## 2 Literature Review
### Hierarchical Routing Approaches  
Hierarchical or cluster-based routing approaches, originally proposed in wired networks, are well-known techniques with special advantages related to efficient communication. The concept of hierarchical routing is also utilised to perform energy-efficient routing in WSNs. In a hierarchical architecture, high-energy sensors can be used to process and transmit information, whereas low-energy sensors can be employed to perform sensing tasks in the proximity of the target. Hierarchical routing is usually a two-layer routing approach, where one layer is composed of cluster heads and the other layer consists of cluster members. Cluster members are responsible for sensing tasks, while cluster heads are used to aggregate packets and transmit the aggregated packets to the sink node. Since the size of aggregated packets is much less than that of raw packets, the energy consumption used for packet transmission can be significantly reduced, and thus, the overall network lifetime can be prolonged.  
The most famous hierarchical routing protocol is LEACH (low-energy adaptive clustering hierarchy) [19]. LEACH randomly selects a few sensor nodes as cluster heads and rotates this role to evenly distribute the energy load among the sensors in the network. In LEACH, cluster heads compress packets arriving from cluster members that belong to the respective clusters. Cluster heads then send aggregate packets to a sink in order to reduce the number of packets that must be transmitted to the sink.  
In [20], an enhancement over the LEACH protocol was proposed, _i.e._, power-efficient gathering in sensor information systems (PEGASIS). PEGASIS is a near optimal chain-based protocol. The basic idea is that in order to extend network lifetime, sensor nodes need to communicate only with their closest neighbours, and these neighbours take turns in communicating with the sink. Unlike LEACH, PEGASIS avoids cluster formation and uses only one sensor node in a chain instead of multiple nodes to transmit the sink.  
[PERSON] [21] presented a coverage-preserving routing protocol that was a modified version of LEACH. Unlike most routing protocols, the aim of his protocol is to preserve sensing coverage when some sensor nodes are no longer available, because, for example, their energy is used up. [PERSON] considered that simply increasing the lifetime of sensor nodes might not automatically achieve good sensing coverage. Thus, unlike LEACH, [PERSON]'s protocol considered not only energy consumption, but also sensing coverage when clusters were formed.  
[PERSON] _et al._[22] devised a non-dominated algorithm to improve a simple hybrid routing protocol (SHRP) [23] in choosing the best route towards the sink node. Their algorithm allows simultaneous analysis of energy- and latency-related metrics to generate a Pareto-optimal solution, which has better time convergence and reliability than SHRP.  
[PERSON] and [PERSON] [24] developed an adaptive energy-aware routing protocol that was expressly designed for many-to-many communication, _i.e._, simultaneously routing from multiple sources to multiple sinks. To increase network lifetime, their protocol minimises the number of nodes involved in many-to-many routing and balances forwarding load, reduces the amount of redundant information flowing in the network and decreases contention on the wireless medium and packet collisions.