.

The rate with which network topology changes, is an important factor in network operation.

The network with lower rate of topology changing has less control messages than a one with

high rate. VAPs affect this factor greatly because they control other nodes and periodically

broadcast beacons, so in VAP determination process, it is important to select a one which

affect the topology as less as possible. As it is said, VAP has a table with some information.

VAP knows how much it takes a vehicle to leave the zone, so it knows which car stays in thezone more than others. This may be a good choice for substitute, but if we want to implement

our network by current wireless equipment, it makes an important limitation.

Current spread spectrum communication technologies are sensitive to speeds of nodes. It

means for nodes with high speed, the efficiency of communication is reduced [3]. This factor

must be considered in substitute determination, so it had better choose a SVAP among nodes

with speeds near to average zone or lawful speed. This is not a defect for GIS-based network

.e. designers are free to consider any optional factors.

V. VAP AND ROUTING MECHANISMn the GIS based in-road information network, data communication has two classes: (1) data

communication within a zone and (2) inter-zone data communication. Nodes (vehicles) in the

same zone can communicate together directly. Before they start data transmission, notify the

VAP and if there aren't any problems, the VAP will give them permission. This is done

because if VAP receives a message request from another zone for one of these connected

nodes, it can interrupt them and get the message to the requested node.

Now suppose a typical node wants to send data packets to another one. At first it sends a

request to send message (RTS) to its VAP and notifies it. The VAP looks up the information

table. If destination is inside the zone, it will forward RTS to the destination node. ReceivingRTS, the destination node sends a reply back to the source. Now the source and the

destination can start data transmission.

f a source and its requested destination are not in the same zone, VAP must find the

requested node in the network, so it sends a RTS to its direct neighbor VAPs. Since all of the

nodes information is saved in VAPs, the source VAP must just search among VAPs. Here,

there is a "some for all" location service, because information of all nodes is saved in some

nodes. It is important to notice, positions of these stations (information data bases) are fixed

and known i.e. each zone has its VAP as a base station, and thus all of nodes can be



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connected to them without any problems.

Fig. 3. Hierarchical concept in GIS-based in- road network.

There is an interesting fact concern to in-road information networks that makes the design and

mplementation easy. As a matter of fact, since vehicles have to drive just in roads not outside

them, locations of nodes of network are nearly predetermined. If a specific car goes out of a

road, it will be eliminated from the network and it's not important, because according to our

aim, we want to make a network just in roads.

According to this fact, it is necessary for a VAP to send RTS messages toward two directions:up-road and down-road. Then unlike other location services and routing protocols in mobile ad-

hoc networks, finding an unknown destination is limited to two directions [2]. Then there is a

kind of directional flooding mechanism in routing phase in road networks.

By using VAPs, the amounts of RTS and control messages are reduced again. In the routing

phase, the only responsible node in each zone is VAP. This means that the maximum number

of hops in a two-level GIS based network is (3+ Nz), where Nz is the number of zones

between a source and destination. If both of a Source and destination are VAPs then the

number of hops will be (Nz+2). As we see, the maximum number of hops in a specific road is

predetermined and known, so the amount of delay originated from the number of hops can not

be increased from a threshold. An important source for delay is networking traffic that makes

data packets wait in queues in VAPs. This is caused from bandwidth limitation and discussed

n physical layer so here is not considered.



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Fig. 4 Route maintenance of In-road network.

After finding the destination and making a route between the source and destination, the

maintenance phase is started. In this phase it is tried to maintain the route during data

transmission. Like base stations in a common cellular communication system, VAPs have to

control and survey movements of nodes and forward data packets to destinations in new

zones. This is depicted in figure 3. As it observed, the source and destination zones are

changed during transfer of data. When a connected node (source or destination) wants toeave its zone, the VAP must pass it to the next VAP and then inform the source VAP.

Consequently this can be done for the source node. Because in an in-road network VAPs

ocate along road such as chains, informing source node may be not necessary. That is, when

a message arrives to the pervious destination VAP, this VAP correct the address and forward

t toward the destination where is in new zone now. It had better be done when a source and

destination move in opposite directions.

As a summery we can say our network is a hierarchical (two level) network in which zones is

divided based on geographical map that has a communication layer.

