## 5 Challenges and Perspectives  
Although the use of sensor nodes equipped with directional antennas represents a promising tool in mitigating the risks of security attacks in WSNs, research in this field is still in the beginning stages. This research status should be significantly improved with the availability of new commercial-off-the-shelf sensor nodes equipped with directional antennas and relying on efficient network protocols. However, the road towards endowing commercial wireless sensor nodes with directional antennas is still long and not free of challenges, and further improvements being expected in both technological and operational aspects. The most important difficulties in providing such sensor nodes lie in:  
1. designing small sized, reasonably priced and energetic-efficient directional antennas able to be integrated in highly resource-constrained sensor nodes;
2. developing efficient MAC protocols to address deafness, directional Hidden Terminal (HT) problem or Head-of-Line (HoL) blocking problem in multi-hop wireless networks [66; 67];
3. providing network protocols able to assure self-localization, self-configuration, self-synchronization and self-optimization in the case of randomly deployed sensor networks using aerial scattering or other similar procedures;
4. designing effective and reliable neighbor discovery mechanisms, being known that traditional approaches either depend on omnidirectional annuncers and on time synchronization or are two complex to be implemented in real large-scale sensor networks [13; 37];
5. adapting the in-network data and message aggregation mechanisms to the directional antenna-based topology of WSN;
6. designing customized topology control mechanisms to increase effective network capacity and conserve energy; and
7. providing appropriate QoS models incorporating both communication-related parameters (e.g., delay, packet delivery ratio, jitter, _etc._) and sensing-related parameters (e.g., network sensing coverage, probability of missed detection of an event, sensor failure probability, _etc._). Despite the fact that some protocols or mechanisms required by operational needs (items (ii)-(vi)) are already reported in scientific literature, their validation in real-world WSNs applications is still pending. Despite all these difficulties, the use of directional antennas in wireless sensor networks has already proved several advantages: it improves the transmission reliability, increases the spatial reuse, extends the transmission range or decreases the overall network power consumption. Moreover, directional antennas offer sensor nodes additional control over signal strength and interference, which allows the use of optimization techniques for providing higher network throughput and transmission reliability. Last but not least, the directional antennas provide significant advantages in coping with various security threats. Studying the factors that can boost the effectiveness of such devices when coping with security attacks, we found that narrowing the radiation region of antennas favors both of the abovementioned types of approaches (direct and indirect). By this the probability of eavesdroppers/jammers to be outside the radiation zone is increased and, moreover, the localization based on signal's angle of arrival becomes more accurate. The endeavor to narrow the radiation region for directional antennas is not a simple task knowing that the antenna's size increases with the increase of angular resolution [17]. From the information security point of view, employing directional antennas for communication purposes inside WSN opens up a wide spectrum of new research opportunities as follows:  
1.