# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 5 Reliable Routing
### Multipath Based
The root can increase a sequence number in the DIO messages to trigger \"global recalculation\" of DODAG. RPL also provides an information dissemination mechanism over a dynamically formed network topology. This dissemination enables a minimal configuration in the nodes, enabling them to operate mostly autonomous. The minimum in router state requirements to run RPL is the following:  
1. The DODAG root identifier
2. The address and rank of the parents
3. The shared DODAG root configuration parameters
4. The maximum rank, which has been advertised by the Wireless Sensor Network router  
In RPL, low energy consumption is achieved. However, only unicast traffic is supported. _Scalable Solution to Minimum Cost-Forwarding_ (SSMCF) [163] is a solution to the problem of delivery to the base station of minimum cost messages, from any given source, along the minimum-cost path in a simple, large, and scalable network. A cost function is used to capture delay, throughput, and energy consumption from every node to the base station. There are two phases during SSMCF:  
1. First phase: Is the setup phase in which the cost value has been set for every node. The cost is diffused from the base station through the network. After that, each node sets the value cost by adding the cost from the node it received and the link cost. This cost setup is not done through the flooding mechanism, but, instead, a back-off algorithm is used to limit the number of messages exchanged. Packet forwarding has a preset delay time to allow messages with minimum cost to arrive. After that, it uses only one cost message to find the optimal cost from node to base station. The next hop states are unnecessary after the cost fields are set, ensuring scalability. 2. During the second phase, source transmits data to its neighbor nodes. Each node in turn adds the transmission cost of them, from the base station to the cost of the packet and checks the remaining cost in it. If the remaining cost is not sufficient to send it to the base station, the packet is dropped; otherwise, it forwards the packet to its neighbors. The design adopted has three goals:  
1. Optimality: Achieve least cost forwarding * Simplicity: Reduce performed operations to the minimum number and the states that participate in data forwarding. * Scalability: Scale to large size network, as unconstrained scale is an inherent feature of sensor network. In SSMCF, with the use of back-off based algorithm and proper set up back-off timer, the medium number of advertisement messages can be reduced. In addition, SSMCF scales up to large network sizes. However, it requires constant time and space complexities at each node. In addition, to achieve optimal forwarding, it needs to have the minimum number of advertisement messages. _Fast Adaptive, and Energy efficient Multi path-multi channel_ (FAEM) [164] utilizes the basketball NET topology that pre-assigns every node with a multi-parent-multi-child connection table along with a different receiving channel per node from their respective neighbor nodes removing transmission interference. FAEM protocol divides operational time in duty cycles with every cycle containing two phases, a scheduling and a forwarding one:  
* The first phase is called Distributed iterative scheduling phase. It schedules the three activities that a node can perform in the network, which are sleep, download to children nodes, and upload to parent nodes. * The second one goes by the name of Slot based packet forwarding phase. It divides transmission time in a fixed number of frames that each contains a fixed number of slots. Every slot lasts enough for only one data packet transmission between two nodes. In FAEM, low energy consumption is achieved with sleep mode. In addition, collision-free data transmission is attained. In addition, interference prevention is attained. However, network is unable to connect mobile parent-children nodes.