Map function for ETX metric.

Map function for ETX metric.

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This paper introduces a novel quality estimator that uses different metrics to decide the best path towards the root in Wireless Sensor Networks. The different metrics are assessed at medium access control layer (MAC), under the IEEE 802.15.4 standard, and are used at network layer, enhancing the best path selection process done by the routing prot...

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Context 1
... to transmit correctly a packet over a link, but in TSCH, retransmissions are made in different channels due to the channel hopping, so it is not possible to give an accurate value of ETX for a particular channel. ETX calculated values are also filtered with EWMA. The mapping feature for the ETX metric, used in the quality function is shown in Fig. 3. As an alternative, PER (Packet error rate) could be used to measure how many packets are lost in each of the channels, however, this would cause the time needed to process this metric to be proportionally larger, depending on the percentage of channels used, compared to the method of obtaining the ETX ...
Context 2
... other solutions that use RSSI values to estimate channel quality, this solution allows nodes to measure the RSSI value for each of the channels involved in the communication. This is relevant because the devices have been developed under the IEEE 802.15.4e standard using the TSCH mode that is specially designed to work in industrial environments. This mode allows the communication to alternate between different frequency channels to mitigate negative effects of the channel, such as interference or fading. As a result, the level of RSSI will vary over time, because not only a given channel changes over time, but the communication also jumps 1 Destination-Oriented DAG (Directed Acyclic Graph) between channels over time. Therefore, when evaluating the RSSI level of a received message, the channel through which the message has been transmitted is also verified, allowing the characterization of this metric to reflect this time/frequency variability. In order to organize all this information, in each of the nodes a series of matrixes have been defined to store the RSSI measured values for each of the neighbors that are within the range, and for each of the channels used in TSCH. Fig. 1 shows a representation of the process of updating values in the commented matrixes. Each RSSI value for each channel (matrix cells) is computed and statistically softened with the EWMA filter, which allows considering past values, to improve stability, and then the corresponding cell of the matrix is updated. This type of filtering uses a structure like the one shown in (1). Then, for each row (representing a link with a neighbor in all available channels), the mean is applied to obtain a meaningful but single value to feed the quality estimator (Table 1). In this case, ? is set between 0.3 and 0.15 depending on the freshness of the actual sample (time limit set to 10 minutes). This allows to characterize each of the channels involved in the communication, allowing, for example, to perform a blacklisting of those channels that are hindering communication. The mapping function to introduce RSSI in the quality function is shown in Fig. 2. Map function for RSSI metric B. Expected transmission count ETX is a link quality measure between two nodes in a wireless network based on the exchange of data packets. This metric is commonly used in routing algorithms in mesh networks, for example in the MRHOF RPL objective function, to minimize packet loss. The ETX is defined mathematically as shown below (2), where N means the number of packets sent or received, and subscripts refer to node i as the sender, and j as the receiver: ? ????? ????? ???? ( 2 ) In this case, ETX values are not resolved for each channel due to the very nature of the ETX metric. ETX counts the number of retransmissions needed to transmit correctly a packet over a link, but in TSCH, retransmissions are made in different channels due to the channel hopping, so it is not possible to give an accurate value of ETX for a particular channel. ETX calculated values are also filtered with EWMA. The mapping feature for the ETX metric, used in the quality function is shown in Fig. 3. As an alternative, PER (Packet error rate) could be used to measure how many packets are lost in each of the channels, however, this would cause the time needed to process this metric to be proportionally larger, depending on the percentage of channels used, compared to the method of obtaining the ETX ...

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