## 3 Data Routing Approaches in WSNs
### Delay-Aware Approaches
#### 3.2.2 Soft Delay-Aware Approaches  
In [47] a simple least time and energy-efficient routing protocol named LEO is proposed which is able to select the shortest path and guaranty minimum time for routing. LEO uses absolute time rather than hope count to reduce both latency and congestion. For selecting the shortest path, first an initialization packet containing the absolute time of data delivery from node to sink and the remaining energy of the node is propagated to the network. Each node upon receiving the initialization packet, by using the information sent from the previous node, calculates the time required for a packet to reach the BS. This process is repeated until the entire network is covered. The outcome of the initialization phase is that, each node will know all its neighbors in its radio frequency (broadcast) region, time to reach the BS of each of neighbors and their residual energy. From such information, the node is able to determine the neighbor which has the least travel time to reach the BS and the highest remaining energy. In this approach, the sensed data are classified as real-time or non-real-time. Real-time data are sent through the nodes with least travel time while non-real time data are sent through neighbor nodes with maximum available energy. Thus, the scheme is able to respond regarding the needed quality of service. The process of classifying and forwarding data at all the nodes repeats till the BS is reached. By doing so, it is assured that the data which has priority reaches the BS within the shortest time and regular type data is forwarded in such a way that the energy of the neighboring  
time of data delivery values respectively. The values of A and B can be selectively decided as per the application and the type of data (priority or routine) is more in the network. This way, the problem of a node starving and losing energy quickly is overcome when the majority of the data are of priority type. The approach in [66] proposed a fast data collection mobility-based mechanism in heterogeneous WSNs. The authors in this paper criticize the latency in previous mobility-based approaches caused by the delay in reaching the sensor nodes by the mobile sink. Thus, they have proposed a strategy for guiding the mobile sink in the sensing area to reduce the delay of data collection. In the proposed scheme, sensor nodes measure the local topology information while moving in the sensor fields and gradually transmit their positional information to the sink. Therefore, the sink(s) can be aware about the current positions of the nodes and adjusts its position according to the nodes' location and finds the shortest path for fast data delivery. For achieving this, two different schemes are proposed: _greedy scheme_ in which the sink moves toward the regions with high densities and _aggregated scheme_ for dealing with the aggregated regions in wider network areas. While the greedy scheme is appropriate for spatially balanced networks area, the aggregated scheme is useful for the areas which are geographically correlated. In the proposed approach, while the nodes moving in the surrounding environment, they carry some local topology information about the sub-regions wherein they are mobile. Such information is measured by considering local density, corresponding position and time. Moreover, the significance of the information by using a ranking function is estimated. When the rank of the information goes down, it means that the information is not updated; therefore, the old information is updated with some new information. By using this mechanism, sensor nodes always transmit the updated information to the mobile sink which makes it aware about the current distribution of the nodes. Hence, it can reach and serve each sub-region quickly. Figure 7: IntraDRC and InterDRC. In [67] authors proposed a QoS-aware hierarchical data routing approach in WSNs. In this approach, the whole network is organized into multiple clusters wherein, each node upon sensing data, transmits its data to its cluster head.