## Main Design of DERM
### Track Routing
#### 4.4.1 Determining the Forwarding Direction
The average number of updates per node in _Multihop_ and DERM increases from 1.2 to 3 and 0.7 to 1.8, respectively, when the speed is varied from 1 to 4 m/s. This is because the location deviation is proportional to the speed with a given pause time. Remarkably, compared to _Multihop_, the overhead of DERM is reduced by about 40% and increases

Figure 9: Impact of the delay constraint on the performance of four routing schemes (\(v_{M}\) = 2 m/s). (a) transmission cost per packet; (b) maximum end-to-end delivery delay; (c) end-to-end delivery delay per packet. at a smaller rate with the speed. For DERM, 0.7-1.8 updates per node during one cycle of the sink movement is acceptable. #### 5.4.2 Impact of \(\alpha\)

In this experiment, we investigate the impact of \(\alpha\) on the performance of DERM and _Multihop_. For each approach, we show a lower bound (i.e., optimal value) of the transmission cost per packet, which can be achieved when the real sink movement is always known. We also present the upper bound, which is obtained when the location calibration method is not adopted. As depicted in Figure 11a, the transmission cost of DERM and _Multihop_ increases with \(\alpha\) since a greater route length dilation is allowed. Figure 11b shows that \(\alpha\) has almost no influence over the maximum E2E delay because the track routing can always guarantee on-time packet delivery. On the other hand, the location calibration overhead is reduced when \(\alpha\) is varied from 1.2 to 2.4, as shown in Figure 11c. We see that adjusting \(\alpha\) can strike a balance between the routing efficiency and the overhead. It is noted that, when \(\alpha\) is set to 1.2, the two approaches achieve approximately optimal transmission cost. For DERM and _Multihop_, the number of transmissions per packet is reduced by about 1 and 2.3, respectively, compared with their corresponding upper bounds. Therefore, under this setting, the reductions are considerable while the control overhead is acceptable. Figure 10: Performance overview under different sink speeds in realistic movement scenarios (\(D_{s}=120\) s, \(\alpha=1.2\), and the pause time is 30 s). (**a**) transmission cost per packet; (**b**) maximum end-to-end delivery delay; (**c**) location calibration overhead. #### 5.4.3 Impact of the Pause Time

In this test, we examine the performance of the four routing schemes under different pause times varying from 10 to 80 s. Figure 12a indicates that the transmission cost of DERM and _Multihop_ change little with the pause time, although longer pauses will incur larger location deviations. This result demonstrates the effectiveness of the location calibration method. Judging from Figure 12b, the maximum E2E delay of _Rendezvous_ gradually increases with the pause time, while that of the other three approaches almost stays constant and satisfies the constraint. As expected, the location calibration overhead is proportional to the pause time, as shown in Figure 12c. Compared to _Multihop_, the overhead of DERM increases more slowly with the increase of the pause time, which implies that _DERM is more applicable for the realistic scenarios._

#### 5.4.4 Effectiveness of DERM for Reliable Packet Delivery

In this part, we verify the effectiveness of DERM for packet successful delivery and on-time delivery. As illustrated in Figure 13, DERM achieves a high delivery success ratio close to 100%, which can be mainly attributed to the modified perimeter routing employed in DERM for handling communications void. Another reason is that the destination in DERM is a region, and the probability of having a route from a node to a region is higher than to a point. When the track routing is adopted, the on-time delivery ratio stays approximately the same as the success ratio despite the increase of \(\alpha\). That is, almost all the received packets are delivered within the delay constraint. For comparison purposes, we also present the on-time delivery ratio of DERM without the track routing.