ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 70)

lish the specifications of the HomePlug AV (for Audio and Video) version which is

much more efficient for the management of the quality of service (QoS).

Figure 3.17 illustrates the organization of the beacon frames in HomePlug AV.

This standard, based on a master-slave architecture, uses CSMA and TDMA

medium access functionalities. CSMA is preferred for data traffic with a medium or

no priority level and TDMA for data traffic with priority, for which the QoS is

important (real-time data flow, like in VoIP, or high data flows, like in VoD).

The QoS management is obtained by means of a very efficient technique specific

to PLC technologies that consists of synchronizing TDMA beacon frames on the 50

or 60 Hz signal of the electrical network. This fully deterministic signal is synchro-

nized over the entire public electrical network and private electrical networks.

Therefore, the PLC devices can be synchronized without a specific clock by using 50

or 60 Hz signal zero crossings.

This technique makes it possible to obtain the efficient determinisms that criti-

cal data communications require. The master of the PLC network manages the allo-

cations for access to the TDMA slots between the slave devices of the network

according to their requirements.

Managing Frame Priorities

The frame priority for medium access is managed by the CAP (channel access prior-

ity) field and the size of the contention window (CW), as illustrated in Figure 3.18.

The CAP variable affects the medium access as we have seen in Figure 3.7,

where the CW and DC parameters are set by the back-off procedure and given by

the correspondence table according to the respective CAP values of the network

PLC devices.

Transmission Channel Functionalities 51

Figure 3.17 Synchronization of HomePlug AV beacon frames on the 50 Hz signal

1 2 ... 65 66 67 68 69 70 71 72 73 74 75 ... 348 349

Žádné komentáře

ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 70