ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 38)

also be expressed in terms of electrical flux (φ) and associated with the electrical

potential between the two surfaces of the dipole:

(expressed in coulombs)

In the case of a sinusoidal voltage (as is the case for household electricity), this

equation is expressed efficiently using Ohm’s law, as a function of the voltage (U),

current (I) and frequency (f):

2π

(expressed in farads)

The impedance (Z) of an electrical circuit is composed of resistive (R), inductive

(L), and capacitive (C) components. Together, they completely characterize the

impedance from an electrical point of view.

These characteristics have an influence on the overall behavior of the electrical

network as a function of the electrical current flow in the network. From a com-

puter science point of view, these characteristics compel a particular modeling of

the physical layer in order to obtain the best quality possible from the transmission

channel.

Using Ohm’s law, the impedance may be expressed in complex values as the

sum of resistive, inductive, and capacitive components, where j expresses the imagi-

nary part of a complex value:

ZRjLf

=+ +2

(expressed in ohms for the absolute value)

As the current passes through multiple electrical circuits, their combined imped-

ances form a complex network of impedances in series and parallel. Sections may be

connected or disconnected at any time. Also, the various impedances induce mutual

magnetic and electrical fields that create electrical currents based on their relative

proportions. From the point of view of a transmission channel, this property can

have surprising implications, as we will see.

Since the inductive and capacitive characteristics constantly modify the physi-

cal transmission channel, PLC transmission techniques must be optimized and

consolidated.

Electromagnetic Noise and Perturbations

The transmission channel acquires a certain noise level from the various electrical

devices connected to the electrical wiring or in its proximity.

The different types of noise that can be identified on and around the wiring are:

•

impulse noise from stops and starts of electrical devices;

•

broadband white noise, whose power spectral density is the same at all fre-

quencies;

•

periodic noise at multiple frequencies;

Architecture of Electrical Networks 19

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 38