## 2 Literature Review
### Local-Based Routing
The basis of AHH-VBF is HH-VBF, which also creates a virtual pipeline at each hop, but as we know, in UASN, the sensors nodes are distributed randomly, as in some local regions the density of the sensor nodes is low, called sparse sensors regions, and in some regions, the density of the nodes are high, called dense regions of the network. Thus, due to this non-uniform distribution of the sensor nodes in a local region, AHH-VBF differentiates itself from HH-VBF, which is discussed now. Firstly, in AHH-VBF, we adaptively change the transmission range at each hop and create a sensor node-oriented pipeline. Due to this, the transmission power of each forwarder node is also adjusted in a hop-by-hop fashion, which efficiently overcomes the energy distribution and enhances the life time of the whole network. Secondly, the transmission is proceeded in a controlled and confined forwarding range, as at each hop, the radius of the virtual pipeline changes adaptively, which guarantees a reduction in the duplicate packet transmission. It also effectively enhances the transmission reliability of the source node and, eventually, of the whole sensor network. Thirdly, due to the adaptive approach of AHH-VBF, the routing is conducted in a restricted forwarding region and in a thoroughly measurable forwarder node present inside the pipeline of the source node. Thus, the selection of the forwarder node is based on its distance from the current source node to the destination node present inside the pipe, which effectively reduces the end-to-end delay. As in AHH-VBF, the whole focus is on the transmission regions that are on the pipeline radius, which effectively improves the important parameters of the whole network, such as a reduction in the duplicate packets, a phenomenal distribution of energy, and an improvement in the end-to-end delay. However, in AHH-VBF, the forwarding region covers more areas in the dense sensor region then is required, which affects the performance of the network, so that another novel protocol called AHHC-VBF [15] works on this. In AHHC-VBF, at each hop, a cone-based forwarding region is made, so that the next forwarded node is selected for transmission according to its relative position with the virtual pipe and the angle of the cone. A predefined angle is defined for the cone, and the angles of the nodes present at the local region are measured relative to the cone angle, and also, the distance of each node from the virtual vector is stated. A node will be present inside the cone if it makes an angle that less than the predefined angle of the cone. Thus, from this, we find which nodes are located inside the cone and which are not. The cone angles are hard coded and increase/decrease at each hop according to the local sensor node distribution. If there are sparse sensor regions, the cone angle is increased to find a suitable node for next forwarding. However, for dense node distribution, AHHC-VBF tries to adjust a small cone angle that takes a low amount of energy to transmit the packet. As from the above discussion, AHHC-VBF gives direction to the transmission of the packets towards the sink nodes. AHHC-VBF improves the important parameters and the performance of the network. Most pertinently, it gives direction to the transmission of the packets and changes its angle according to the local node distribution, due to which, the reliability of the network in the low density node area increases. Additionally, due to the smart selection of the forwarded node, as the source node angle adaptively changes, this reduces the duplicate packet creation and the end-to-end delay.