## 3 Comparison of Telecommunications Technologies to Support the Protection Scheme
Thanks to the presence of redundant paths, it is able to guarantee a high data rate, high communication reliability, and automatic network connectivity against potential problems, e.g., wireless node failures and path failures. However, these peculiarities, which are very important for managing SGs characterized by a high grade of connectivity and working in normal states, are not useful in our case. On the contrary, due to the presence of multi-hop communications through wireless links, typically based on the standards IEEE 802.11 and IEEE 802.16, the end-to-end delay caused by using an WMN can reach values a lot higher than the aforementioned maximum accepted values of 100 ms in the specific case considered in this paper [21; 22; 23]. Moreover, if the cheaper IEEE 802.11 is used to realize single-hop links, the number of consecutive hops can be high because of the limited range of coverage of each link (a few hundreds of meters) and the distance between each switch (CBS, sectionalizer) and the DMS installed at the PS. Consequently, besides unacceptable delays, this causes high vulnerability to failure. This problem could be overcome by the more recent standard version IEEE 802.11y, which allows one to transmit the signal up to 5 km at a rate of 54 Mbps but requires a licensed frequency band of 3.7 GHz, very high transmission power, and a specially designed antenna--conditions that often make it unfeasible. The other wireless technology that allows major coverage and higher data rate is IEEE 802.16. It allows high data rates on distances of some kilometers, but requires Line Of Sight (LOS) between transmitting and receiving stations. This can be easily achieved in rural areas, while it is a problem in urban areas. Similar considerations hold for microwave and free-space optical communications, both suitable for point-to-point communications. Microwave communications allow use of conveniently sized directional antennas to obtain transmissions at high bandwidths. Free-space optical communication, on the other hand, is a communication technology that uses light propagation in free space to transmit point-to-point data at high bit rates with low bit error rates. However, due to the LOS requirement, the transmission quality achieved with them is greatly affected by obstacles (e.g., buildings and hills)and environmental constraints (e.g., rain fade), making them unsuitable during particular weather conditions and not applicable in urban areas. Instead, they are very useful in rural areas, where using other wireless or wired technologies is costly or even impossible. Other wireless technologies like Bluetooth, ZigBee, WirelessHART, and ISA100.11a, all based on the IEEE 802.15.4 protocol stack, although strongly applied at the edge of SGs in the customer home networks, are not applicable in our context for their very short range of coverage. In addition, Bluetooth has a very high network joining time, around 3 s, compared to 30 ms for ZigBee. On the other hand, satellite channels, widely considered as backup channels in case of disaster [24] to maintain the normal state of an SG, cannot be considered when a fault occurs because of their unacceptable latency as compared with the maximum acceptable delay of 100 ms. Finally, another wireless solution is given by the communications channels based on cellular communications, that is, the short message system and the more complex Wireless Wide Area Networks [25]. The first technique is widely used today to execute restoration procedures by means of telecontrolled sectionalizers. On the other hand, the very huge end-to-end delays, on the order of some seconds [26], limit its application to CBs. The second technology is realized as networks of cellular channels following the evolution of the cellular network standards, starting from the first version of General Packet Radio Service (GPRS) up to the modern 4G.