V. SIMULATION AND RESULTS

Since movements of vehicles in roads are random and unpredictable, statistical software

ARENA has been used to simulate and model the network. As mentioned, the rate that the

network topology is changed with is an important factor that predicts the amounts of control

messages and beacons. In high rate, it is expected that there are plenty of control messages

n the network and vice versa.

To analyze this factor in our in-road network, first we must consider a probability function that

presents the rate of vehicle entrance in a special zone. An exponential function can be a good



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choice. In this function an average of time when a car enters a zone ( = ) and the variance

( ) are given. It has the form like the below function:

n simulation the value of has been selected as 8, 30, 70, 110 and 160 second.

Determination of probability distribution function is another issue. The maximum speed whichcars must not pass it, is given in each road, so we can estimate the average speed of cars in

a special zone as a value near to maximum allowed speed. We chose a triangle probability

distribution function for speeds of vehicles. Since the numbers of cars, which have higher

speeds than allowed velocity, are less than vehicles, which have speeds less than allowed

one, the ramp in higher speed is sharper then the ramp in lower speed.

Fig. 5. The vehicle velocity probability distribution function

The average speed of vehicles in this simulation has been chosen equal to 115 km/h. In

addition, it is considered that vehicles have fixed velocity along the zone just for simplicity.

Another factor in our simulation was the length of a zone. It supposed equal to 3 km.

Simulation shows an interesting result. For example for as figure 6 indicates, the average

rate of topology change is equal to average cars speed divided by the length of the zone.

This is because this rate is proportional to VAP change rate. As mentioned before, VAP

broadcast control messages more than other nodes, so its movements have the most effect

on topology.



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Fig.6. Simulation result for .

After all, in our in-road information network the rate of topology change is a predetermined

value and can differ by cars average speed and the length of zone (if physical layer has no

imitation), so maximum delay and queuing problems can be estimated well. As another note,there is an interesting point relative to topology change rate: zones length and car velocity in

many roads are inverse, in other words in mounting regions cars derive slower than flat roads

and on the other hand in flat places the length of zones are shorter than one in even regions

(because of the range of radio communication). Therefore we expected that topology change

rate does not differ greatly from flat to even roads.

VI. COMPARISON & CONCLUSION

Comparison between our method with other identical researches and methods such as GLS,

SOTIS [4], DSR and ZPR indicates that all methods suggest using position data in the

address field of data packets [1] [2]. However, previous works have not used roadcharacteristics and especially their maps [4] [5]. Generally GIS can be used in in-road

networks, simplify models and reduce the amounts of computations. In addition, it guarantee

unique zone IDs for network. This is very important and efficiently affects reliability of data

delivery.

Table1. Comparison between mobile ad hoc networks in road applications.

Table 1 compares our GIS-based in-road information network with other mobile ad hoc

networks qualitatively. The main result of this comparison is VAP attribute in network



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performance. By using this content in addition to road map it is expected that the complexity of

n-road network be reduced.

VII. REFERENCES

1. George Aggelou, Mobile Ad Hoc Network , Mc Graw Hill, 2005.2. Martin Mauve and Jorg Widmer, University of Mannheim, Hannes Hartenstein, NEC

Europe, Heidelberg, "A Survey on Position-Based Routing in Mobile Ad Hoc Network",

pp. 30-39, IEEE Network, November/December 2001.3. Keith Biesecker, Broadband Wireless, Integrated Servieces and Their Application to

Intelligent Transportation Systems, Center for Telecommunication and Advanced

Technology, McLean, Virginia, June 2000.

4. Lars Wischhof, Andr Ebner, Hermann Rohling, " Self-Organizing Traffic InformationSystem based on Car-to-Car Communication: Prototype Implementation", WIT 2004-

1st International Workshop on Intelligent Transportation, Hamburg. www.et2.tuharburg.

de/Mitarbeiter/Wishhof_WIT2004.pdf

5. Walter Franz, Christian Wagner, Christian Maihofer, Hannes Hartenstein,

DaimlerChrysler AG, Research & Technology 3, 89081 ULM, Germany, "FleetNet:Platform for Inter-Vehicle Communications, 1st International Workshop on Intelligent

Transportation 2004. www.fleetnet.de

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an introduction to a gis-based in-road information network

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