A RQUITECTURA DE LA SOLUCIÓN

This section approaches the use of WMNs in the scope of smart grids as a potential solution in supporting a last-mile communications infrastructure that is able to cope with the control schemes discussed so far.

A set of scenarios was defined based on the geographic characterization and probability distribution candidates that were assessed in the previous section. An evaluation of a WMN based on WiFIX follows with the analysis of the network requirements in terms of communicating nodes to ensure a proper coverage of each of the considered scenarios are evaluated along with the network performance in terms of delays and information losses.

5.4.1 Distribution Grid Scenarios

A set of distribution network types have been established in previous sections and a set of PDFs have been assessed to provide the basis for random generators to be used in the definition of different distribution grids that allow exploring the impact of the variability of node placement, under different contexts, in the performance a WMN based on WiFIX. In Fig. 5.18 a random generation of node positions for each type of LV scenario is illustrated. The node positioning and involved distances between nodes, represented in meters, are in accordance with the graphic representation of feeders presented in Appendix D.

300 200 100 0 100 200 300

The GW node is always located at the center (0,0); in MV scenarios it represents a HV/MV substation whereas in LV it represents a MV/LV secondary substation with which all remaining nodes exchange data.

The dashed circle signals the farthest MAP node from the GW, thus illustrating the geographic span of the communications network.

Similarly in Fig. 5.19 a random generation of node positions for each type of MV scenario is portrayed.

A set of randomly generated scenarios can hence be used to evaluate the performance of WiFIX using a Monte Carlo implementation, once the connectivity among all communicating nodes is ensured.

4 2 0 2 4

5.4.2 WiFIX in Last-Mile Communications Scenarios

For each of the generated scenarios it is necessary to deploy a WiFIX wireless mesh network. The nodes from the previously generated scenarios are also communicating nodes of the WiFIX network, Mesh Access Points (MAP), but since the involved distances may leave some MAPs without connectivity, relay nodes have to be added. Unlike MAPs these relay nodes do not generate traffic and are only responsible for relaying information from and to a GW node. The ns-3 WiFIX module processes the different scenarios and, according to the propagation characteristics, establishes the number and position of the relay nodes if they have to be added.

4 2 0 2 4 Figure 5.20: MV Scenarios With Relay Placement

In all cases of the considered LV scenarios no relays were necessary to ensure the coverage of all MAPs. As it will be shown, the installed MAPs are able to exchange data directly with the GW node.

Conversely in the MV networks a significant amount of cases requires relays to be added due to the involved distances. Fig. 5.20 illustrates the placement of relay nodes for a single case of each of the MV scenarios presented earlier in Fig. 5.19.

The average number of relays necessary for each case of the Monte Carlo simulation for the MV Rural scenario is shown in Fig. 5.21. It is possible to observe that the number of relays to be installed can vary significantly from 36 to 91 relays. This is due to the varying distances between neighboring nodes. On average, as illustrate by the red line in Fig. 5.21, about 61 relays are needed to ensure the coverage in each of the MV rural cases.

0 200 400 600 800 1000

Case # in rural scenario 0

20 40 60 80 100

# Relays

*average: 60.74

*

Figure 5.21: MV Rural - Number of Installed Relays

The number of installed relays for the each of the simulated cases of the MV Mixed scenario is represented in Fig. 5.22.

0 200 400 600 800 1000

Case # in mixed scenario 0

5 10 15 20 25 30 35

# Relays

*average: 11.28

*

Figure 5.22: MV Mixed - Number of Installed Relays

Despite the higher number of MAP nodes when compared with the rural counterpart, the smaller distances between neighboring MAP nodes means that less relays have to be installed. Hence, more MAPs are able to forward information among them without needing relays to overcome the established

RF distance between nodes. There is also a significant variation on the number of installed relays for each case, which ranges between 2 and 28. On average about 11 relays are necessary to ensure connectivity in each case.

Fig. 5.23 depicts the average number of relays installed in the MV Urban scenario. In most cases no relays are necessary to ensure the coverage, since in general the position and distance between MAPs allow them to forward information without relays. The number of relays to be installed varies from 0 to 3.

0 200 400 600 800 1000

Case # in urban scenario 0

2 4 6 8 10 12

# Relays

*average: 0.11

*

Figure 5.23: MV Urban - Number of Installed Relays

The main ns-3 simulation parameters used throughout this section are presented in Table 5.9. The number of nodes considered for each scenario was previously defined in Table 5.8. It should be mentioned that a few changes were introduced to the underlying IEEE 802.11g to make it compatible with its use outside buildings. Hence the frequency was reduced to 900 MHz and the data rate was assumed to be between 6 and 54 Mbps. Although this means a non-compliance with the IEEE 802.11g standard the objective is to show the compliance of a multi-hop wireless sub-GHz solution with the requirements of smart grids.

Table 5.9: Main Simulation Parameters used in ns-3

Control

RTS/CTS disabled

Radio

TxGain 5 dB RxGain 5 dB

TxPowerStart 16 dBm TxPowerEnd 16 dBm

EnergyDetectionThreshold -63 dBm CCaModelThreshold -68 dBm

Frequency 900 MHz Range 410 m

Models

Propagation Open Space Standard IEEE 802.11g

Loss NIST Error Ratio Model Mobility None

Antenna height 1.5 m Data Rate [6-54] Mbps

WiFIX

Hello Interval 10 s Message Size 1 kb

Warm-up Time 500 s Sim Time Variable (1000 msgs/node)

The Round Trip Time (RTT) of a complete polling sequence is presented in Fig. 5.24 for each type of LV scenario. It is visible that the RTT is similar for LV rural and urban, respectively ∼56 ms and ∼58 ms on average, since the number of nodes is similar, whereas in the mixed scenario the higher number of nodes has an impact on the also higher RTT, which on average totals ∼127 ms. The scheduling nature of the polling process and the absence of relays in any of the LV scenarios allows the RTT values to be quite stable despite the placement of nodes.

0 200 400 600 800 1000

Figure 5.24: RTT in LV scenarios

Similarly, Fig. 5.25 depicts the RTT of the complete polling sequence for each MV scenario. As expected the RTT is lower for the urban scenario where the number of nodes is smaller. The RTT values for the rural and mixed scenarios are visibly close despite the fact that in the latter the number of MAPs is more than the double. This can attributed to the fact that the average number of relays added to the rural scenarios is higher than the number of relays to be installed in the mixed scenario to ensure the necessary connectivity.

Figure 5.25: RTT in MV scenarios