# Energy Sustainability in Wireless Sensor Networks: An Analytical Survey
## 4 Algorithm-Based Energy Sustainability in WSNs
### Algorithm-Based Methods for Energy Sustainability in WSNs
#### 4.3.3 Energy Efficient Routing  
By default, the transmission and routing of data in WSNs consume great amounts of the energy that sensor nodes retain [73]. Therefore, in order to preserve the lifetime of nodes and consequently of WSNs, it is critical to implement routing protocols that are energy efficient. The energy efficient routing protocols are generally classified, in terms of their organizational or functioning characteristics, into four main categories that namely are: Communication Model, Network Structure, Topology, and Reliable Routing [51; 74]. * The protocols belonging to the Communication Model category typically can deliver more data for a certain amount of energy. Nevertheless, the delivery of data is not assured. They are classified as Query-based, Coherent/Non-Coherent, and Negotiation based. * Query-based protocols use enquiries to support the transfer of data from nodes that own information to nodes that request specific pieces of this information. Protocols of this type enable both multiple path routing and dynamic network topologies [51];
* Coherent protocols perform minimum processing of the sensed data and then they send these data to other nodes, called aggregators, which further process them. In Non-Coherent routing protocols, nodes process sensed data locally before they transmit them [74];
Negotiation-based protocols use meta-data negotiation patterns in order to reduce the quantity of redundant data at destination network nodes. In this way, energy efficiency is achieved. * Energy efficient routing protocols of Network Structure category are classified as either Flat or Hierarchical. * In Flat protocols there is not any hierarchy adopted and every sensor node has the same role with all of the rest network nodes. Protocols of this kind perform well in networks constituted from a small quantity of sensor nodes [51];
* In Hierarchical protocols, the role of each one of all network nodes depends on the position that it holds within the overall hierarchical structure of the sensor network [75]. In this way, data aggregation is enabled, and great scalability is achieved. Additionally, load balancing is achieved [76]. * Energy efficient routing protocols belonging to the Topology category use position related information in order to route data. They are further classified into three subcategories, namely: Location-based, Mobile Sink-based, and Mobile Agents-based. * In Location-based protocols, all nodes know not only their own location but also the positions of both their neighboring nodes and the destination nodes during data routing. Consequently, the most energy efficient routing paths are followed [74];
* Mobile agents-based routing protocols presume that a movable entity collects the sensed data from the individual network nodes in order to convey these data to the BS. The arrival of mobile agents near the network nodes that sense data reduces the energy expenditure for data transmission of these sensor nodes. Additionally, the traffic load in the entire network is reduced [51];
* Mobile sink-based protocols, suppose the existence of one or more sinks (i.e., base stations) that move around the FoI in order to collect data sensed by the network nodes. In this way, the energy consumed by the network nodes in order to transmit data is considerably reduced [51]. * Reliable Routing protocols pursue the attainment of increased trustworthiness in data routing either by satisfying specific QoS metrics or by using a number of alternative paths in order to route data. They are categorized into two corresponding subcategories, i.e., QoS-based protocols and Multipath-based protocols depending on whether they chase QoS metrics or implement data routing via multiple paths. * QoS-based protocols consider not only energy consumption, but also other metrics such as end to end delay and quality characteristics of the data transmitted.