ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 46)

signal and not on the data frames. Therefore, this type of repeater does not

reduce the bandwidth of the overall PLC network.

•

“Logical” PLC repeaters repeat the signal at the level of the data frames. This

type of repeater is composed of two PLC devices connected by their Ethernet

interface. The first device is connected to one segment of the electrical net-

work and the second device is connected to another segment that is inaccessi-

ble to the PLC signal due to excessive attenuation. This type of repeater

reduces the bandwidth of the overall PLC network by a factor of two because

it creates two distinct logical networks on the same physical electrical net-

work.

Layered Architecture

The OSI (open systems interconnection) layered model provides a common base for

the description of any data network. This model is composed of seven layers, each

describing an independent protocol that furnishes a service to the layer above it and

requests services from the layer below it.

In the context of this model, PLC networks correspond to layers 1 (physical)

and 2 (data link), supplying an Ethernet connection service to the layers above.

Figure 2.9 illustrates the position of PLC technologies in the OSI model. Layer 1

(physical) is materialized by the electrical wiring that carries the PLC signal. The

PLC equipment provides a terminal (typically a PC) with an Ethernet connection

service corresponding to layer 2 (data link), using a MAC protocol and RJ-45 con-

nectors. The terminal uses PLC network services to access services in higher layers

(IP, TCP, HTTP, and so forth).

The Physical Layer

The physical layer of PLC technologies is materialized by electrical wiring and,

more generally, by electrical networks. In order to transport the PLC signal on this

medium, the line frequency (for example, 110 V/60 Hz) of the electrical circuit is

supplemented by a modulated signal of low amplitude around a center frequency

(carrier frequency) F.

The physical layer therefore consists of this low amplitude modulated signal,

transported on electrical wiring at a frequency determined by the PLC technology

employed and the applicable regulations. We will go into detail on modulation tech-

niques in Chapter 3.

Figure 2.10 illustrates the sum of the PLC and power signals, which are super-

imposed on the electrical wiring, creating the physical layer of a PLC network.

Frequency Bands

The PLC signal is modulated in amplitude, frequency, or phase around a carrier fre-

quency F. National or international standards organizations have set down rules

that should be followed for the utilization of each frequency band, from zero to tens

of gigahertz.

Layered Architecture 27

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 46