Evaluación del modelo de diseño propuesto

There are other wireless solutions and technologies that can be considered for smart grids but their effec-tive use is still under discussion. So far the presented technologies, which can be regarded as conventional, concern wireless communications between devices within a single hop. As an alternative, multi-hop tech-nologies can be considered to support SG applications, which is the case of the IEEE 802.15.4g, which defines a mesh network solution for smart metering, using a flat network of smart meters. Although the mesh network has already been used in WPANs/ZigBee, namely through the IEEE 802.15.5, this section discusses the use of mesh networks in the last-mile segment which is beyond their scope. This section discusses a wireless mesh alternative to support applications beyond the simple smart metering, where the exploration of enhanced management schemes can allow multi-hop wireless networks to become more flexible and appealing in the SG context.

2.10.1 Wireless Mesh Networks

One of such solutions makes use of Wireless Mesh Networks (WMN) as a means to extend the coverage of a wireless network over multiple wireless hops and to provide access to infrastructure networks, wired or wireless. These are networks composed of Mesh Access Points (MAP) organized according to different topologies.

In the last-mile segment of electric grids the deployment of communications networks has been tradi-tionally scarce, meaning that WMNs can in fact be considered as a solution to extend communications networks from other segments, in this case typically from upstream utility related backbone network. It

can be regarded as a potentially cost-effective alternative to the long lasting narrowband PLC preference for the electric distribution grid.

The different wireless mesh networks topologies are depicted in Fig. 2.19. In a simple vision the WMN is composed of MAPs that act as End Stations (STA) where the exchanged information is relayed towards an external network by any node as illustrated in Fig. 2.19 (A). A more complex and structured system can implemented with the introduction of a two tiered approach, where higher level MAPs form a mesh network as illustrated in Fig. 2.19 (B). Hence, MAPs form a wireless mesh backbone network to which STA devices are attached. The devices at the lower level communicate directly with a neighbor MAP, which provides access to the mesh network. This allows a scalable and robust higher level mesh network structure ensuring connectivity between STAs and with the external network, through a MAP gateway. An even more complex topology can be implemented in a hybrid approach, were mesh networks are formed at different tiers, with more relaying alternatives, as portrayed in Fig. 2.19 (C).

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Figure 2.19: Wireless Mesh Networks Topologies

The expected traffic pattern in the distribution segment will most likely involve data aggregation in the upstream direction, from the end devices/customers towards the electric system operator. Such pattern is compatible with WMN implementations where the data traffic concentration is higher near a GW node, which will exchange data with an external network. The data traffic inside the mesh network that is, between end devices itself is expected to be reduced.

Mesh networks offer higher levels of redundancy and robustness in terms of data communications in the event of a link or node loss, either temporary or permanent, or link degradation. Routing algorithms can adapt to dynamic network topologies to deal with variations in communications channel characteristics.

The estimation of the best data paths and routing schemes rely on the definition of proper metrics.

Despite the advantages of such routing schemes in the flexible operation of these data networks, there is also an increase in the overhead associated with operation signaling required to discover and maintain routes at network level. However mesh networks can be managed at data link level, making use of conventional bridging techniques that rely on simple packet forwarding along a logical tree, although the established routes might not be the best ones. The data traffic between internal pairs of nodes of the

WMN is reduced, meaning that the largest part of the traffic concerns a WMN external entity. As such the mesh gateway node is almost always involved in data exchanged with the exterior, acting as a root node, which makes a spanning tree optimal for this type of traffic.

There are however other drawbacks associated with WMN that need to be addressed with proper mechanisms. One of the issues is the competition among nodes, which is a 1-hop problem typical of 802.11 networks. Since the communication channel is a shared media this issue is usually address by contention based mechanism such as CSMA/CA. Both hidden node and exposed node are also related issues, which are typically addressed through RTS/CTS mechanisms. These issues are aggravated in WMNs due to the existence of multi-hops in the data path, which contribute to the performance degradation of the communications network. In these scenarios the use RTS/CTS can even become counterproductive.

Beside the increase in latency another problem is the existence of additional collisions since the interference (inter and intra-flow) phenomena can have impact over several hops. This means that unfairness will occur where nodes farther away from the GW will have to contend more times for accessing the channel.

