ZyXEL Communications PLA-470 V2 - V3.0.5 Guia de Instalação descarregar pdf (Página 132)

Necessary Rates for Video Routing

The video devices mainly use the most recent MPEG standards. DVB (digital video

broadcasting) is also widely used.

MPEG uses inter- and intraframe compression algorithms. The rate can be as

low as 1.5 Mbit/s for television quality with very few losses in comparison with the

original image. New developments improved the image quality with bit rates for

MPEG-2 of around 4 Mbit/s. An even higher compression can be envisaged with the

MPEG-4 standard by including, where applicable, the elements that are necessary

for reconstructing the image at the other end.

The difficulty with broadcast television resides in the fact that the bit rate is very

variable over time and must adapt itself to the transport network. The algorithms

more or less compress the information according to the time and resources available

on the medium. If the network is almost fully available, the image quality can be

highly improved. If, on the contrary, it is congested by miscellaneous information

coming from various sources, a degradation of the video transmission must be

envisaged if the quality of service demanded by the user allows this. A control mech-

anism is essential to fully optimize the application transfer.

High-definition digital television (HDDTV) requires a bit rate of around 5 to 10

Mbit/s according to the quality demanded by the user. This 5 Mbit/s rate is almost

too big to be supported by the HomePlug 1.0 and Turbo networks. With HomePlug

AV (40 Mbit/s), only two users have access to the service.

However, HDDTV broadcasting over PLC networks is now available but is

restricted to a maximum of ten users.

Capacity Problems

A PLC network must be capable of providing connections enabling a video applica-

tion to use the optimum bit rate while allowing it to maintain an acceptable quality

of service.

Let’s first examine the difficulties raised by capacity. For telephone speech,

there is no problem since, once compressed, the stream is 8 Kbits/s, even 5.6 Kbits/s.

On the contrary, for video, the capacity required for a MPEG-2 television quality

image varies between 2 and 8 Mbits/s. With the MPEG-4 generation, it goes down

to 1 Mbit/s. In any case, it is currently 2 Mbits/s. These values can drop to some

hundreds of kilobits per second by reducing the video quality.

If the bit rate of a HomePlug 1.0 network proves insufficient to broadcast good

quality video, the bit rate of a HomePlug Turbo or HomePlug AV network should

suffice. Since the useful bit rate is around 10 Mbit/s and 40 Mbit/s for these two

technologies, having a rough estimate of its own stream and of the stream of other

applications on the network is enough not to exceed these values.

Giving streaming flows a higher priority level is possible by using the same pri-

ority techniques as in the transfer of the telephone speech. In this case, there are no

longer bit rate problems by using the HomePlug Turbo and AV networks.

If the capacity is adequate, i.e., if the number of users is small enough with

respect to the capacity required, or if a priority scheme is implemented, the second

problem to solve concerns compliance with the latency for byte resynchronization.

This is the reason why the latency is generally of the order of several seconds, even

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