# Energy-Aware Forwarding Strategies for Delay Tolerant Network Routing Protocols
## 2 Related Work
### Energy Efficiency of DTN Routing Protocols  
Different routing protocols with their own diverse characteristics and features have been proposed for DTNs. The existing DTN routing protocols do not consider the available energy resources when making routing decisions. Only a few researchers have investigated energy-aware protocols. [PERSON] et al. [22] investigated the performance of four existing DTN routing protocols in terms of energy consumption by varying the number of nodes, message sizes, message generation intervals and the nodes' speeds. They only measured energy consumption and did not consider energy information in DTN routing decisions. The authors in [11; 23] proposed energy-efficient add-ons to existing routing protocols such as the Epidemic routing protocol and Two-Hop routing protocol (2 HR), which uses a forwarding strategy based on the message lifetime and delivery probability requirements. They analyzed the performance of these two protocols using analytical models such as an Ordinary Differential Equations (ODE) model in a heterogeneous DTN. In [11], an ODE model was used to model the performance of these routing protocols with individual node selfishness. The energy-efficiency improvement of their work is revealed by comparing the performance of both Epidemic and 2 HR protocols. The applicability of these protocols in realistic scenarios representing practical applications needs further investigation. Another body of literature focuses on maximizing network lifetime. [PERSON] et al. [24] proposed EXLIOSE (EXtending network LIFetime in Opportunistic Sensor Networks), a routing protocol in mobile WSNs based on a new metric called Energy Shortage Cost (ESC). The aim of this approach is maximizing network lifetime by choosing the next hop based on ESC, which considers the energy consumption and residual energy of each sensor node. A decision for transferring the message to other nodes is based on a calculation of the ESC in each node, and messages are forwarded to the neighbour with a lower ESC. [PERSON] and [PERSON] [25] proposed a novel context-metric queuing method for Delay Tolerant Networks to achieve high delivery probabilities with higher-priority messages and utilize the node's energy efficiently in transmission. A new metric is introduced to sort and queue a message in the buffer by using a node's remaining energy, speed and estimated distance to the destination. To achieve effective utilization of a node's energy in transmission, an energy-aware transmission scheme is implemented to assign priority levels of messages for forwarding the messages to nodes. Both EXLIOSE and CEAMS (Context-metric queuing and Energy-Aware Message priority Scheme) methods provide higher message delivery rates and performance over protocols that do not take energy into consideration. However, the energy consumption and residual energy of nodes are calculated based on radio scanning and transmitting energy without considering energy consumption when generating messages and without considering energy harvesting. Energy-Aware Epidemic Routing (EAER) [26] is an extension of the n-Epidemic routing, which aims to improve packet delivery ratio and energy consumption. The n-parameter strategy is proposed to optimize the possibility of transferring packets from a node to its neighbors when they are within the transmission range. By using this technique, a node will transmit its packet to the next node only when it is within the range of at least n neighbors. They proposed an energy-aware heuristic algorithm called the Prevalence Strategy (PS), which manages the value of n based on the Current Energy Level (CEL) and Current Neighbour Node (CNN). When the residual energy of a node is low, the n-parameter value will be increased so the probability of being within the range of many nodes is low, allowing the node to conserve energy. Previous work on DTN routing protocols mostly focused on the energy consumption.