Performance Evaluation of 2-D Optimised grids vs 2-D Equal grids and 2-D

One of the key differences between the linear 1-D and 2-D network is that all the wireless sensor nodes are included in the simulation. These wireless sensor nodes add extra burden on the network near the base station as the cluster heads not only have to receive and forward all the data from the previous cluster heads, but also have to communicate with its local wireless sensor nodes. This extra traffic and interference caused by the local wireless sensor nodes can have detrimental effects on the total throughput as they will be trying to communicate with the cluster head, and decreasing its performance in receiving packets from previous cluster head and forwarding them to the next cluster head.

Figure 6.3 2-D 100% Traffic Throughput comparison between all the three 2-D networks

Figure 6.3 compares the throughput achieved by the all the three 2-D linear sensor networks. The 2-D Optimised grids network has a much higher throughput of 225789 bits/s as compared to 180596 bits/s and 185707 bits/s compared with 2-D Equal grids and 2-D COTS network. This proves that the 2-D Optimised grids network has much better spatial reuse where cluster head nodes do not receive message from grid (i+2) due to Optimised grid’s asymmetric spacing hence reducing network congestions and collision

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while improving throughput. When compared with 1-D networks, all the three 2-D networks suffered a drop in throughput by approximately 7.5%. With respect to 2-D COTS and 2-D Equal grids network, the 2-D Optimised grids network showed an increased throughput between 21% and 25% respectively.

As the throughput is affected due to the addition of wireless sensor traffic in the network, Figure 6.4 (abc) compares the packet delivery, average latency and jitter for the three 2-D networks. The 2-D Optimised grids network exhibits a higher percentage of packet delivery (over 77%) for all the cluster heads in the network as well as nearly over 98% for the first three grids nearest to the base station. The 2-D Equal grids and 2-D COTS network show great performance for the first 3 grids, but then comes a sharp drop in packet delivery up and until grid 12 and onwards, where an average just below 55% is maintained for the rest of the grids. Due to inefficient Equal grids spacing, the network suffers from large number of collisions, retransmissions and elongated waiting times. The conclusions are justified by Figure 6.4(b) that shows the 2-D Optimised grids network having around 9 times less latency compared with the other two networks.

For the 2-D Equal grids and 2-D COTS network, the trouble starts, when cluster heads, 1,2 and 3 are forwarding all their sensor data, and queues start to build up on cluster heads 4 and onward till the end of the two networks.

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The jitter for the 2-D Optimised grids network is also around 1/3 compared to 2-D Equal grids ½ compared to 2-D COTS network. A key point to remember is that while comparing these results with 1-D linear networks, all the three networks suffered a reduction in packet delivery and an increased average latency and jitter.

Moving away from cluster heads, Figure 6.5 compares the network QoS parameters for all the wireless sensor nodes in each of the 2-D network. Please note, that nodes 1to 19 are cluster head nodes, that are not included in Figure 6.5, due to their higher traffic capacity. As all the three networks have 85 nodes, nodes from 20 to 85 are sensor nodes. All these nodes produce the same amount of data irrespective of their grid size or location in the network. Each node has to transmit 0.003 Erlang data which is equivalent to generating a 1000 byte packet every 2.67 second. Depending on grid size and number of nodes in each grid from Table 6-1, for the 2-D Optimised grids network, nodes 20 and 21 will be in grid 1, nodes 22 and 23 will be in grid 2. Hence for all the three networks, nodes from 20 and 30 lie between the first 4 grids nearest to the base station. Another thing to note is that these wireless sensor nodes only report to their respective cluster heads. They have no contact with the base station nor do they transmit any data directly to other wireless sensor node (this does not exclude that other nodes within their transmission range will not hear them). Some peaks can be seen where sensor nodes are transmiting over 100%, as the packet delivery percentage is measured as the No of packets received over total expected packets, per second at the cluster head node. The wireless sensor nodes transmission rate is set as random. E.g from above a sensor node has to send a packet every 2.67 second. However when set to random, it can send that packet any time between 0 and 2.67 second. And again in the next cycle period it has to send one packet within that time period. Hence in some cases, the packets can reach very close to each other when the packets are sent back to back as shown as a peak in the graphs.

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Figure 6.5 Wireless sensor nodes QoS parameters in 2-D networks.

The 2-D Optimised grids’s wireless sensor nodes have nearly 99% packet delivery, while the other 2 network have slightly less. The latency and jitter between wireless Sensor nodes 30 to 40 are relatively high for 2-D Equal grids and 2-D COTS network. This where these networks, start to become inefficient, and cluster head nodes start dropping majority of the packets. From Figure 6.5 (b,c) the latency and jitter decreases for all the three 2-D networks towards the furthest end away from the base station due to less traffic.

6.1.2 Sensor Nodes, Cluster head and Network Lifetimes for 2-D