# Energy Efficient Policies for Data Transmission in Disruption Tolerant Heterogeneous IoT Networks
## 5 Results  
In this section we evaluate numerically the energy efficiency of the packet transmission policies produced by the OPI and Rollout algorithms. To facilitate insight into the energy efficiency of the aforementioned policies we introduce another heuristic policy, which is often used in contemporary DTNs, whereby the mobile node will transmit every packet in its backlog whenever a network connection is available. In case more than one network connections are available in a cell the mobile node will use the most energy efficient one and, in case no network connection is available in a cell, it will drop packets in order to be able to accommodate \(W_{m}^{\max}\) incoming packets. We term this policy the Empty-Backlog policy. We begin with the illustrative example presented in Figure 1 that involves a surveillance area of \(20\times 20\) cells, i.e., \(X=20\) and \(Y=20\), which includes five LP-WAN base stations and nine LP-PAN base stations. LP-PAN and LP-WAN base stations have a communication range that extends to an euclidean distance of one and five cells respectively. The capacity of the mobile node's backlog is nine packets and the maximum number of packet arrivals within a stage is uniformly distributed between zero and three packets, i.e., \(W_{m}^{\max}=3\). We assume that the transmission of a single packet through a LP-PAN and a LP-WAN connection costs one and two energy units respectively, while packet drops account for ten energy units. Finally, we set the discount factor to 0.9. Figure 2 presents, for four different policies, the number of packets \(u^{m}\) that the mobile node will have to transmit in each cell whenever there are six packets in its backlog. To facilitate presentation we do not include in Figure 2 the values of \(u^{w}\) and \(u_{l}\) since for all four policies the mobile node will use the most energy efficient network connection available in each cell. More specifically, in Figure 2a we present the Empty-Backlog Policy according to which the mobile node will transmit all six packets stored in the backlog whenever the node enters a cell that has network connectivity, i.e., a value of 1, 2 or 3 in Figure 1. The policy does not involve any packet drops for cells with no network connectivity, i.e., cells with a value of 0 in Figure 1, because the backlog's capacity is nine packets and only six packets are stored in it, thus there is enough space for the worst case scenario of three new packet arrivals (\(W_{m}^{\max}=3\)) at the current stage. In Figure 2b we present the myopic policy according to which the mobile node will not transmit in any cell of the surveillance area, since, myopically, this is the most energy efficient thing to do. We note again here that only six packets are stored in the backlog and thus there is enough space to accommodate the worst case scenario of three new packet arrivals (\(W_{m}^{\max}=3\)), thus the mobile node is not forced to transmit or drop packets. In Figure 2c we present the policy derived by the Rollout algorithm with a single policy improvement step. As seen in Figure 2c, the mobile node will transmit all packets in its backlog whenever it enters a cell that lies on the edge of an area with network connectivity. This is due to the Rollout algorithm identifying that the mobile node will probably make a transition to a region that offers no network connectivity where costly packet drops are the only way to empty the backlog in order to collect new packets. On the other hand, while the mobile node is within an area with network coverage it will transmit packets preferably when the most energy efficient interface is available, e.g., when in cell (1,1), and will avoid transmitting packets otherwise, e.g., when in cell (3,1). Finally, in Figure 2d we present the approximately optimal policy produced by the OPI algorithm when executed with \(r_{\max}=50\) and \(q_{\max}=50\). The OPI algorithm's policy is an improved version of the policy presented in Figure 2c.