ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 130)

nal to another one in the same PLC network. After going across the outgoing PLC

network, the stream of telephone packets is routed in a fixed IP network, which can

be an operator network, then goes via a dedicated gateway, PABX IP, before cross-

ing the conventional telephone infrastructure. PABX IP converts IP addresses into

telephone addresses and carries out the necessary code conversions of a compressed

stream to a 64-Kbit/s operator telephone stream.

The Asterisk software is typically used to create an IPBX (PABX), which man-

ages the local IP calls and the outgoing calls to the STN (switched telecommunica-

tions network) at the server level.

Hi-Fi Quality Telephony

PLCs are used to carry speech of much higher quality than the conventional tele-

phone voice. Indeed, since they do not have constraints on the bit rate, they can

absorb a high bandwidth likely to carry hi-fi or almost hi-fi quality.

Suppose you have a 512 Kbit/s speech compressed to 64 Kbit/s. To fill the 64

bytes with telephone data, only 8 ms are necessary. Globally, the rate of the IP

packet stream is the same as before but, failing it filling with padding bytes, it only

contains useful bytes. Therefore, speech with much higher quality can be trans-

ported at the same actual rate.

This technique is still not widely used since the telephone devices are not always

compatible with such quality. The compatibility could be found by using a micro-

computer with a sound board. Unfortunately, this solution does not prove to be

better, since the sound boards on the market are very slow and require a processing

time of about 50 milliseconds that, when two devices must be crossed (that of the

sender and of the receiver), makes the transit time unacceptable.

In any case this example shows that an interesting extension of telephony over

PLCs could be high quality telephony.

Voice, Video, and Multimedia 111

Figure 6.2 Devices crossed by a PLC digital speech stream

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 130