ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 119)

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Variable field specific to this delimiter, which contains the priority level of the

PLC station (indicated by the CAP parameter).

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FCS, which uses a 16-bit CRC for the frame integrity check. The FCS is calcu-

lated both on the frame header and body. The techniques used in FCS are usu-

ally defined in the main standards on frame transport over a link.

MAC Layer Frames

The MAC (medium access control) layer frames, situated just above the physical

layer, allow a link with the layers of the upper levels.

As indicated before, the PLC technology can be viewed as a MAC encapsulation

since MPDU frames are encapsulated into long PPDUs. Likewise, all data coming

from layers above the MAC layer is encapsulated into the MAC frame.

MAC HomePlug 1.0 Frames

In the case of HomePlug 1.0, the encapsulation of the IEEE 802.3 or MPDU (MAC

protocol data unit) frame is included in the frame body of the PLC frame between

the start and end delimiters.

The Ethernet HomePlug 1.0 frames can be easily identified on an Ethernet net-

work since, for all of them, the hexadecimal 0x887b value is indicated in the MAC

ETHERTYPE frame type field. This parameter is used to create applications at the

data link layer level dedicated to HomePlug PLC technologies. In the case of

HomePlug AV, the value of the ETHERTYPE field is 0x88e1.

In addition to the 72-bit encryption check, the data body is encrypted with the

NEK (network encryption key) exchanged between the various PLC stations of the

network.

The MPDU form what is called a service block (BS). If the BS exceeds the limit

size of the MAC frames (1,500 bytes), the BS is fragmented into segments sent in

sequence by the source stations. The MPDU are then subjected to a fragmenta-

tion-reassembly sequence during the transmission and receipt by the various PLC

stations of the network.

Each segment of a MPDU is numbered and sequenced to be reassembled by the

destination station.

MAC Header Format

The MAC frame begins with a rather complex header containing three fields of total

length 17 bytes as illustrated in Figure 5.16.

Block Check Field

The first field of the header comprises 40 bits subdivided into eight subfields. The

purpose of this field is to convey check information that the MAC layer requires.

Figure 5.16 illustrates the frame check field and its division into subfields. The

purpose of the various subfields is the following:

100 Frames

1 2 ... 114 115 116 117 118 119 120 121 122 123 124 ... 348 349

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 119