Wireless Ad Hoc Networks (WAHN). Much of the attention has gone into MAC and Routing protocols that have either been modified or redesigned based on protocols similar to 802.11. Some of the earliest protocols designed included Power-Aware Routing Optimization protocol (PARO) (Gomez, Campbell et al. 2003) in which if node ‘A’ wanted to transmit a message to node ‘E’, it would calculate a multi-hop path that required the minimum energy consumption. Several other protocols have been reviewed in Chapter 2.0 that rely on the principles of active energy saving.
In passive energy saving protocols, the node often sends the radio into a sleep state, when it’s in an idle state for more than a predefined period of time. In the GAF algorithm the network is divided into virtual grids and the node with the maximum energy is chosen to be the cluster head. The chosen cluster head can talk to all its neighbouring cluster head nodes and sufficient numbers of cluster head nodes are created to achieve network connectivity. In Sensor MAC (SMAC), a ‘sleep awake’ schedule is set up among a group of nodes that allows them to go to sleep after a predefined wakeup time known as duty cycle. The duty cycle can be set between 10% and 100%. Nodes lying on the border of two grids have two schedules to follow so connectivity can be maintained between the whole of the network.
5.1.1 Adding Sleeping Mode to the Model Devised in Chapter 4.0
In chapter 4.0 it was noticed that the cluster head nodes nearest to the base station spent the majority of their time in either transit or receive state and hence used more energy compared to cluster head nodes that were further away from the base station. This idle time and hence idle energy was becoming larger as the network traffic was gradually reduced from 100% to 10%. A further study was carried out to see the behaviour of network lifetime by introducing a Sleep Mode. At present a sleep model is only introduced in SMAC and is not implemented in 802.11 protocols that is the most reliable protocol in NS2. The network traffic levels used by SMAC are much lower as compared to the model developed in Chapter 4.0. A way to overcome this bottleneck in NS2 was to decrease the idle energy consumption to a fixed lower level. At the moment the idle energy consumed by a node in the idle state is 0.05 joules per second. It was assumed that during the idle
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time the node goes to sleep for 90% of the time and only remains awake for 10% of the total idle time. This idle energy consumption was reduced to 0.005. This meant that the nodes will only consume 10% of the energy while they were awake. This also assumes that when nodes have gone to sleep (90% of the idle time) or are transitioning from idle to sleep and vice-versa, they were not consuming any energy at all.
This does not affect the network throughput, packet delivery, latency and jitter. The study is aimed to get an insight on how the network lifetime could be improved if sleep energy is introduced into the Optimised grids, Equal grids and COTS networks.
The Idle energy consumption of the Optimised grids, Equal grids and the COTS network was modified to accommodate for sleep energy and the simulations were run for 1010 seconds. To avoid confusion, the three models (Optimised, Equal and COTS) will have a prefix of ‘Sleep’ where the Sleep Mode is introduced and prefix of ‘Idle’ where there is no Sleep Mode. Figure 5.1 compares the cluster head lives of the three network models (Optimised grids, Equal grids and COTS network) with idle energy developed in Chapter 4.0 section 4.2 with the same models including the sleep energy. Apart from idle energy consumption as described earlier, nothing else is changed in the simulation run.
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The lower zoomed graph of Figure 5.1 shows the cluster head lifetime of the first five nodes. It can be seen that all the three networks have an increase in cluster head lives. The Sleep Optimised grids network shows an average increase of 26% for the first five cluster head nodes, while for Sleep Equal grids and Sleep COTS network it has been 13.3% and 16.4%. Keeping in mind from Chapter 4.0 section 4.4, the Sleep Optimised grids network has approximately 24.7% higher throughput and much higher packet delivery with lower latency and jitter. As the nodes spend less time being idle, the highly efficient Sleep Optimised grids network uses the saved idle energy more efficiently, by providing extended cluster head lifetime compared to the other two networks.
Figure 5.2 100 % Traffic Total Network lifetime comparison with and without Sleep Mode
The average total grid lifetime for the first five grids of the Sleep Optimised grids network (black line) has increased by 39% as compared to Idle Optimised grids network. It also shows that the increase in grid lifetime has only been 18.4% for the COTS and Equal grids network. Another key point noted is that without Sleep Mode, the Idle Equal grids network had overall 6.8% longer lifetime as compared with Idle Optimised grids networks. By using the Sleep Mode and intelligently utilising transmit energy, the Sleep
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Optimised grids network now has at least 9.1% more network lifetime as shown by node 3 of ‘Sleep Optimised grids network’ compared with node 5 of ‘Sleep Equal grids network’. Please note from Figure 5.2 (b) that nodes in grid 5 and grid 7 will die much quicker for both the Sleep Equal grids and Sleep COTS networks. This is because of the congestion in those networks near the base station. Cluster head nodes in grid 5 and 7 are repeatedly transmitting packets that are being lost for majority of the time to cluster head nodes of grid 4 and 6. Hence cluster head nodes of grid 5 and 7 are continuously using more transmit energy as compared to cluster head node of grid 4 and 6 that are consuming less energy as they are only receiving packets. The next section compares the effects on network lifetime for three networks when the traffic is fluctuating with the integrated sleep mode.
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