Experiments for this section use the sensor network with 160 nodes randomly de- ployed in a 250 meter by 250 meter area. Fig. 4.7 and Fig. 4.8 plot performance results of the synchronous protocols for 10% and 30% physical layer loss rate, re- spectively. Values for τ are chosen manually within the range of [0.13, 1.19] seconds. Aggregating the same data at different nodes improves reliability, but also in- creases the packet size for broadcast-based aggregation with increasing packet size. When the physical layer loss rate is 10%, the cost of the retransmissions is rela- tively modest in unicast-based aggregation. As seen in Fig. 4.7(a), synchronous unicast-based aggregation outperforms synchronous broadcast-based aggregation for increasing packet size. For 20% physical layer loss rate, synchronous unicast-based aggregation also outperforms synchronous broadcast-based aggregation for increas- ing packet size, as shown in Fig. 4.3(a). As the physical layer loss rate increases, however, the cost of the retransmissions in unicast-based aggregation becomes more substantial. Fig. 4.8(a) shows that with 30% physical layer loss rate, synchronous broadcast-based aggregation is able to achieve lower maximum data age than the synchronous unicast-based protocol for increasing packet size.
For fixed packet size, 4.8(b) shows that the synchronous broadcast-based protocol is able to lower the maximum data age by about 50% with moderate end-to-end loss rate (5% end-to-end loss rate, for example). Fig. 4.7(b), Fig. 4.3(b), and Fig. 4.8(b) plot performance results of the synchronous protocols for 10%, 20%, and 30% physical layer loss rate, respectively. The results suggest that the relative performance of synchronous broadcast-based aggregation improves as the physical layer loss rate gets higher. The reason behind this can be traced back to traffic volume. As mentioned before, in broadcast-based aggregation, each node broadcasts
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end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X unicast synch., 4X unicast synch., 8X broadcast synch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X unicast synch., 4X unicast synch., 8X broadcast synch.
(b) Fixed Packet Size
Figure 4.7: Impact of Lower Physical Layer Loss Rate on Synchronous Ag- gregation (real-time, varying τ , N = 160, PLR = 10%)
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end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X
unicast synch., 4X unicast synch., 8X broadcast synch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X
unicast synch., 4X unicast synch., 8X broadcast synch.
(b) Fixed Packet Size
Figure 4.8: Impact of Higher Physical Layer Loss Rate on Synchronous Ag- gregation (real-time, varying τ , N = 160, S = 250m × 250m, PLR = 30%) at most twice for each round regardless of the loss probability. In unicast-based aggregation, however, the number of retransmissions increases as the loss rate gets higher, and the delay increases accordingly as more traffic is generated. As seen in Fig. 4.7(b), Fig. 4.3(b), and Fig. 4.8(b), the physical layer loss rate has a greater impact on the maximum data age of unicast-based aggregation than on that of broadcast-based aggregation. As a result, the relative performance of synchronous broadcast-based aggregation improves as the physical layer loss rate gets higher
Fig. 4.9 and Fig. 4.10 plot the performance of the asynchronous protocols for 10% and 30% physical layer loss rate, respectively. Fig. 4.9(a), Fig. 4.5(a), and
Fig. 4.10(a) show that asynchronous broadcast-based aggregation is outperformed by asynchronous unicast-based aggregation for all three physical layer loss rates in the case of increasing packet size. For fixed packet size, asynchronous broadcast- based aggregation is able to achieve lower maximum data age for all three physical layer loss rates, but the performance gain is not as significant as with the syn- chronous broadcast-based aggregation protocol. Similarly, the relative performance of asynchronous broadcast-based aggregation improves as the physical layer loss rate increases.
4.3.2.3 Impact of Density
Fig. 4.11 and Fig. 4.12 plot performance results of the synchronous aggregation protocols, for the sensor fields with 120 nodes and 240 nodes over a 250 meter by 250 meter area, respectively. Values for τ are chosen manually within the range of [0.13, 1.5] seconds.
As the network density increases, the broadcast-based protocols are expected to achieve higher reliability as a broadcast is likely to be received by more nodes. However, for broadcast-based aggregation with increasing packet size, larger packets are produced as the same data is aggregated by more nodes, which means a longer packet transmission time and greater network congestion. Meanwhile, packet loss recovery is quite feasible for unicast-based protocols with the packet loss rate of 20% that is used for these figures. As the result of these two effects, Fig. 4.12 shows that synchronous broadcast-based aggregation is outperformed even more by synchronous unicast-based aggregation for increasing packet size, when the number of nodes (and density) is increased. For fixed packet size, however, performance with the synchronous broadcast-based protocol improves as network density gets higher, but performance with synchronous unicast-based aggregation degrades as network density increases. The relative performance of synchronous broadcast-based aggregation improves with increased density.
Fig. 4.13 and Fig. 4.14 show the performance of the asynchronous aggregation protocols, for the sensor fields with 120 nodes and 240 nodes, respectively. Similar to
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maximum data age (in seconds) unicast asynch., 3X unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast asynch., 3X unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(b) Fixed Packet Size
Figure 4.9: Impact of Lower Physical Layer Loss Rate on Asynchronous Aggregation (real-time, varying τ , N = 160, S = 250m × 250m, PLR = 10%)
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end-to-end loss rate
maximum data age (in seconds) unicast asynch., 3X unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast asynch., 3X unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(b) Fixed Packet Size
Figure 4.10: Impact of Higher Physical Layer Loss Rate on Asynchronous Aggregation (real-time, varying τ , N = 160, S = 250m × 250m, PLR = 30%)
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end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X unicast synch., 4X unicast synch., 8X broadcast synch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X unicast synch., 4X unicast synch., 8X broadcast synch.
(b) Fixed Packet Size
Figure 4.11: Impact of Lower Density on Synchronous Aggregation (real- time, varying τ , N = 120, S = 250m × 250m, PLR = 20%)) 0.1% 0.5% 1% 5% 10% 20% 0 0.5 1 1.5 2
end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X
unicast synch., 4X unicast synch., 8X broadcast synch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast synch., 3X unicast synch., 4X unicast synch., 8X broadcast synch.
(b) Fixed Packet Size
Figure 4.12: Impact of Higher Density on Synchronous Aggregation (real- time, varying τ , N = 240, S = 250m × 250m, PLR = 20%)
synchronous broadcast-based aggregation, for increasing packet size, asynchronous broadcast-based aggregation is outperformed even more by asynchronous unicast- based aggregation when the number of nodes (and density) is increased. For fixed packet size, however, the relative performance of asynchronous broadcast-based ag- gregation improves with higher density. For 1% end-to-end loss rate, asynchronous broadcast-based aggregation is able to lower the maximum data age by about 30% with higher density, as seen in Fig. 4.14(b).
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end-to-end loss rate
maximum data age (in seconds) unicast asynch., 3X unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast asynch., 3X unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(b) Fixed Packet Size
Figure 4.13: Impact of Lower Density on Asynchronous Aggregation (real- time, varying τ , N = 120, S = 250m × 250m, PLR = 20%) 0.1% 0.5% 1% 5% 10% 20% 0 0.5 1 1.5 2
end-to-end loss rate
maximum data age (in seconds) unicast asynch., 3X
unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(a) Increasing Packet Size
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end-to-end loss rate
maximum data age (in seconds) unicast asynch., 3X unicast asynch., 4X unicast asynch., 8X broadcast asynch.
(b) Fixed Packet Size
Figure 4.14: Impact of Higher Density on Asynchronous Aggregation (real- time, varying τ , N = 240, S = 250m × 250m, PLR = 20%)