ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Pobierz pdf (Strona 127)

Telephony over PLC

The bit rate is not a problem in itself to convey telephone speech, since it can be as

low as 5.6 Kbit/s and that such a value is supported by PLC networks to a large

extent.

On the contrary, since telephony application is interactive, more than 300 ms

must not elapse between the moment when the information is sent by a user and the

moment when it is received by the recipient. If this is a symmetrical network, the

maximum round-trip time must therefore not exceed 300 ms. This is the maximum

permitted value for an application with human interaction.

Synchronization represents the second constraint when conveying telephone

speech. The information must be available to the receiver at precise times. In partic-

ular, the bytes originating from the digitization must be delivered at fully deter-

mined synchronization times. For example, if the compression generates a 8-Kbit/s

flow, this involves synchronizing every microsecond. Therefore, a byte must be

delivered to the receiver on each microsecond. If speech is not compressed, a

64-Kbit/s channel is synchronized every 125 μs.

The third main characteristic of PLC telephony is the use of the VoIP (voice over

IP) technique. The speech bytes are routed in IP packets and use the same network

resources as the packets routing other applications. Therefore, telephony over PLC

is integrated into the conventional framework of speech over IP.

Figure 6.1 illustrates the synchronization constraint at the remote telephone

level. Although the packets are regularly transmitted by the sender, they are received

at irregular intervals; because of this, delivering speech bytes to the receiver at pre-

cise times is rather difficult. This irregularity on receipt is due to the crossing of the

PLC network, which makes the speech packets arrive at random times.

108 Applications

Figure 6.1 Telephone communication constraints

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ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Strona 127