## 6 Deployment Guidelines for NB PLC System Deployment
### PLC Signal Repetition  
The repetition of PLC signals has been proven at a massive scale in new generation HDR NB PLC systems, based on PRIME standard. As it has been mentioned before, the PRIME Base Node is the element injecting the PLC signal in the SS in a PRIME system. The Base Node is in charge of creating the PLC topology (Figure 5), which is composed of service nodes, some of them acting as terminals (_i.e._, just connecting to the Base Node) and some others collaborating with the Base Node as switches (repetition elements) to gain access to places of the network (terminals) that are not able to receive or make their packets be heard by the Base Node. Thus, the switches are the elements appointed by the Base Node to make a layer 2 repetition of the signal. Figure 5: Power Line Communications (PLC) topology in a SS. In PRIME systems, the concept of repetition is just a specific state in the state machine of any network element. Generally, network elements are the service nodes inside electricity meters, but nothing prevents that some elements could exist, detached from the smart meters, and placed anywhere in the network (grid), whose basic function would just be to repeat (retransmit) PLC signal in difficult parts of the network. One clear example of such an element is the Auxiliary Node used in multi-transformer SSs. These Auxiliary Nodes are specifically configured to act as \"switches\" to repeat PLC signal in poorly or non-PLC connected transformers or bus-bars inside a SS. The generalization of that sample device could lead to specialized devices placed in different parts of the grid to cover specific areas of it. These specific areas could be classified in two groups: the first one would be areas where SNR is low due to the presence of a high noise level; the second one would be areas where SNR is low due to the reduced level of the signal received from either the Base Node or any natural switch that could be created. The value of the specialized repeater function is based on virtually \"shortening\" PLC links, placing an intermediate repetition point to help the PLC signal reach the desired end-point. Thus, SNR in both links (_i.e._, the two segments configured with the repetition) would be improved by the increase of the signal level at the target end-point, and simultaneously increasing signal power and decreasing noise level at the link intermediate point. Reference [119] offers the results of such an experience, focusing on the results of an experiment in a real LV grid, in a grid location showing an abnormal noisy condition. Two repeater configurations are tested, one with single-phase injection and another one with three-phase injection. In both cases, availability results are improved (from 16.2% to 37.8% with a single-phase repetition device, and to 47.9% with a three-phase device), demonstrating the feasibility of this procedure. Apart from the pure functional feasibility, the more important aspect to consider in this strategy is the applicability of the concept on different real LV grids. The implementation of this strategy depends on their nature of the LV grid, as it needs grid accessible points that could be used if they present suitable characteristics to host this kind of repetition devices. In North-American style grids, LV is usually laid on poles, and the fitting of accessories for signal repetition would be straightforward, as the cables are easily accessible. However, in European style grids, with an extensive combination of overhead (on poles and on walls) and underground cables, the latter may pose a challenge due to the difficulty to access the cables needed to connect these devices. Fortunately, in Europe there are locations such as street cabinets and fuse boxes (e.g., Germany and UK for the former case, and Spain for the latter; all of them with suitable enclosures) where access to cables is granted, and that are located at a certain distance from noise sources (generally the customer premises) to produce the kind of configuration tested in [119].