# Efficient Opportunistic Routing Protocol for Sensor Network in Emergency Applications
## 2 Related Work  
The Low Energy Adaptive Clustering Protocol (LEACH) [18] is a well-known clustering protocol of sensor networks. In this protocol, the cluster head (CH) is randomly selected from all nodes based on a metric called probability of CH. Once clusters are constructed, the member nodes of each cluster transmit data to its CH using a time division multiple access (TDMA) scheme. The CH collects all data, aggregates and transmits them to the base station for further processing. In LEACH, the cluster member nodes transmit data periodically and the cluster formation takes place in every round, which is not energy efficient. In the Threshold Sensitive Energy Efficient Sensor Networks Protocol (TEEN) [16], each sensor node senses and transmits an event of interest (e.g., pressure, humidity) based on the minimum attribute value set into to the sensor. The next time the node senses and transmits another event of interest, if the attribute value changes by a certain threshold, that is set into the node. Hence, the number of sensed and transmitted events is less in the TEEN protocol. However, LEACH and TEEN protocols may not work well for emergency and disastrous situations. The work done in [12] introduced an opportunistic routing protocol, namely EXOR. In this protocol, if the sender needs to transmit data to a destination it first transmits data to a node from its forwarding list, which has the least distance to the actual destination. Once the sender sends the data packet to all forwarding nodes, it determines the best forwarder based on the receiving ACK packet from the forwarder nodes. Although EXOR has higher throughput, it only uses the information during the time of transmission but does not use the updated information during transmission. Another work in [12] introduces an opportunistic routing that selects the next hop forwarding node based on the data delivery ratio and priority of nodes to transmit data. In [12], the authors present a Trust Opportunity Forwarding Mechanism that considers both distance of forwarding nodes to the destination (cost) and the trust factor/degree. The trust factor of a node is calculated by neighboring nodes through direct observation or recommendations. A node is selected as a forwarder if it has better trust degree as well as less distance to the destination. On the other hand, among several routing protocols Social Relation Opportunistic Routing (SROR) identifiesthe forwarding nodes based on social relations and profiles among nodes. The work done in [20] introduces a planned opportunistic delay tolerant routing protocol by creating a subset of opportunistic links from source to destination, as opposed to a deterministic single copy or multiple copies of links between source to destination. This paper optimizes the multiple deterministic links to obtain only the best one in terms of lower data delivery rates. However, this approach does not employ clustering and is not efficient, as the creating paths require lot of computations and consumes energy of sensor nodes. The work done in [21] introduces LORA--a load-balancing opportunistic routing for asynchronous duty-cycled WSN. To avoid the flooding of packets from source nodes, through a number of preselected forwarder nodes, this approach creates candidate zones where all possible data forwarder candidates reside. The source node only chooses a candidate based on a metric calculated using direction distribution, transmission distance distribution, perpendicular distance distribution, and residual energy distribution. Again, this approach seems computationally expensive and does not allow clustering to achieve more energy efficiency. In opportunistic routing, each node decides about the forwarder node locally using greedy approaches and collects lot of information (i.e., overhead) for making such decision. To mitigate this problem, the work done in [22] introduces SeeR--a simulated annealing-based routing protocol for opportunistic mobile networks.