ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 59)

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Two types of mechanisms are used in the VCS:

•

detection of fields at the beginning of the frame;

•

wait for response information provided by the frame control fields.

Figure 3.6 illustrates these two medium listening techniques before the data

frames are transmitted over the electrical network.

Access to the Medium

The access to the medium is controlled using a mechanism called IFS (interframe

spacing). This spacing corresponds to the time interval between the transmission of

two frames. In fact, the IFS intervals are idle periods on the transmission medium

used to manage medium accesses for the stations and to establish a priority system

during a transmission.

The values of the various IFSs depend on the physical layer implementation.

Three types of IFS are defined by the HomePlug 1.0 standard:

•

CIFS (contention distributed interframe spacing). The CIFS is used by stations

wishing access to the medium when it is free, leading to the end of other trans-

missions during 35.84 μs. The CIFS is followed by the priority solving phase

for each station.

40 Functionality

Figure 3.6 Listening to the medium in HomePlug 1.0

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 59

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