ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 75)

Fragmentation Reassembly

In a PLC transmission using a shared medium disturbed by other uses with a wired

Ethernet link using a cable dedicated to data communications, the error rate for the

electrical wiring is higher (10

–5

for the electrical wiring against 10

–9

for the Ethernet

cable).

The PLC link may be subjected to various constraints, such as attenuation due to

interference, multipath over the electrical wiring, or electrical wiring crosstalk

effects. These constraints result in the attenuation of the signal power, which no lon-

ger makes it possible for the PLC link to deliver data correctly.

A high error rate results in the retransmission of all the erroneous data sent over

the network. This retransmission entails a high cost in terms of use of the band-

width, especially when the size of the data sent is high.

To avoid wasting the bandwidth to a large extent, a fragmentation mechanism is

used; it reduces the number of retransmissions in environments with a high noise

level like PLC.

Fragmentation

The data frames of the network layers (IP, and so forth) or of upper layers are seen

by the data link layer as successive MPDUs (Mac Protocol Data Unit) forming SB

(service blocks). The SB are then sliced into segments with 1,500 bytes maximum.

Therefore, the size of a segment can be 1,500 bytes or less. In the latter case, it is

filled with padding bits in order to obtain a MPDU with a fixed size. The 1,500 byte

size corresponds to 160 OFDM symbols for the physical layer.

Each of the segments forming the SB is numbered for its recognition; this makes

it possible to reconstitute the SB sent by the source station (source address at MAC

level) to the destination station (destination address).

Figure 3.24 illustrates the various segments sent by the source station and num-

bered for their identification by the destination station. As we shall see with the

ARQ functionalities of the MAC layer, if one of the SB segments is not received by

56 Functionality

Figure 3.23 MAC encapsulation in HomePlug 1.0

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 75