ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 68)

SACK Response in HomePlug AV

In the AV version of the HomePlug standard, an additional response, the SACK (Selective

ACK) response, has been added to compensate for the fact that the PLC links between two

stations are not necessarily symmetrical in terms of useful throughput. Due to the charac-

teristics of the electrical network, the data transmissions are not under the same influences

in one direction as in the other one. The SACK response is used by the central device of the

PLC network, the CCo, to manage global links, i.e., the various links between PLC stations

of the network, and the transmission time allocations within the framework of the TDMA

medium access technique.

Synchronization and Frame Controls

The frames are checked using the FCS field that is included in the data block of the

frame. This field is used by the destination station to resend the suitable response

type (ACK, NACK, or FAIL) to the source station.

The source station then checks the integrity of this response using the RFCS

field of the response frame, as illustrated in Figure 3.15.

The response frame is sent by the destination station after an interframe period

of 26 μs minimum and 1,695 μs maximum (see Figure 3.16). Since the defined size

of the response frame is much shorter than the data frames, it is much more likely to

be transmitted and does not occupy much of the total bandwidth.

Transmission Channel Functionalities 49

Figure 3.14 FAIL response in HomePlug 1.0

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 68