This subsection evaluates benefits of the proposed duty cycle management scheme (that provided in section 7-7) over the two MAC protocols: the IEEE 802.15.4 and the Gamma based CSMA-CA protocol. These evaluations consider three performance metrics: average lifetime of nodes, average end-to-end delay of packets and throughput of network. The results of these assessments as illustrated in figures 7.10-7-12 are normalised to the unity duty cycle scheme of the IEEE 802.15.4 protocol. In general, it can be seen from results of these figures that the proposed duty cycle scheme is able to prolong the lifetime, increase the throughput and reduce the end-to-end delay over different network configurations for both IEEE 802.15.4 and Gamma based CSMA-CA protocols. These improvements demonstrate the advantages and wide applicability of the proposed duty-cycle scheme which is attributed mainly to the fact that the proposed duty-cycle scheme employs a non- parametric inferential procedure. In particular, the proposed scheme is not based on pre-assumptions about the dynamic behaviours of networks but it infers the potential communications based on the interacting mechanism of nodes, which in turn makes this approach effective under different configurations.
Figure 7.10 Average lifetime of AC/IEEE 802.15.4 and AC/Gamma normalised to unity duty cycle/IEEE 802.15.4
The results of figure 7.10 show that employing the proposed duty cycle management scheme over the Gamma based CSMA-CA protocol provides better performance
1 2 3 4 5 6 7 8 9 10 A vera ge lifet im e norm al ised to u nity duty cy cl e schem e of IEEE 802.15.4 Number of hops, h 11 12 13 14 15 16 v ; IEEE 802.15.4 ; Gamma ; IEEE 802.15.4 ; IEEE 802.15.4 ; Gamma ; Gamma ; IEEE 802.15.4 ; Gamma ; Gamma ; IEEE 802.15.4 ; Gamma ; IEEE 802.15.4
M. Baz, PhD Thesis, University of York 2014
readings compared to the cases when the proposed duty-cycle scheme is used over the IEEE 802.15.4 protocol. The main reason for this characteristic is the ability of the Gamma based CSMA-CA protocol to minimise the service times of packets as well as the probability of collision. As a result, a node spends a large portion of its life in idle state, which in turn saves the consumed energy and expedites the inferring of the waking and sleeping intervals. Another main reason for the significant increasing in the lifetime of nodes that are yielded from incorporating the proposed duty cycle management scheme with the Gamma CSMA-CA protocol is that the Gamma distribution is the conjugate prior for the probability distribution that are used to generate the inter-arrival times between packets. Hence using the Gamma distribution to generate the back-off intervals as well as the waking and sleeping durations enhances the consistency of interference process and reduces the uncertain space of interference process. In comparison, the IEEE 802.15.4 protocol suffers from high level of end-to-end delay and significant variation, which in turn requires a node to be awake for longer durations in order not to miss the routed traffic. This characteristic is attributed to the fact that the IEEE 802.15.4 protocol is based on a power law process, which has a heavy tailed distribution with higher probability of rare events [6].
The average end-to-end delay of the proposed duty cycle scheme for different network scenarios under both the IEEE 802.15.4 and Gamma based CSMA-CA protocols are illustrated in figure 7.11.
Figure 7.11 Average end-to-end delay of AC/IEEE 802.15.4 and AC/Gamma normalised to unity duty cycle/IEEE 802.15.4 1 2 3 4 5 6 7 8 9 10 A ver age end -to -end del ay norm al ised to unity duty cy cl e sche m e of IEEE 802. 15. 4 Number of hops, h 11 12 13 14 15 16 ; IEEE 802.15.4 ; Gamma ; IEEE 802.15.4 ; IEEE 802.15.4 ; Gamma ; Gamma ; IEEE 802.15.4 ; Gamma ; Gamma ; IEEE 802.15.4 ; Gamma ; IEEE 802.15.4
M. Baz, PhD Thesis, University of York 2014
It can be seen from results of figure 7.11 that using the proposed duty cycle management scheme over either the Gamma based CSMA-CA or IEEE 802.15.4 protocol can reduce the average end-to-end delay considerably compared to the cases when the unity duty cycle scheme is used over the IEEE 802.15.4. The main reason for this behaviour is that the proposed duty cycle scheme prolongs the lifetime of network hence it can maintain the shortest paths between nodes for longer time. Forwarding packets over these shortest paths leads to reduce the end-to-end delay. Conversely, in the unity duty cycle scheme, the topology of the network is changed frequently due to exhaustion of batteries of some nodes. These changes in the topology require a node to route its packets over longer paths which in turn increase the end-to-end delay of packets. Another key reason for the reduction in the end-to- end delay of packets is attributed to the mechanism of the proposed duty cycle scheme which requires a node to not to sleep unless it has forwarded its queued packets.
Figure 7.12 Throughput of AC/IEEE 802.15.4 and AC/Gamma normalised to unity duty cycle/IEEE 802.15.4
The results of figure 7.12 assess the advantage of the proposed duty cycle management scheme on the throughput of the networks, these results show that the proposed scheme can at least double the throughput of the network operating a highly variable MAC protocol (i.e., IEEE 802.15.4) under extremely operational conditions, i.e., the case 〈1,50, WBL(0.1), 0.002, U〉. This characteristic is attributed
1 2 3 4 5 6 7 8 9 10 Throughput norm al ised to unity duty cy cl e sche m e of IEEE 802. 15. 4 Number of hops, h 11 12 13 14 15 16 ; IEEE 802.15.4 ; Gamma ; IEEE 802.15.4 ; IEEE 802.15.4 ; Gamma ; Gamma ; IEEE 802.15.4 ; Gamma ; Gamma ; IEEE 802.15.4 ; Gamma ; IEEE 802.15.4
M. Baz, PhD Thesis, University of York 2014
to the fact that the proposed scheme does not impose overheads on the channel as each node infers the parameters of its duty cycle without a need for control packets. Hence prolonging the lifetime of nodes enables them to deliver more packets over channels that are free from control packets, which in turn increases the throughput of the network. Moreover, prolonging the lifetime of node maintains the shortest path for longer period which in turn avoids the higher collision probability that would be resulted from sending packets over longer paths as the case in the unity duty cycle scheme.
7-9 Conclusion
This chapter has proposed a novel duty cycle management scheme for multihop wireless networks. The proposed scheme is able to achieve three concurrent goals: prolonging the lifetime of nodes, minimising the end-to-end delay of packets and leveraging the throughput of networks. The scheme is applicable to a general multihop network without restrictions on the topology configurations, traffic patterns or routing policies. Moreover, this scheme is lightweight and does not require any modification to the MAC or routing protocols. The approach that is used in deriving the proposed scheme is based on drawing analogies between the reactions of chemical species and the communication of wireless nodes. Based on this metaphor, the powerful modelling and optimisation techniques of artificial chemistry (AC) are exploited to design the proposed scheme. The molecular statistical mechanics of AC are used to quantify the energy consumed and the artificial decision making techniques of AC are employed to develop an effective duty cycle management scheme for multihop networks. The discussion and results reported in this chapter demonstrate the accuracy of the proposed model as well as the benefits of the proposed duty cycle management scheme compared to a unity duty cycle scheme over two MAC protocols: the IEEE 802.15.4 and Gamma based CSMA-CA protocols. The contributions of this chapter together with the contribution of chapter 6 represent the first two main objectives of this thesis. The next chapter will present the third objective by designing an effective routing protocol.
M. Baz, PhD Thesis, University of York 2014