# IoT for Smart Cities: Machine Learning Approaches in Smart Healthcare--A Review
## 3 Wireless Sensor Networks for Smart Cities
As a result, a wide variety of sensors can be integrated and addressed relatively flexibly. A wide variety of sensors are compatible, ranging from basic temperature and humidity sensors to high-precision vibration sensors or GPS modules for position determination. Thanks to improved manufacturing processes, standardization and advances in the field of microsensors (MST), very small, inexpensive, but nevertheless, very precise measuring sensors, so-called MEMS, can be integrated into such systems today. The requirements for sensor networks are usually very different from those of conventional networks. These requirements must be met by special communication protocols, and algorithms are treated and simply cannot be taken over. Thus, all nodes need a high level of self-organization to build a functioning network structure. Here, nodes of the network fail and others join, without affecting communication, otherwise the network may collapse. Conventional communication principles such as client-server models are unsuitable for this. To be monitored by such sensors, event-based solutions are more suitable. Here, a message is sent by a sensor as soon as a certain limit value of the sensor is exceeded. To get the information about these events through the network to self-organizing routing nodes requires special cooperative algorithms. Sometimes, the collected information in the node is already preprocessed to announce information about the entire topology of the network. In addition to these dynamic requirements, sensor networks are subject to restrictions as the individual sensors only have limited energy resources, and these, if possible, are to be used sparingly. Thus, the knots should work effectively and, with a minimum, get by on control packages, e.g., parts of the node or the node itself is put into sleep mode if no activity in the monitored area is recorded and only wakes up when communication is necessary [28]. Figure 2: Graphical representation of WSNs for smart cities [26]. In addition to the problems that have to be considered when developing MAC protocols, questions of routing are also of great importance. Here, too, conventional technologies and algorithms cannot simply be adopted. The main problem researchers struggle with is the strong dynamics typically found in sensor networks. Accounts can permanently change their position and thus make the creation of stable routing paths impossible. All nodes have limited energy resources and can suddenly fail, and yet, the data traffic routed from accounts to nodes must not come to a standstill. Although the routing protocols have a number of difficult problems to solve, they still have to be relatively simple and easy to implement so that none of the nodes have to spend a lot of energy on routing [25]. Despite all of the changed conditions compared to conventional networks, tried-and-tested mechanisms have also been adopted. In this way, every node can fall back on a flood of its data in the event of an error. The packet in question is simply sent to all nodes within range with the hope of reaching the destination. Of course, this is by no means an optimal solution, as too much network traffic is generated and an enormous amount of energy is wasted. In addition to very simple processes such as shortest-hop-first, in which the packet is sent to the node that is closest to the destination, or hierarchical processes in which tree-like structures are built, one also tries to break new ground. Thus, probabilistic methods are often used to keep the routing effort in each individual node low. Similar to MAC protocols, it remains to be seen that no routing protocol will fully establish itself because the context of the application determines the most suitable protocol. In addition to the actual application, the characteristics of the network traffic are often analyzed and design decisions for the routing protocol are made on this basis [28]. One of the major challenges faced by WSNs is cheap production of small, robust sensor nodes for widespread deployment.