ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 131)

Video

Video is another application that should develop in the future in PLC networks. This

application especially requires a high rate that becomes accessible in PLC environ-

ments.

Depending on the video application type being considered, the time constraint is

more or less strong. The two main cases, streaming video and videoconferencing,

are examined below.

Streaming

With streaming without a return channel, like video on demand (VoD) and televi-

sion, between the time when the video stream is sent from the source and the time

when this video is played on the screen, a rather long time can elapse in the order of

several seconds up to about fifteen seconds. The viewer does not necessarily have the

feeling that the video source sends correctly before he or she views the images.

The single constraint to be observed for these applications is the waiting time at

the beginning of the video. It is rather irritating to have to wait while the application

initializes itself whenever the channel is changed due to resynchronization at the

receiver level. The purpose of streaming is to leave some advance for the packet

stream to reach the receiver and to have enough packets in memory in the receiver so

that there is no interruption in the packet delivery to the client. This constraint is

illustrated in Figure 6.3.

The video can come from an analog signal that is digitized then compressed, or

from a digital signal which is compressed already. It can be highly compressed or

may require a high rate; this depends on the network possibilities and the computing

power of the emitters and receivers.

The higher the bit rate and the lower the compression, the higher the image qual-

ity. This requirement concerning the rate is a major feature of the transmission of a

video image. This characteristic poses no particular problems for PLC networks as

long as the network is not saturated. Let’s first analyze the necessary rates for rout-

ing a video channel.

112 Applications

Figure 6.3 Streaming video application over a PLC network

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 131