Besides a reduction of the actual network capacity is to be expected, since the spatial contention goes beyond 1-hop. As such, spatial reuse strategies and scheduling schemes are used to mitigate the impact of interference, but this is a challenging process since there is not a clear optimization procedure to adopt, usually meaning that a trade-off between performance and complexity need to be reached. Due to these issues congestion phenomena occurs, especially when large volumes of data are exchanged through the GW node, namely due to data aggregation towards the GW.

Hence, the use of WMNs introduces challenges that need to be tackled to prevent the performance degradation of such networks. Issues like fairness [76], scheduling in terms of fair-sharing [77, 78] or resource optimization [79] and cross-layer mechanisms like scheduling with congestion control [80] are topics of research in the area of WMNs.

There are some characteristics in the distribution segment that may promote and facilitate the use of WMNs to support applications within the context of smart grids. Most communications devices deployed in distribution networks are expected not to have mobility characteristics and the planning for the WMNs can be made in a medium/long term perspective, since major changes will only be introduced when considering expansion or reinforcement of communications networks. Power constraints are very low given the nature of the power grid, but battery support may be required in case survival of communications network is necessary in the event of a power failure or more severe blackout events. The geographic span of last-mile SG communications networks may not require a higher number of mesh nodes, which has clear benefits in terms of data path hop-count and requires less complex management schemes. However the use of WMNs in MV distribution networks may have to deal with larger distances and a significant number of nodes that already include data aggregation of larger data volumes thus requiring higher bandwidth.

2.10.2 WiFIX

The growth in broadband services has been stimulating investments in access networks, like those using IEEE 802.11 based technology. On one hand, the success of these solutions in supporting a wide variety of applications has been triggering the exploration of other possible scenarios of application. On the other hand the known limitations in terms of radio range and consequent geographic coverage has posed

several challenges when new applications are considered. In fact these limitations are transversal to wireless communications technologies in general; the use of mesh networks is suggested to overcome these limitations at the expense of some performance degradation and routing complexity. Hence, as pointed out in [81], WMNs are a cost-effective solution to extend the coverage of wireless networks, like 802.11.

They can also be used as an extension of wired communications network infrastructures.

A proposal for the extension of wired communications networks using simple and efficient WMNs is advanced in [81], which described a solution called WiFIX. This solution is presented as an alternative to the IEEE 802.11s which defines mesh networking within the 802.11 family. The original WiFIX system has evolved [81, 82, 83] and it consists of a simple and efficient tree-based algorithm for stub WMNs that runs over 802.11 legacy MAC. An active tree topology is built, starting from the root node to deal with path auto-configuration issues. Hence, it is possible to run WiFIX over 802.11 compliant wireless cards, by processing the incoming and outgoing data packets without any modification to the 802.11 MAC.

Instead an IEEE 802.1D bridging mechanism is used with the addition of a single message protocol scheme. Like those described earlier, a WiFIX WMN is composed of MAPs with bidirectional forwarding capability defining a multi-hop communications infrastructure. These nodes can also act as STAs or GWs ensuring an interface with external and possibly wired backbone networks.

Over WiFIX it is possible to schedule transmissions from each MAP in a master-slave scheme, so that collisions are prevented, but the CSMA/CA mechanism is still active to deal with residual collisions, in particular those involving control packets. One of such examples is a polling-base mechanism through which it is ensured that each node has the same opportunity to transmit a packet.

In order to improve the overall efficiency of a mesh network, like WiFIX, spatial reuse can be exploited.

Although this is an orthogonal subject it can be incorporated in WiFIX to increase the performance of the network. The network capacity is one of the examples where spatial could be used to mitigate the degradation with the hop count (e.g., in chain of nodes). It can allow the WMN to reach between 1/4 to 1/3 of the capacity of common wireless channels.

2.10.3 Wireless Communications Opportunities for Smart Grids

Some of the scenarios that are likely to be found in SG will raise some challenges for the use of wireless communications in terms of coverage and RF penetration, especially in the aforementioned last-mile segment where a combination of outdoor and indoor communications is often required.

