## 2 PLC Technologies  
### Overview of PLC Technologies and Their Application to AMI  
The very basic elements that compose AMI are the smart meters, which work both as sensors and actuators with bidirectional communications capabilities, and the information systems (also referred as MDMS--Meter Data Management Systems--in the literature [13]), where the gathered data is processed and the appropriate decisions are made. The smart meters and the information systems can be connected directly through backhaul communications technologies, resulting in monolithic communications architectures, as shown in Figure 1a.  
Monolithic communications architectures have pros and cons [13] and, in order to improve scalability, data concentrators are typically deployed, resulting in hierarchical communications architectures, as shown in Figure 1b. The place where the data concentrators are deployed may vary depending on the features of the power distribution infrastructure (e.g., number of smart meters per SS, length of the low voltage cables, number of SS per primary substation, etc.). For instance, in Europe (and China) such data concentrators are typically located at the SS since low voltage cables are long and power distribution grids are reasonably dense; whereas in the US (and Japan) they are  
Figure 1: Overview of Advanced Metering Infrastructure (AMI) communications architectures. (**a**) Monolithic; (**b**) Hierarchical with two network segments; (**c**) Hierarchical with three network segments.  
located upwards in the power distribution hierarchy because the low voltage cables are shorter and less populated [42; 43].  
No matter where the concentrators are, the use of concentrators split the communications architecture into two main communications segments: (1) the \"last mile\" or NAN (Neighboron Area Network) [44; 45], which comprises the smart meters and the data concentrators; and (2) the backhaul, which comprises the data concentrators and the information systems. This paper focuses on the \"last mile\" or NAN (in red in Figure 1), notably when the concentrators are deployed at the SS, so smart meters and concentrators are directly connected through the low voltage cables.  
PLC can be mainly classified in three different types [10; 12]: UNB (Ultra NarrowBand), NB (NarrowBand), and BPL (Broadband over Power Lines). UNB is not the preferred solution for AMI in the considered communications segment since the data rate may be too low, although there are solutions available and running in the market for quite a few years, such as Aclara TWACS (Two-Way Automatic Communications System).  
On the opposite side, the two main BPL standards (namely IEEE 1901 and G.Inem [46]) may exceed the communications and economic requirements of this segment, although there are examples where it is used as well [47]. In the case of Spain, for instance, DSO Iberdrola used this technology to provide broadband access some years ago, but with the launch of VDSL (Very-high-bit-rate Digital Subscriber Line) the business case stopped being economically justifiable. Nevertheless, Iberdrola learned from this experience and now they use this technology over the medium voltage or FAN (Field Area Networks) for AMI and tele-control purposes [48; 49], resulting in the communications architecture shown in Figure 1c.  
Thus, the most widely used PLC technologies for this segment are definitely NB. Table 1 summarizes the most relevant NBPLC technologies together with their main features.  
Notably, this paper focuses on PRIME. Hence, the next section explains more in detail how this technology works.