ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 133)

several tens of seconds if this is necessary. In this case, once the streaming applica-

tion is started, the first image only appears at the end of this latency.

Visioconferencing and Videoconferencing

Visioconferencing and videoconferencing are applications with human interactivity,

which requires a 150-ms latency. As explained previously, the data

resynchronization process must be observed to reconstitute the isochronous applica-

tion to the receiver. For this purpose, a quality of service must be associated with

transporting these applications.

The difference between the two application categories comes from the quality of

the broadcast image.

In visioconferencing, the image can be black and white and jerky due to the

number of images per second being lower than normal. A low resolution screen can

be used by this application in order to reduce the bit rate. These characteristics only

require a transport capacity lower than 100 kilobits per second.

Videoconferencing requires a much higher bit rate (several megabits per second)

to obtain an image quality comparable to that of television. To obtain cinema qual-

ity, about 50 megabits per second must be achieved, which cannot be envisaged

within the framework of current HomePlug networks but can become possible with

the HomePlug AV generation.

The main difficulty for these two applications is to control synchronizations to

replay images in time. The same two techniques used for telephone speech can be

implemented to carry out this synchronization priority management at the IP level

or HomePlug frame level. The second solution makes it possible to assign a priority

on the random access of the MAC layer and grant shorter timers to stations with pri-

ority. In other words, the priority stations take precedence over the other ones as

long as they have frames to transmit. The only condition to be observed is that the

total bit rates of the priority stations remain lower than the value of the useful

throughput available.

In HomePlug 1.0 PLC networks, broadcasting good quality videoconferencing

cannot be envisaged easily. With HomePlug AV extensions to 200 Mbit/s (nominal

transfer rate), if the number of clients is reduced, it then becomes possible to trans-

mit over one or two good quality videoconferencing channels although the probabil-

ity of desynchronization quickly increases with the traffic.

Multimedia

Multimedia applications generally use at least one speech or video stream superim-

posed onto other data streams. These applications do not pose more problems for

PLC networks than telephone voice or video. The only additional constraint that

they bring about comes from the synchronization of the simultaneous applications

that achieve the multimedia process.

A compromise must be achieved between the complexity and the transit time in

the networks to convey multimedia applications. To find again the quality of the

original signal for digital documents, we consider that the compression must be lim

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 133