The radio frequency used in wireless communications systems is one of the main characteristics that affect performance in these types of scenarios. The problem is that the available RF spectrum is be-coming a scarcer resource for newer applications, like SGs, especially in the sub-GHz segment. Given this constraint the strategy so far has been to conserve or reuse this resource and explore the use of more efficient RF technologies. The use of more convenient frequencies can allow the implementation of cost-effective solutions and less complex planning activities in order to deploy wireless communications infrastructures. One particular example can be found in WMNs, which can largely benefit from the use of technically more favorable radio frequencies, in terms of transmission range, which contributes to the reduction in the number of hops that are necessary to ensure the same level of coverage.

Within the RF reusability spirit, TV White Space (TVWS) communications are considered in [73] for SUNs, where unused locally assigned TV broadcast frequencies are used to convey SG data. The TVWS

consists of unused VHF/UHF bands which are licensed to TV broadcasting companies. However, the US Federal Communications Commission (FCC) has recently defined regulations for the use of TVWS where several conditions and restrictions are imposed, which limit the objective of reusing these frequencies.

One of such restrictions is associated with the allowable transmission power according to the type of device. However, the main obstacle in the implementation of SUNs in these bands is related with the required geolocation awareness capability that these nodes must have [73]. In the UK, the Office of Communications (Ofcom) is defining the rules for TVWS whereas the Electronic Communications Committee (ECC), which is part of the Conference of European Post and Telecommunications (CEPT), defines the rules in Europe.

In this regard, cognitive radio networks seem to find in TVWS a potential infrastructure to support several applications such as medical, public safety, cellular or smart grids [84]. The use of cognitive radio has been envisaged to increase the efficiency in the use of radio frequency bands. These inefficiencies can be associated with design issues, technological limitations or the intention of regulating entities to protect spectrum allocation, ensuring exclusivity in its usage. The use of cognitive radio requires a coexistence approach since it will operate in scenarios were communications services are deployed, which are referred to as primary services, whereas cognitive radio provides secondary services [85].

Cognitive radio-based networks are thus a potential candidate, as mentioned in [84], to implement the infrastructure for AMI/FAN, with advantages in terms of bandwidth, coverage range and cost. Smart grid communicating devices based on cognitive radio hardware are hence able to access the underutilized spectrum to exchange information. The frequencies associated with TVWS have propagation character-istics that can allow cognitive radio devices to communicate directly with a GW node or in case of a wireless mesh implementation to use less hops to reach the GW node. It also allows a higher degree of penetration within buildings, making it flexible and suitable for last-mile combined indoor and outdoor scenarios.

The IEEE 802.22 standard defines the MAC and PHY layers of point-to-multipoint Wireless Re-gional Area Networks (WRAN) in the VHF/UHF TV broadcast bands. According to [86] the standard purpose is to enable the use of WRAN devices in diverse geographic areas within TV broadcast bands without interfering with incumbent licensed services. The expected coverage can vary from 10 to 30 km and can reach up to 100 km under exceptional RF propagation conditions. The IEEE 802.11af and IEEE 802.15.4m are current amendments for TVWS, respectively for WLANs and WPANs. A specific coexistence standard for TVWS is defined under IEEE 802.19.1.

Despite the advantages of cognitive radio systems, there are reliability and applicability issues, which are also transverse to other systems that make use of license exempt bands. It can lead to congestion scenarios when cognitive radio devices are operated in a heterogeneous and non-coordinated fashion [84]. These challenges can compromise smart grid applications, especially those depending on real-time requirements; nonetheless, they can be used to implement low-latency data links [87].

The disconnection of analog TV broadcasting systems, which is freeing in large areas a significant amount of RF band within the VHF/UHF spectrum is another opportunity for smart grids [88]. Unlike TVWS and cognitive radio, where a co-existence rationale is used, here specific bands could be assigned to SG applications only, but a generalized dependence on specific regulatory issues that will further define the use of theses frequency bands is still pending. The EU is committed to define a pan-European

approach towards SGs implementations [89] where harmonization in terms of the usage of TV analog bands for communications is being considered.