# Data Gathering Techniques in WSN: A Cross-Layer View
## 5 Routing for Data Gathering in WSN
### Common WSN Routing Protocols for Data Gathering
#### 5.1.3 Routing Protocol for Low-Power and Lossy Networks (RPL)
[PERSON] et al. [134] suggest another MRHOF-based objective function that takes into account three routing metrics: ETX, energy, and delay. Energy consumption, in order to increase the lifespan of the network, is also considered by [PERSON] et al. [135], which suggests cluster-tree-based routing protocol to maximize the lifetime of IoT (CT-RPL). As the name suggests, CT-RPL is a cluster-based routing protocol that involves three processes: cluster formation, cluster-head selection, and route establishment. CT-RPL first scans the nodes and group (cluster) nodes whose Euclidean distance from their centroid point is bounded, adding one node at a time. Next, each cluster selects its cluster-head (CH), utilizing a game-theoretic approach in which the node with the maximum payoff \"\(p\)\" value--which considers parameters such as residual energy, sensing energy, receiving energy, aggregation energy, and transmission energy--is selected as the CH node for each round. Finally, the route is established using the metrics residual energy ratio (RER), queue utilization (QU), and expected transmission count (ETX). Another RPL enhancement termed weighted random forward RPL (WRF-RPL) is proposed by [PERSON] et al. [136]. WRF-RPL suggests a load balancing over RPL mechanism, which distributes the traffic between multiple transmission paths, trying to avoid one preferred parent's congestions. WRF-RPL OF relies on the composition of two metrics, the remaining energy and the count of parent nodes, where the latter aims at prioritizing parent nodes with more optional paths to the destination. Forwarding decisions are probabilistic according to the defined metric, such that nodes with a higher number of parents or hops are more likely to be selected than others. The authors utilize the Cooja simulator for evaluation. [PERSON] et al. [137] leverage a wired data center network labeling protocol to suggest IoTrii, a routing protocol for LLNs. IoTrii supports multiple paths between sender and receiver, and requires fewer table entries and control messages, with similar performance compared to the standard RPL. [PERSON] [138] provides a graph-theoretic solution to the general problem of QoS-constrained routing in WSN that relies on RPL. The authors stress the difference between multi-objective optimization and multi-constrained problem setting. [PERSON] et al. [139] examine the integration of RPL to IEEE 802.15.4e with time-slotted channel-hopping (TSCH) medium access mechanism. In particular, [PERSON]. [139] characterizes the long deployment delays required for such networks to become operational and able to start exchanging data messages. The article proposes an analytical model that estimates the average time that the synchronization process can take for a new node to join a TSCH-based network, as well as an estimation of the maximum time required for the formation of a complete network of this kind, and the additional time required to set the RPL-based routes. The paper validates the analytical model via simulations. A recent comprehensive survey on routing protocols for LLN networks in IoT (not exclusive to RPL) can be found in [140].