# Reliability-Aware Cooperative Node Sleeping and Clustering in Duty-Cycled Sensors Networks
## 2 Related Works
### Geographic Routing  
Due to good scalability and efficiency, geographic routing is an attractive localized routing for WSNs. Most existing geo-routing protocols require every fixed node in the network to broadcast its accurate position information, so that all its direct neighbors can precisely locate the node. The greedy perimeter stateless routing (GPSR) [14] is a representative stateful geo-routing protocol wherein every node periodically exchanges beacon messages to maintain the location information of its neighbors. A packet holder chooses the closest neighbor to be the next relay node. A right-hand rule is used for bypassing the void region after a void region is reached.  
In dynamic network environments due to node mobility, node sleeping and link failures, to reduce packet losses, a forwarding node will sets up multi-backup next-hop nodes so that it can have alternative (backup) nodes to choose. For instance, the medium access control (MAC) layer will fail to deliver a packet when a primary next-hop node dies and becomes unreachable. After several retransmission failures, the MAC layer will drop the packet and notify the network layer. When receiving the notification, the routing protocol immediately selects a backup next hop and transmits the same packet, which is stored in the cache, down to the MAC layer. If the backup next hop also dies, such a retransmission mechanism will be repeated. When the node failure rate is higher than the threshold, such a multiple backup-node strategy along with data caching will severely increase network ETE delay, reduce bandwidth utilization and waste nodes energy due to unnecessary retransmissions.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Items** & **GPSR** & **BLR** & **REER** & **RCR** \\ \hline \multirow{3}{*}{Categories} & Stateful geo-routing protocol & Stateless routing protocol & Reliable and energy-efficient routing & Reliability-aware cooperative routing \\ \hline \multirow{4}{*}{Features} & Waste communication resource; significant energy consumption & Acknowledgment collision; worse robustness of forwarding, latency and energy efficiency & Relaying candidate selection is independent from data & Adaptive in dynamic network environments; from data control energy consumption; guaranteeing communication overhead & ETE reliability \\ \hline \multirow{4}{*}{Related Works} & Receiver-oriented load-balancing and reliable routing in wireless sensor networks [13]; GPSR: greedy perimeter stateless routing for wireless networks [14] & Contention-based forwarding for mobile ad hoc networks [15]; routing protocol in dense networks [18]; wireless sensor networks [19] \\  & & & & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of greedy perimeter stateless routing (GPSR), beaconless routing schemes (BLR), reliable and energy-efficient routing (REER) and RCR.  
On the other hand, the stateful routing intrinsically uses a 'transmitter-oriented' [13] approach based on the following steps: firstly, the next hop node is selected based on the neighbor information table, and then, the packet is transmitted forward to the selected node until the network reaches a predetermined number of transmission failures. Therefore, this routing suffers from several drawbacks in a highly dynamic network: (1) maintenance of neighbor information causes too much communication resource waste and results in significant energy consumption [20]; (2) neighbor information collection is often quickly outdated, which leads to frequent packet drops; and (3) the maintenance of neighbor information consumes scarce memory resource in WSNs.