ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 128)

The access method used to obtain the right to transmit to the access point, the

CSMA/CA (carrier sense multiple access/collision avoidance), makes the PLC net-

work crossing time random. In addition, to reach the recipient, the packets must

cross wider networks and go via intermediate transfer nodes that are also crossed

randomly.

Speech Packetization and Depacketization

Let’s suppose that speech is compressed to 8 Kbit/s, which is the most usual stan-

dard in telephony over IP environments.

The telephony bytes must be packetized into an IP packet which is itself encap-

sulated into an Ethernet frame or, to be more precise, into a PLC frame for transmis-

sion over the electrical network.

The synchronization takes place on each microsecond at a speed of 8 Kbit/s. If n

represents the number of bytes that can be used in a PLC frame, the filling time is n ´

1 ms. Since the minimum length of the PLC frame is 64 bytes, the packetization

requires 64 ms.

Depacketization does not actually require additional time since it is carried out

at the same time as packetization. Therefore, the packetization-depacketization

time is equal to at least 64 ms. In fact, the tendency is to add the packetization and

depacketization time to take account of the latency found in most

packetizers-depacketizers.

It is acceptable for this 64-ms time to remain below the 150-ms outgoing path.

However, this 64-ms value may prove to be too high if the packet has to go across

networks other than the PLC network or if the packetizers-depacketizers are much

too slow. This is the reason why speech packets are only filled with 16 byte speech

and the remainder is completed by padding bytes to achieve the minimum frame

size. The packetization-depacketization time can still be on the order of 16 ms with

these 16 bytes.

Actual Rate

The actual rate over the network is in fact much higher than 8 Kbit/s since the packet con-

tains a lot of additional information like headers and padding bytes. It is considered that

the actual rate over a PLC network or any other packet transfer network is around 60 to 70

Kbit/s using the IPv4 standard and after encapsulation into an Ethernet frame.

If the IPv6 standard is used, the supervision fields are even bigger and we consider that

a speech channel exceeds 100 Kbit/s.

The time that the coder-decoder (codec) requires to digitize the signal from an ana-

log signal or vice versa can be estimated to be around 5 ms. Therefore, 26 ms are

obtained for coding, decoding, and packetization-depacketization. The total allowable

transport time therefore becomes 124 ms (maximum transport time of 150 ms, as indi-

cated at the beginning of this section, minus 26 ms for the various times). The technique

for MAC access to the PLC network is included in this transport time.

Voice, Video, and Multimedia 109

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 128