# Software Defined Networks in Wireless Sensor Architectures
## 3 Wireless Sensor Networks
### Wireless Sensor Network Challenges  
WSNs present some general challenges [54] in terms of sensor management and data processing unification. One of the problems of WSN is the Network Discovery, since the network topology has to be built in real time and it has to be updated permanently against adding new sensors or sensors' failures. Another challenge is Network Control because each network node changes dynamically in terms of energy, bandwidth, etc. Routing is also an important aspect of WSN in contrast to IP routing, since the routing paths are established from geoinformation depending on the needs. Moreover, Collaborative Signals and Information Processing require the communication between nodes, and, at the same time, do not lose any signal during the transmission. Tasking and Querying include the methods to query data gathered from the network. Finally, the most important challenge is the Security since the network can be deployed in a hostile environment, and, therefore, it is exposed to physical attacks. As described before, several surveys have discussed the aspects on WSN. [PERSON] et al. [55] provide a comprehensive review on most recent developments and challenges that the WSN need to overcome with some solutions to mitigate them. In particular, Ref. [55] also deals with the increasing importance of IEEE 802.15.4 standard [56] in WSNs. Such standard defines the characteristics of the physical and Medium Access Control (MAC) layers for Low-Rate Wireless Personal Area Network providing reliable data transfer, short-range operation and reasonable battery life while maintaining a simple and flexible protocol stack. Four key requirements must be fulfilled for a creditable WSN deployment in industrial environments: energy efficiency, scalability, reliability, and timeliness. The first requirement, energy efficiency, is extremely important since the sensor nodes are typically powered by batteries that some of them cannot be replaced or recharged due to the environmental circumstances. In this case, the 802.15.4 standard includes a power management mechanism, based on duty cycle, to minimize the activity of sensor nodes. The second feature, scalability, is another important factor that has to be considered in a WSN since the number of deployed sensor nodes may be high, especially when large deployments cover big geographical areas. Finally, reliability and timeliness are very critical factors in the industrial field. If a given set of data packets are not correctly delivered to the sink, the correct behaviour of the system can be compromised. In these terms, the IEEE 802.15.4 standard provides a very low reliability in terms of packet delivery ratio when power management is enabled. This behavior is caused by the 802.15.4 MAC protocol, and, therefore, throughout, we will refer to it as the 802.15.4 MAC unreliability problem. [PERSON] et al. [57] propose a solution for this issue taking advantage of the flexibility in choosing Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA) parameters that the IEEE 802.15.4 standard offers. [PERSON] et al. [58] resume the main security challenges and problems in WSN. They state that a secure system is the one that each one of its components are secure since the ones that do not provide security are potential point of attacks. In addition, they affirm that a secure system has to fulfill some features such as key establishment and trust set up, authentication, privacy, robustness to communication, secure routing and resilience to node capture. Regarding key establishment, the easiest solution is the establishment of a shared key across the network, but, unfortunately, all network traffic can be decrypted if the key is intercepted. The most used choice to provide security between nodes is the public key cryptography. In order to provide protection against attackers who wants to spoof, eavesdrop or inject malicious code, WSNs have to provide authentication using cryptography. Using it, senders and receivers are identified in the network as reliable entities.