ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Pobierz pdf (Strona 77)

This is achieved with the dynamic adaptation of the data rate at the physical

level according to the quality of the PLC links.

Optimum use of the global bandwidth can also be made by sending the data

only to the PLC devices involved. These functionalities correspond to those found in

other network technologies, such as Wi-Fi.

Dynamic Adaptation of the Bit Rate

As indicated before, the PLC technology permanently readjusts the condition of the

links between network stations.

Since the PLC links depend on the medium condition and interference with the

other electrical devices connected to the network or inductive, the transmission

speed must be permanently readjusted by choosing the modulation mode for OFDM

symbols forming the frames.

For the user, the useful bit rate between the terminals connected to the PLC net-

work dynamically varies according to the PLC links.

Table 3.5 lists the various transmission speeds or PHY bit rate of the PLC

devices of the HomePlug 1.0 standard according to the tone map determined for

each station with regard to the other stations of the network.

Unicast, Broadcast, and Multicast

Insofar as the PLC can be seen as MAC encapsulation techniques, the various modes

for MAC frame sending, whether these are unicast, broadcast, or multicast modes,

are authorized.

In the unicast mode, a network station transmits data to a single other station

using its MAC address. In broadcast mode, on the contrary, a station transmits its

data to all the stations of the network using a MAC address dedicated to this mode

and with all the bits to 1. In multicast mode, a station transmits to a group of other

network stations using a single MAC address for the entire station group. For this

purpose, the station group with the associated MAC addresses must have been pre-

defined. A prefix is used by the multicast MAC addresses for their recognition on the

network. This prefix uses the first twenty-four bits (out of 48) of the MAC address.

As we’ll see in Chapter 5, the broadcast and multicast modes are supported

using the multicast flag (on one bit) of the block control field of the MPDU data

frame.

58 Functionality

Table 3.5 Dynamic Bit Rates of the HomePlug 1.0 Standard

MODULATION

TECHNIQUE

ENCODER

PARAMETER

FEC (CODING RATE OF

CONVOLUTIONAL CODE

PHY BIT RATE (Mbit/s)

DQPSK 23/39 to 238/254 ¾ 14.1

139 possible

PHY bit rates

between 0.9 and

DQPSK 23/39 to 238/254 ½ 9.1

DBPSK 23/39 to 238/254 ¾ 4.5

ROBO (DBPSK) 31/39 to 43/51 ½ 0.9

1 2 ... 72 73 74 75 76 77 78 79 80 81 82 ... 348 349

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