## 3 Comparison of Telecommunications Technologies to Support the Protection Scheme
Since all the cellular technologies deployed up to now are characterized by round-trip delays ranging between 800 ms and 8 s (in some bad cases) [27], they cannot be applied to the considered case where the fault messages have to be delivered within 100 ms.  
An alternative solution to wireless links is given by wired technologies, mainly dominated by fiber-optic communications and power-line communications (PLC). Both technologies are able to guarantee very low end-to-end delays, given that signals propagate with a speed comparable to the speed of light. Since in our application the amount of data to be transmitted is very small, the end-to-end latency, that is the sum of the transmission delay and the propagation delay, is strongly acceptable and on the order of a few msec. Nevertheless, the main advantages of fiber-optic communications, _i.e._, their very high transmission rates, very low delays, and immunity to electromagnetic and radio interference (matters that are very important for high-voltage operating environments like the one considered in this paper), are not well balanced by their expensive installation costs, also taking into account the large amount of spare capacity. Therefore their application is justified in our scenario if the energy provider installs them for other goals, for example a leasing agreement with telco operators, or to support the normal state of an SG populated with a huge number of devices. The other wired technology that is very powerful for the SCS of an SG is PLC [28]. It is a technology for carrying data on the same conductors of the electrical grid, and for this reason it has deployment costs comparable to wireless technologies since the lines are already there. This technology, already widely applied in low-voltage distribution grids (close to homes), is well suited for smart metering infrastructure and enables communications between electric vehicles and a power grid (narrowband PLC), as well as transferring data seamlessly from SG controllers to home networks and _vice versa_. However, its application to manage the clearing process in the distribution networks is very complex because it needs to carry data through the electrical line when it is open. The application of the PLC technologies is still possible but requires some deployment expedient to also maintain connectivity in the presence of a fault in the distribution network. A way to apply it is to use a two-step double technology. More specifically, if the fault is on one phase of a three-phase conductor, the PLC channel can work on the two other phases to communicate the fault to the DMS in order to operate the CBs. Once the CBs are opened, the following communications cannot be realized with PLC because the conductors are open. Then another technology (e.g., GSM) can be used for the telecontrolled sectionalizers because the end-to-end delay is not yet a stringent constraint for safety purposes, although the portions of the network outside of the faulty zone must be restored as soon as possible considering that the DNO is affected by the reward/penalty mechanisms. On the other hand,such a two-step double technology fails when a three-phase fault occurs and consequently cannot be adopted to implement the considered protection scheme.