## Main Design of DERM
### Track Routing
#### 4.4.1 Determining the Forwarding Direction
* _Rendezvous-based Routing (Rendezvous)_: Each data packet is transmitted to an RN via the shortest path using geo-routing and then collected when the sink arrives. * _Delay-Constrained Rendezvous-based Routing (DC-rendezvous)_: Each packet is first sent to the closest RN as in the traditional rendezvous-based routing. However, if the sink has not arrived within the delay constraint, the packet will be delivered to it via the track routing. For fair comparisons, the aforementioned three solutions and DERM adopt the same routing metric (i.e., remaining ETX) as defined in Equation (4). We first evaluate their performance in ideal scenarios where the sink movement exactly complies with its mobility pattern. Then, we show the simulation results in realistic scenarios where the real movement deviates from the estimated one,

Figure 7: An example of the track routing in DERM. and demonstrate the effectiveness of the void handling and the track routing mechanisms in DERM for reliable packet delivery. ### Simulation Setup

We use the _setdest_ tool to generate 100 different topologies randomly in a 1000 m \(\times\) 400 m rectangular area with four vertices located at (0 m, 0 m), (0 m, 400 m), (1000 m, 400 m), and (1000 m, 0 m). The network consists of 1000 nodes and the communication ranges are set as 40 m. We adopt the IEEE 802.15.4 MAC layer protocol and measure the PRR with the Nakagami fading model [51] where the link quality is related to the distance between two nodes. Each node randomly generates one sensory data packet per minute, which is required to be delivered within the delay constraint. The mobile sink moves from the start point (0 m, 200 m) to the end point (1000 m, 200 m) and then returns back to the start point with a constant speed. It sends out a Beacon message every second when moving in the network. The sink location check timer interval is 5 s and the deviation threshold \(\Delta_{th}\) is 1 m. Unless otherwise stated, we set the sink speed \(v_{M}\), the delay constraint \(D_{s}\), and \(\alpha\) as 2 m/s, 120 s, and 1.2, respectively. All the results have been averaged over 100 rounds of simulations. ### Evaluation Metrics

We evaluate the performance of our design in terms of the following metrics:

* _Transmission Cost per Packet_: measured as the average number of transmissions for an end-to-end (E2E) packet delivery. * _E2E Delivery Delay per Packet_: the average elapsed time from a data packet being sent out by the source node to finally being collected by the mobile sink. * _Maximum E2E Delivery Delay_: the maximum of end-to-end delivery delays of all packets generated at different sensor nodes. * _Location Calibration Overhead_: measured as the average number of sink location updates per node during one cycle of sink movement. * _Packet Delivery Success Ratio_: the ratio of the number of packets successfully received by the sink to the total number of packets sent from source nodes. * _On-time Delivery Ratio_: the ratio of the number of packets delivered to the sink within the delay constraint to the total number of packets sent by source nodes. ### Performance Evaluation in Ideal Movement Scenarios

In ideal scenarios, the sink location can be accurately estimated using the mobility pattern, and thus the location calibration mechanism is not required. We mainly investigate the impact of the sink speed and the delay constraint on the performance of DERM and the three baseline solutions. #### 5.3.1 Performance Overview in Ideal Movement Scenarios

We evaluate the performance of DERM in ideal movement scenarios, varying the sink speed from 1 to 4 m/s. Figure 8a reports the transmission cost per packet of the four routing schemes under different sink speeds. Compared to the _Multihop_ and the _DC-Rendezvous_ approaches, DERM reduces the transmission cost by 13%-44% and 26%-38%, respectively. The energy savings are significant considering the cost is calculated by averaging the transmission number per node per packet. This is because DERM takes full advantage of the sink mobility.