## 1 Introduction
### QoS Based Connectivity in Heterogeneous Wireless Sensor Networks  
When a mobile wireless sensor (MS) can link to more than one router of the same network, we can establish a similarity to the well-known procedure of Horizontal Handoff or horizontal handover (also known as intrasystem handoff or handover) in general wireless networks. Horizontal handoff (HH) involves the change of the link of a mobile node to other wireless access points using the same technology. Nevertheless, the increasing complexity of the WSNs and their associated technologies forces us to consider heterogeneous scenarios where more than one network are present and where the nodes can be of different natures, technologies and behaviors. In these cases, and following with the previous similarity, we can follow a scheme of Vertical Handoff (VH) where the mobile node can link to other wireless access points of the same or different sensor networks and technologies.  
In this work we study the more general and complex case: the VH process of a mobile wireless sensor moving along heterogeneous wireless sensor networks. In order to explain better the framework of our study, let us consider an example next.  
Figure 1 shows a case where three different networks are present. Each network has a set of routers as access points to the network for mobile wireless sensors, and a collector node which receives the information of the sensor and supplies it with different services. All the routers in this scenario could be sensors too, configuring thus a heterogeneous wireless sensor network of fixed routers (with different routing paths) ready to provide support to mobile wireless sensors where the connections are established under Quality of Service (QoS) requirements.  
Let us suppose a mobile wireless sensor moving through this scenario (Figure 2). In its route, some router nodes are within its reach, where these routers can belong to different networks. In this situation, the mobile wireless sensor must decide at any time what the best router is to connect to, in other words, what network provides it the best quality of service. As we can see in Figure 2, the mobile sensor can find more than one router in determined times, where the possible routers can be of the same network (horizontal handoff) or different networks (vertical handoff). For this case, some situations take place. For positions #3 and #25 there is not possible to link to any router, the terminal falls inside areas without coverage and the transmitted values by the mobile sensor (for example temperature) are not collected in no way. Vertical handoff happens, for example, in position #8, where three different networks can be reached because there are up to five routers belonging to these networks in the coverage area of the mobile sensor; in this case, the mobile sensor must decide what network to link to, according to the quality of service provided. On the other hand, horizontal handoff takes place, for example, in position #16, where two different routers belonging to the same network can be reached;  
Figure 1: Heterogeneous wireless sensor network with three networks based on sensor routers of different technologies, giving different quality of service, and ready to provide support to mobile wireless sensors.  
in this case, the sensor continues connected to the current network. Figure 3 shows the complete scenario, combining Figures 1 and 2, in order to have a complete sight of the possible vertical and horizontal handoff processes.  
Figure 3: The complete scenario for the heterogeneous wireless sensor network where different processes for vertical and horizontal handoffs can be given.  
Figure 2: A mobile wireless sensor configures a path where several routers belonging to different heterogeneous networks can be reached in order to establish a link. This scheme is based on the scenario of Figure 1. The first column indicates the mobile sensor position in the path; the second column tells us how many available routers can be reached.