ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 21)

(intruder alarm, fire detection, gas leak detection, and so forth). Much less power

needed to be injected, since the power was reduced to levels of approximately a hun-

dred milliwatts.

The expression “power line carriers,” usually abbreviated to PLC, appeared at

the end of World War II in 1945. By that time, many telephone and electrical lines

had been destroyed and there were more infrastructure electrical lines than tele-

phone lines. For communication purposes, systems were designed for data transmis-

sion over high or medium voltage wiring by imitating remote meter readings already

carried out on the electrical lines.

Figure 1.1 illustrates the changes in the PLC technologies classified by speed

since the beginning of the 1990s.

Standard Organizations

The various standardization bodies as well as the concepts of standards and specifi-

cations which we will clarify are presented in this section.

The word “standard” covers several types of documents.

There is quite a difference between a norm and a standard, although, in English,

most of the people use the same word: standard.

A “norm” is a document from an international body, such as the ISO (Interna-

tional Standardization Organization). It is sometimes called “standard de jure.” In

the following pages, we will call them “standard.”

2 Introduction

High

Passport

Figure 1.1 Low and high speed PLC technologies

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 21