# Extensive Experiments on Opportunistic Routing in Wireless Sensor Networks
## 2 Related Work  
### Opportunistic Duty Cycle Based Routing in WSNs  
Many works [14; 24; 25] have applied the opportunistic routing to save energy in WSNs. Many metrics are proposed in the literature. In [24], ORW applies asynchronous low duty cycle and selects the potential forwarders according to EDC: the Expected number of Duty Cycled wakeups until a packet has reached its intended destination. However, packet duplication still be possible. In [26], authors propose ORIA protocol which is an improved version of ORW supporting in-network aggregation. The protocols proposed in [24; 26] require the exchange of duty cycle information to update the EDC metric. In [14], the residual energy is used as a metric to select packet forwarders which contributes to balance energy consumption among all nodes. In ASSORT (Asynchronous Sleep-wake Scheduling and Opportunistic Routing Technology) [25], whenever a node has data to transmit, it broadcasts beacons to collect potential forwarders. If no acknowledgement is received, the node retries to find a forwarder after a sleep period. This waiting time increases data delivery delays. In ASSORT, the routing metric is based on: residual energy, link reliability, and sleep-wake schedules. Similarly, the paper [27] considers the problem of relay selection. The objective is to reduce energy consumption and to avoid routing through nodes with low residual energy. Thus, like [14] for instance, nodes are prioritized according to two metrics: their distance to the sink and their residual energy. The authors evaluate the performance of their proposal via a real testbed using Zigbee nodes in both indoor and outdoor scenarios. In RI-MAC (Receiver-Initiated Medium Access Control) protocol [15] when any node turns on its radio, it transmits a short beacon frame to announce that it is ready to receive data. To avoid collisions, the receiver informs its neighbors about its collision window, that is the period that must be used to communicate with it. In the same category of receiver-initiated MAC protocols, CD-MAC (Contention Detectable MAC) [28] has been proposed more recently. In CD-MAC, once a sender receives a probe from a receiver, it acknowledges it. Then, the receiver will poll one after the other each of the potential forwarders having sent such an acknowledgement. The proposed scheme has been implemented in TinyOs on TelosB motes. Unlike RI-MAC, B-MAC [11] is sender-initiated. Each node willing to send data, transmits a \"wakeup signal\", called a preamble which lasts longer than the receiver sleep interval. Each node periodically wakes up to check if there is any preamble, in which case it remains active to receive possible incoming packets. Despite its advantages, opportunistic duty cycle comes with challenges that should be faced. In particular, the tradeoff offered by these two techniques has been extensively studied. For instance, Reference [29] focuses on the throughput and the energy efficiency while considering the duty cycle in mobile wireless networks. The authors establish an analytical model to determine the evolution of these two metrics. Results show that throughout and energy consumption depend on listening durations. As many other papers, the paper ends by proving the advantages of the duty cycle on energy consumption. Reference [30] deals with another aspect in opportunistic routing which is packet duplication. Through a coordination method between a source and its potential forwarders, authors significantly reduce the amount of duplicate data. For our case, ODYSSE avoids the duplicate packet overhead and ensures packet unicity in the network. In [31] authors focus on the tradeoff between energy gain and delay loss when applying the duty cycle. They propose a method to enable nodes to dynamically adjust their duty cycle depending on the application delay requirements. The work in [32] mainly focuses on the load balancing issue in opportunistic routing.