ZyXEL Communications PLA-470 V2 - V3.0.5 Guia de Instalação descarregar pdf (Página 134)

ited to a factor 3. This is the case of imaging applications, in which the quality is

essential, such as X-ray radiographies, for example. Factors varying from 10 to 50

for fixed images and from 50 to 200 for video are obtained. The compression aver-

age is 20 for fixed images and 100 for video.

These compressions distort the image very slightly but use the recovery capaci-

ties of the human eye. This is because the eye is much more sensitive to luminance,

i.e., image brightness, than to chrominance or color. This characteristic is found

again in the coding of the high definition television, where the luminance resolution

is based on an image definition of 720 by 480 points, whereas a definition of 360 by

240 points is used by the chrominance signal. The luminance requires more coding

bits per point than the chrominance.

We have seen that the PLC networks can support the necessary bit rates to

transmit the streams of multimedia applications. For this purpose, the number of

clients with access to an electrical network just needs to be limited (see Chapter 3).

Therefore, the problem resides less in the network capacity than in the manage-

ment of time constraints. The two constraints (real time and synchronization) are

very difficult to achieve with asynchronous networks such as PLC networks, in

which there is no time management and where data is not transported in a determin-

ist way (see Chapter 3).

In this respect, HomePlug AV is essential for transporting multimedia applica-

tions, since it is the only one that can classify packets according to priorities in order

to obtain the quality of service necessary for the applications transported by each

stream.

Quality of Service

As we have seen in Chapter 3, no quality of service is proposed by HomePlug 1.0

and Turbo in their technology, since the data transfer times are not determinist. The

quality of service must be implemented by the application layers above the MAC

layer to compensate for this nondeterminism.

An implementation of the quality of service is proposed by HomePlug AV with

a guarantee for the various services requiring a bit rate and a stable data transfer

time. This quality of service is provided by the allocation of TDMA timeslots for

each type of data service.

Table 6.1 gives examples of subscriber premises PLC networks according to uti-

lization scenarios (lone couple, couple with three young children, and couple with a

young child and two teenagers).

PLC Local Networks

The use of PLCs to build a local area computer network is the most visible and wide-

spread among the general public and professionals. Families are keen to equip

themselves with several personal computers to share a number of applications and

access to the Internet, whereas professional environments exchange occupational

and Internet applications.

PLC Local Networks 115

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