ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 129)

Transit Time

In PLC, the waiting time to access the power line medium can be relatively long. If,

for example, five clients are connected to the same electrical network by using

1,500-byte frames and integrating access times related to CSMA/CA, a waiting

time on the order of 10 ms, or even more, is obtained. If the telephone speech is

supposedly intended for another employee of the same company connected to a

PLC network, about ten milliseconds for access to the network must be added again.

Altogether, the transit time remains around 100 ms assuming that the traffic is

relatively high but without collisions. This time makes it possible to transport tele-

phone speech under good conditions over a PLC network.

With the exception of the HomePlug AV standard and developments in progress

by competitors, because priorities are not managed by the current PLC generation,

the packets of other users circulate with the same priority, even if they convey data

that is not of immediate interest. For example, the packets of a client working under

a peer-to-peer (P2P) application and recovering a video file with several gigabytes

randomly circulate ahead of the packets a user on the phone. This is the reason why

drastically limiting the number of users or the global traffic is essential for the cur-

rent PLC generation. The next HomePlug AV PLC generation will be capable of

managing the priorities of telephony and video packets, thus ensuring the quality of

service over the data network.

If the number of users exceeds ten, or if the useful bit rate exceeds 5 Mbit/s,

transporting a telephone speech successfully using a Homeplug 1.0 PLC network

cannot be guaranteed, i.e., with the necessary quality of service. In this case,

another technique must be used to assign priorities to the packets carrying telephone

speech.

Differentiating IP Packets

Two solutions can be deployed in the short term to implement this differentiation

between packets passing through PLC:

•

A technique for IP packet control at the IP protocol level. In this case, the PLC

network manager slows down the incoming acknowledgments of nonpriority

packets delivered by the receiving stations in such a way that these streams are

maintained in a slow-start condition, in which the sending stations can only

send a few packets and must wait for acknowledgments.

•

Use the HomePlug AV standard, which was released in 2007. This standard

determines priorities at the MAC layer level. In this case, assigning the highest

level priority to the telephone terminals is enough.

The second solution is clearly the best one, as it can be applied at the lowest level

of the architecture and clearly favors telephone speech streams. The other solution is

more artificial, as it consists of restricting non-priority streams without estimating

the actual bandwidth requirements of clients having priority of the telephone speech

type.

Figure 6.2 illustrates the various components crossed when transporting tele

phone speech within a broader framework than a simple conversation from a termi

110 Applications

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