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

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.

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 are smaller 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.

It is possible one thinks that if there isn’t 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 kind of map for all roads. Since there aren’t 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 cost procedure. To overcome these limitations, a virtual access point (VAP) in each zone will be introduced in the next part.

III. 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 its duty to another car.

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 a little 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; when it enters and exits, but VAP have to send its messages periodically because it must announce its presence to other nodes all the time. For example, if VAP is damaged, other nodes can understand it, because they won’t 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 identify which nodes are inside its zone, quickly. Also VAP uses this information to determine its 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 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, it will 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 information to SVAP.