## 1 Introduction  
Wireless sensor networks (WSNs) are having a very significant growth in segment markets such as industrial, automotive, defence, intelligent transportation and smart homes. However, such networks have resources of limited capacity. For instance, the bandwidth of a ZigBee network is restricted to 250 kbps. Hence, a challenging issue is the design of an efficient medium access protocol. We can distinguish three types of medium access protocols: TDMA, CSMA/CA and hybrid ones. TDMA ensures a deterministic access without collision and exhibits very good performances in case of heavy loads evenly distributed. CSMA/CA is very efficient in case of moderate traffic load and provides spatial reuse of the bandwidth. That is why hybrid solutions tend to combine the advantages of both. For instance, they behave like TDMA in case of heavy load and like CSMA/CA in case of light load. In this paper, we propose an hybrid medium access protocol called TDMA/CA (Time-Division Multiple Access with Collision Avoidance) and evaluate its quantitative performances. TDMA/CA keeps the determinism of TDMA and the spatial reuse of CSMA/CA. TDMA/CA aims at providing an efficient medium access in wireless sensor networks where bandwidth is the first resource that must be efficiently used. The second resource is energy that must be saved to prolong network lifetime. Since the sleep state is the least consuming, the MAC protocol should support sleeping nodes. Furthermore, communication being energy expensive, wasteful transmissions should be avoided. The third resource to consider is time. The delay to reach the sink should be minimized in data gathering applications. In TDMA/CA, node coloring provided by an activity scheduling module called SERENA (SchEdule RoutEr Node Activity) allows several nodes to transmit simultaneously without interfering, whereas the nodes that are not one-hop neighbors of the transmitters save energy by sleeping. Moreover, in data gathering applications, colors are assigned in such a way that data collected from the sensors reach the sink in a single cycle.  
This paper is organized as follows. We first give a brief state of the art in Section 2 dealing with MAC protocols in wireless networks and node activity scheduling based on graph coloring. In Section 3, we present our solution consisting of two modules: the node coloring module SERENA and the medium access module TDMA/CA. We show how this solution can be adapted to various application requirements and different environment constraints. We consider two types of applications: general ones and data gathering applications. As a first contribution, we prove that the three-hop coloring problem is NP-complete. As a second contribution, we show in this paper how to reduce the overhead induced by the coloring algorithm. Thus, SERENA can be used even in dense networks with limited bandwidth. The properties met by each module are given in an ideal environment. The performances of SERENA and TDMA/CA are evaluated by simulation in Section 4 for general applications and data gathering ones. Our third contribution shows that if links of poor quality are used, collisions can occur in any slot assignment algorithm providing spatial reuse based on node neighborhood. We give a rule that prevents this phenomenon and quantify the benefits brought by SERENA and TDMA/CA in terms of throughput,delivery rate and response time. We focus on data gathering applications and quantify the optimization of the end-to-end delays, our fourth contribution. In Section 5, we consider a real environment where color conflicts can occur due to the occurrence of new links resulting from the dynamicity of the wireless topology. We evaluate the impact of such conflicts on the throughput, delivery rate and response time. We propose a solution to detect and solve the color conflicts, our fifth contribution and determine when such a solution should be applied. Finally, we conclude in Section 6.