# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 5 Reliable Routing
### QoS Based
On the other hand, delay of data packet transmission is significantly greater in MLDA than in Hierarchical-PEGASIS. In addition, MLDA requires more computational power for large sensor networks. Minimum Cost Bandwidth-Constrained Routing (MCBCR) [142] protocol, which is a simple, scalable, and efficient protocol, solves the minimum cost routing problem of WSNs. The most proper paths for routing data from nodes to the base station are found. In addition, the routing cost is minimized, without the load on each wireless link, exceeding its capacity. The polynomial-time minimum cost flow algorithm achieves high scalability. By using MCBCR, both long network lifetime and high scalability are achieved. However, high computational overhead is produced. Energy Efficient Structure-free Data Aggregation and Delivery (ESDAD) [143] is an energy efficient and structure free protocol that aggregates redundant data. ESDAD divides the network area into several sub-regions each with a unique reliability requirement. In every sub-region, there is one or more sender nodes, depending on the sub-region data traffic that collects and sends data to the other sender nodes. Applying the reliability requirement scheme, ESDAD, can choose the number of sender nodes per sub-region, achieving energy saving and decreasing message delays. ESDAD performs better than the traditional structure-free protocols regarding energy efficiency and reliability. Additionally, the miss ratio is minimized. However, mobility is not considered. Voronoi Neighbor Based Energy Efficient Event Detection (VNBEEED) [144] proposes an interpolation method to minimize in-network traffic which is particularly high in dense networks due to redundant sensed data of neighboring nodes. VNBEEED models a WSN as a Markov Random Field (MRF) and applies local inference at every intermediate node to identify the shortest path with minimum energy consumption. In order to do so, VNBEEED uses messages of two types, i.e., data messages and sleep messages. In addition, it induces random delays into nodes of the same level that sensed an event, so only a subset of them will transmit the data message to the base station via the shortest path and a sleep message to their neighbors that sensed the data, so they will enter sleep mode. Thus, great energy saving is made. On the other hand, important messages may not reach the base station. Efficient Dynamic Authentication and Key management (EDAK) [145] is an efficient dynamic authentication and key distribution protocol for heterogeneous WSNs, designed to safely transfer data packets between the base station and sensor nodes. EDAK combines energy efficiency and data protection and a dynamic key generator for new data. In addition, to identify the valid sensor node, it applies the authentication mechanism DMK (Dynamic Matrix Key). The use of this method results in no need for exchanging keys, which achieves energy efficient operation and faster communication, with reduced complexity and overhead. However, energy holes still exist. The Stable and Predictive Energy-aware Coverage Scheduling (SPEC) [146] is a protocol, which utilizes a one-way scheduling strategy, a wake-up strategy and reduces unnecessary repetitive eligibility executions among the nodes. This scheduling aim to remove the risk of network isolation by allowing some sensor nodes within a cluster to be in sleep mode. According to this strategy, nodes operate in four different states:  
1. Discovery, where nodes exchange messages to discover their neighbors,
2. Active, where nodes sense the environment and communicate with their neighbors,
3. Sleep, where they are switched off waiting to be activated, and
4. Exhausted, where nodes have consumed their energy reservoirs. After the discovery phase, nodes have discovered their neighborhood and SPEC knows which nodes should be switched in and out of sleep mode within the clusters. Once this is determined, clusters elect a cluster head and SPEC continues to employ it as the cluster head until it depletes its energy reserves.