# Energy Sustainability in Wireless Sensor Networks: An Analytical Survey
## 4 Algorithm-Based Energy Sustainability in WSNs
### Algorithm-Based Methods for Energy Sustainability in WSNs
#### 4.3.2 Duty Cycling  
Energy saving in a WSN can be accomplished through managing the activity status of the communication module of the nodes. Specifically, the transceiver of the node should be powered off when not receiving or transmitting data, and powered on once there are available data to be handled by the radio submodule. The process of switching the activity status of the transceiver among various modes (i.e., transmission--Tx, reception--Rx, idle, and sleep) depending on the current network requirements is known as duty cycling [59]. It is implemented by using appropriate wake-up oscillators that perform clock generation [60; 61] and wake-up receivers that perform the idle listening while keeping the main radio completely off [62; 63; 64]. Duty cycling schemes are categorized into three main classes: Topology control, sleep/wake-up mechanisms, and MAC protocol. Duty cycling approaches are implemented
* Topology control protocols correlate with the redundancy of network. In some applications, nodes are randomly deployed, and additional nodes are used to confront likely to occur node failures. These protocols intend to dynamically adapt the network's Topology to each application's needs and seek for the minimum number of nodes that ensures the connectivity of the network by utilizing redundant nodes [19]. The nodes that have no crucial role in ensuring the coverage and the connectivity requirements, can temporarily fall into sleep mode in order to preserve their energy levels, and wake up once needed. Topology control protocols are distinguished in location driven and connectivity driven [23]. Location driven protocols determine the activity status of a node, i.e., whether and when this node should be activated or deactivated (sleep mode), by taking into consideration the exact location of this node and all of the rest network nodes (which is known); Connectivity driven protocols ensure the preservation of connectivity by adaptively managing the activation or deactivation of the network nodes. Specifically, only the sensor nodes that are required in order to maintain the network connectivity, remain active while all of the rest network nodes remain in sleep mode, thus saving energy. * Sleep/Wake-up schemes aim to save energy reserves by lessening the periods that the radio submodule of nodes remains inactive, since even when inactive they still consume energy. There are three types of such protocols, which are differentiated regarding transmission and reception patterns. They are: on-demand, scheduled rendezvous and asynchronous [23]. In On-Demand mechanisms nodes should be awake only when it is necessary to communicate with other nodes. Informing a sleeping node that an adjacent one is trying to reach it so as to initiate communication, can be achieved by utilizing multiple radios with different operational characteristics (i.e., rate and power). On-demand mechanisms, are ideal for applications that are defined by a low duty-cycle, such as the detection of a special event (i.e., fire), since, in such cases the sensor nodes monitor the environment and wake up as soon as they detect an event. So, nodes remain active only when needed [19]. Yet, utilizing on-demand mechanisms, usually requires the presence of two different channels, one that is used for the normal data communication and one that is responsible for waking up the nodes when required [23]; Scheduled rendezvous methods determine a wake-up schedule that is the same for all the nodes of a WSN. Nodes simultaneously wake up and once they are awake, they remain so for a definite period of time and go back to sleep all together until their next rendezvous. In order to ensure simultaneous wake-up, nodes must be synchronized. Additionally, to maintain the same wake-up schedule, nodes use deterministic, random or specific wakeup patterns [20]; In asynchronous duty cycling mechanisms, each node selects when to either wake up or sleep, regardless of the activity status of its neighbors.