Evaluation with different Traffic Load

In the following experiment, we will evaluate the performance of each MAC protocol under different traffic load. This experiment helps us to understand the adaptive ability to different traffic load of each protocol. As before, we change the traffic load by varying the interval period of packets. If the packet generation interval period is 5 s, a packet is generated every 5 second by each sensor node. In this experiment, the packet generation interval period varies from 1.2 second to 120 second. For the highest rate with a 1.2-s generation interval, the wireless channel is nearly fully utilized due to its low bandwidth. We have done 500 independent tests when using different MAC protocols with each different traffic pattern. In this experiment, we also use four metrics including average transmission latency, energy

consumption per packet, throughput, and protocol work efficiency, to compare the performance of each MAC protocol.

Figure 5-12 shows the average transmission latency of each MAC protocol under different traffic load. The result shows that all MAC protocols all have higher latency under the higher traffic load. This is because before elder packets are transmitted to the base station, the new packets are generated due to the short generation interval.

Thus, the elder packets need to contend the transmission medium with these newborn packets. This causes the transmission latency increases significantly. When the traffic load becomes medium, LAMAC and 802.11 CSMA both have ideal transmission latency reduction. This is because elder packets can be transmitted to the base station before next new packets generation time. In other words, the transmission latency of each packet is shorter than the packet generation interval. However, S-MAC can not achieve stable transmission latency even if the traffic load is medium. Until the generation interval is 72 second which means a low traffic load, S-MAC get the shortest latency. This is because S-MAC needs much more time to move the elder packets to base station. Only when the packet generation interval is long enough, e.g., 72 second, S-MAC can have enough time to transmit all the elder packets.

0 20 40 60 80 100 120

1.2 3.6 8.4 15.6 24 36 54 72 96 120

Data Generation Interval (Second)

Average Tranmission Latency (Second)

LAMAC S-MAC 802.11 CSMA

Figure 5-12： Average transmission latency under different traffic load

Figure 5-13 shows the energy consumption per packet under different traffic load. The energy consumption of each MAC protocol all increase with the increasing of the packet generation interval. This is because each protocol use more time in idle listening when traffic load is light. Before next new packets are generated, all sensor nodes still need to keep the wake-up schedule. And each active period all waste energy in idle listening because there are no packets to transmit. However, 802.11 CSMA uses the most energy compared with LAMAC and S-MAC. This is because its full active schedule. S-MAC also consumes more energy than LAMAC because it needs all sensor nodes to wake up for one time packet transmission. And the less probability of transmission also increases the energy consumption. Although LAMAC uses a little more energy to transmit a packet under low traffic load, the average energy consumption is still much better than the others.

0 500 1000 1500 2000 2500 3000

1.2 3.6 8.4 15.6 24 36 54 72 96 120

Data Generation Interval (Second)

Energy Consumption per Packet (mJ)

LAMAC S-MAC 802.11 CSMA

Figure 5-13： Energy consumption per packet under different traffic load

Figure 5-14 shows the throughput of each MAC protocol under different traffic load. The definition of throughput is the average amount of data received by Base Station every second. Thus, the value of throughput is affected by traffic load significantly. Reasonably, if the traffic load becomes light, the throughput will decrease with the same ratio. On the contrary, if the traffic load becomes high, the throughput will increase with the same ratio. However, not only traffic load will affect the throughput. The transmission latency also affects the throughput. If a MAC protocol has higher latency than others, its throughput must also lower than others. If with the same protocol, there are still some differences of transmission latency under different traffic load. Thus, the throughput may not have the same ratio of change with the increasing or decreasing of the traffic load. The result shows that the throughput of LAMAC and 802.11 CSMA both decrease slowly under the high traffic load.

However, after the packet generation interval becomes longer than 8.4 second. The

throughput of LAMAC and 802.11 CSMA both have the same ratio of change with the variation of traffic load. The result also shows that S-MAC much less throughput than LAMAC under the light traffic load. This is because S-MAC has high transmission latency when traffic load is high and medium. This high transmission latency significantly reduces the throughput of S-MAC.

0 200 400 600 800 1000 1200 1400

1.2 3.6 8.4 15.6 24 36 54 72 96 120

Data Generation Interval (Second)

Throughput (Bytes/S)

LAMAC S-MAC 802.11 CSMA

Figure 5-14： Throughput under different traffic load

Figure 5-15 shows the protocol work efficiency under different traffic load. The result shows LAMAC has the best performance. There are two turning points in the curve of LAMAC. The first point is in the value of 3.6 and the second is in 15.6.

When packet generation interval is shorter than 3.6, the traffic is heavy. Elder packets need to contend with the newborn packets. The extra contention wastes lots of energy.

Thus, although traffic load is high, the amount of packet received can not increase before level-1 nodes exhaust their energy. The higher the traffic, the more energy

wastes in the contention. When packet generation interval is higher than 3.6, the contention times decrease. The work efficiency increases with the decreasing of traffic load. However, when the generation interval is longer than 15.6, the work efficiency decreases again. Most sensor nodes waste energy on idle listening because there are no packets to transmit. The situation is getting worse when the generation interval becomes longer. Thus, we have the second turning point at 15.6. S-MAC has the second performance next to LAMAC. The reason which makes S-MAC have almost the same value of work efficiency between 1.2 and 36 is similar to the reason of LAMAC between 1.2 and 3.6. The result shows S-MAC has high transmission latency when packet generation interval is shorter than 36. This means there are a lot of contentions among sensor nodes in this situation. When generation interval is longer than 36 and shorter than 54, the contention times decrease and the work efficiency increases. However, when the generation interval is longer than 54, S-MAC suffers the idle listening problem. The performance decreases with the increasing of packet generation interval. 802.11 CSMA has the worst performance. Because of its low transmission latency, 802.11 CSMA suffers idle listening problem when packet generation interval is longer than 3.6. When the interval is longer than 3.6, the performance of 802.11 CSMA decreases with the traffic load.

0 20000 40000 60000 80000 100000 120000 140000

1.2 3.6 8.4 15.6 24 36 54 72 96 120

Data Generation Interval (Second)

Data Get before System Shut Down (Bytes) LAMAC S-MAC 802.11 CSMA

Figure 5-15： Data get under different traffic load before system shut down