## Main Design of DERM
### Track Routing
#### 4.4.1 Determining the Forwarding Direction
As \(\alpha\) is varied from 1.0 to 2.0, the ratio decreases from 96% to 67% because more nodes suffer sink location errors,

Figure 11: Impact of a (\(\
u_{M}\) = 2 m/s, \(D_{s}\) = 120 s, and the pause time is 30 s). (**a**) transmission cost per packet; (**b**) maximum end-to-end delivery delay; (**c**) location calibration overhead. but no fault-tolerant mechanism is used. Note that, when \(\alpha\) is set to 1, the on-time delivery still cannot be guaranteed without the track routing. This is because the future sink location deviations cannot be predicted, although the real current location is always known. From the above observations, we can see that the track routing is effective as well as necessary. Figure 12: Impact of the pause time (\(v_{M}\) = 2 m/s, \(D_{s}\) = 120 s, and \(\alpha\) = 1.2). (**a**) transmission cost per packet; (**b**) maximum end-to-end delivery delay; (**c**) location calibration overhead. Figure 13: Delivery ratio of DERM with and without the track routing (\(v_{M}\) = 2 m/s, \(D_{s}\) = 120 s, and the pause time is 30 s). ### Performance Summary

From above observations, we can conclude that DERM achieves the optimal energy efficiency under the delay constraint. Compared with _Rendezvous_ and _DC-rendezvous_, DERM needs to estimate the sink location and thus introduces the extra control overhead for location calibration in realistic movement scenarios. However, DERM has much better energy efficiency than _DC-rendezvous_, and can always meet the delay requirements compared to _Rendezvous_. The overhead can be amortized by energy savings on the delivery of continuously generated data packets. _Multihop_ can achieve real-time packet delivery, but it also brings more transmission costs and requires more frequent location calibration compared to DERM. ## 6 Conclusions

In this work, we propose DERM, a protocol aiming at achieving delay-aware energy-optimal routing in WSNs with a path-fixed mobile sink. DERM employs a location-based greedy forwarding technique, enabling each node to relay packets to a destination region accessible to the mobile sink within the delay constraint. An energy-efficient location calibration method is introduced to deal with the deviation between the estimated and the real sink movements. Moreover, we present the track routing to guarantee packet on-time and reliable delivery. Extensive evaluation results demonstrate the effectiveness of our proposed approach in both ideal and realistic movement scenarios. Compared to _Multihop_ and _DC-rendezvous_, DERM reduces the transmission cost by 13%-44% and 24%-38%, respectively, and requires about 40% less control overhead than _Multihop_ for the sink location calibration. Compared with _Rendezvous_, it can always meet a wide range of delay constraints under different settings. The energy profit brought by DERM is proportional to the speed and the delay constraint. Therefore, DERM can be applied in many industrial applications with various delay requirements for energy conservation due to its scalability and flexibility. In the future, we plan to validate the proposed approach in WSNs with multiple path-fixed sinks. Furthermore, we also plan to enhance our design by comprehensively considering the end-to-end delay, the transmission cost as well as the total amount of collected data. This work was supported by the National Natural Science Foundation of China (61633002, 61572060, 61772060), the National Basic Research Program of China (973 Project) under Grant 2013 CB035503, and the CERNET Innovation Project 2015 under Grant NGI20151004. [PERSON] and [PERSON] designed the study; [PERSON] and [PERSON] performed the experiments and wrote the paper; [PERSON] enhanced the organization and presentation of the paper. The authors declare no conflict of interest. ## References

* [PERSON] and [PERSON] (2009) [PERSON]; [PERSON] Industrial wireless sensor networks: challenges, design principles, and technical approaches. _IEEE Trans. Ind. Electron._**2009**, _56_, 4258-4265. * [PERSON] et al.