ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Pobierz pdf (Strona 71)

The CAP variable is used by a PLC station to inform the other stations of its

medium access priority. This variable determines the values of the PRP1 and PRP2

priority frame data that is read by the network PLC stations to determine the various

priority levels. Therefore, the other stations are informed in advance of the priority

of each of the PLC devices.

This entire process, called VCS (virtual carrier sense), is used in conjunction

with PCS (physical carrier sense) during medium access attempts.

Managing Frequency Channels (Tone Map)

As we have seen before, there are several OFDM symbol modulation techniques

according to the quality of the PLC links between the devices. Unlike Wi-Fi, where

each station can configure the frequency channel over which it wishes to transmit

data, in PLC, the entire frequency band is used.

Figure 3.19 illustrates a simple network with four PLC stations. Each of them

assesses the quality of the PLC link connecting it to the other stations. It then stores

this information in the correspondence table of a register of the PLC device. This

tone map. Each station regularly updates the tone map table; the updating time may

vary from 10 ms to several seconds according to the PLC station parameterization.

It may happen that some stations see each other at the PLC level whereas other

stations do not see each other. It is important that the stations used as a gateway to

other networks see all the stations involved by the other network. For example, in

the case of Figure 3.19, PLC1 cannot have access to the Internet since, though the

links to PLC2 and PLC3 are correct, the transmission channel cannot be used

towards PLC4. If the electrical wiring is too long or if the electrical network is too

disturbed, this results in an attenuation and a degradation of the PLC signal making

upper layer data communications impossible.

Figure 3.20 illustrates all the fields of the start delimiter. In the variable field, the

first five bits are used by the tone map table. This tone map is used to store the status

of the links toward fifteen other PLC stations. This determines the limit relating to

52 Functionality

Figure 3.18 Frame priority management by the CAP (channel access priority) variable

1 2 ... 66 67 68 69 70 71 72 73 74 75 76 ... 348 349

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ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Strona 71