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This chapter focuses on identifying the parameters that are specific to this frequency band, with reference to ZMD’s transceiver, then testing the system with exhaustive simulations to reach a point to judge the feasibility of this technology in its current operating band for the perceived applications. The report further comments on the performance metrics, while highlighting the technology’s performance areas with a focus on its grey areas explaining reasons where ever possible.
Several metrics can be defined to grade the performance of a technology against the elements of wireless networking. Some of these metrics have been carefully chosen to give an idea of behavior and the reliability of the Zigbee networks. Before we go on to describe these metrics, it is reminded that the analysis focuses only on data transmission, and the metrics measure their features with respect to data packets, hence other controlling overhead, are overlooked. An detailed explanation of these metrics follows:
It is a measure of the amount of data transmitted from the source to the destination in a unit period of time (second). Considering the low datarates and throughputs supported by this technology at 868Mhz frequency band, the throughput is measured in total bits received per second. Also to be noted is that this metric only measures the total data throughput, ignoring all other overhead, over the network. The throughput of a node is measured by first counting the total number of data packets successfully received at the node, and computing the number of bits received, which is finally divided by the total simulation runtime. The throughput of the network is finally defined as the average of the throughput of all nodes involved in data transmission.
Therefore, throughput can be stated as:
Throughput_of_a_Node =(Total_Data_Bits_Received) / (Simulation_Runtime) (5.13)
Similarly the throughput for the network can be defined as:
Network_Throughput = (Sum_of_Throughput_of_Nodes_Involved_in_Data_Trans.) / (Number_of_Nodes) (5.14)
The end-to-end delay is the time taken for a data packet to reach the destination node. The term, ”End-to-End” delay does not have much significance in this scenario. The simulations are performed with a star network topology, and the routing mechanism is disabled, because the standard doesn’t support routing of data among peers. Therefore, the maximum number of hops for any data packet before reaching the destination node can be only 1. Hence the term End-to-End Delay can be safely interchanged with Delay.
The delay for a packet is the time taken for it to reach the destination. And the average delay is calculated by taking the average of delays for every data packet transmitted. The parameter comes into play only when the data transmission has been successful.
Packet_Delay = Receive_Time_at_Destination − Transmit_Time_at_Source (5.15)
Average_Delay = Sum_of_all_Packet_Delays / Total_Num_of_Received_Pkts (5.16)
It indicates the percentage of the transmitted data packets that are successfully received. It is an important metric which can be used as an indicator to a congested network. Again the delivery ratio is only considered for data packets. Firstly the total number of transmitted packets is counted, followed by the total number of received packets and the total number of dropped packets. The delivery ratio is calculated as the percent of packets received to the packets transmitted. The number of packets dropped does not take into account retransmissions. As an example, if a data packet with an ID: 4 is transmitted and has been dropped for the first time. The packet is retransmitted, as long as we have a successful transmission or the maximum number of retransmissions has been reached. But the drops due to retransmission is not taken into account and are only counted when the packet is ultimately dropped, and even in those cases its counted as a single drop. However, if the packet is successfully received by the destination after several retransmissions, the drops are not considered. This would effectively make the number of transmitted packets equal to the sum of the number of received packets and number of dropped packets.
Delivery_Ratio = (Number_of_Received_Packets / Number_of_Transmitted_Packets) � 100 (5.17)
The metric is measured as the percent of energy consumed by a node with respect to its initial energy. The initial energy and the final energy left in the node, at the end of the simulation run are measured. The percent energy consumed by a node is calculated as the energy consumed to the initial energy.And finally the percent energy consumed by all the nodes in a scenario is calculated as the average of their individual energy consumption of the nodes.
Percent_Energy_Consumed = (InitialEnergy − FinalEnergy) / InitialEnergy �100 (5.18)
Average_Energy_Consumed = Sum_of_Percent_Energy_Consumed_by_All_Nodes / Number_of_Nodes (5.19)
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