# Edge-Aided Reliable Data Transmission for Heterogeneous Edge-IoT Sensor Networks
## 4 Evaluation
### Evaluation Results  
The link quality distribution of the testbed is shown in Figure 6. From the cumulated distribution function (CDF) of link quality values in the network, we can see that 80% of links have link quality smaller than 80%, which means 20 packets are likely to be lost in every 100 transmissions. We first compare our work with the existing work, \(\gamma\) protocol, proposed in [4]. Figure 7 shows the CDF of transmission counts in both our work and the \(\gamma\) protocol. First, the transmission count of most network nodes is smaller than 400 with our work while only around 60% of nodes have a transmission count smaller than 400, and nearly 30% of nodes have a transmission count larger than 500. We can see that all network nodes have much fewer transmissions compared to the \(\gamma\) protocol. Second, some nodes have more transmissions compared to other nodes. We analyze the experiment logs and identify that the sensor nodes that are not covered by edge servers have more transmissions compared to the nodes covered by edge servers. The reason is that the nodes covered by edge servers have  
Figure 5: The experimental testbed with 8\(\times\)10 sensor nodes. Figure 6: The link quality cumulated distribution function (CDF) of the testbed. a much larger chance to offload their transmission tasks to the edge servers and thus the transmission count is reduced. Figure 8 shows the transmission count with varying number of destination nodes. We set the number of edge nodes as two in this experiment (we change the number of edge nodes later in Figure 9). We can see that when there are more destinations for each packet, more transmissions are required. It is reasonable because more destinations need more forwarding of the packets on more hops. In addition, the transmissions grow linearly in general. The reason is that we randomly set the destinations for each packet, thus the average hops for each destination is similar. We also notice that when the number of destinations is too large (>14), the number of transmissions decreases because when there are too many destinations, those destinations are close to each other and can be covered by the same transmissions (due to the nature of wireless broadcast). Figure 8: The transmission count with varying number of destination nodes. Figure 7: The transmission count compared to the existing work. To clearly understand the contributions of each building block in the proposed framework, we then conduct simulations to separately evaluate the performance of forwarding and retransmissions. Figure 9 shows the transmission count with a varying number of edge servers. We can see that as the number of edge servers increases, the number of packet transmissions decreases because the edge servers' forwarding and retransmissions can save a number of transmissions. Additionally, the degradation of the transmission count becomes smaller as the number of edge servers increases. The reason is that when there are few edge servers, they are deployed without any overlapped areas and each additional edge server is able to cover a large number of sensor nodes. When there are many edge servers, there is overlap among different edge servers and thus the transmission reduction becomes smaller. From the results, one can also tell how many edge servers should be deployed given the level of transmission efficiency in sensor networks. Figure 10 shows the delivery delay with varying number of destination nodes. The delivery delay is calculated as the time difference from the source node to the destination nodes via multiple hops. We can see that the delay increases almost linearly along with the number of destination nodes. However, different from transmissions, the delay continues increasing when there are more than 14 destinations. The reason is that unlike transmissions, the delivery delay needs to account for the feedback delay from all destinations. Although packet transmissions can arrive at different destinations at the same time, their ACKs cannot be sent back to the sender at the same time.