ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 63)

If CW and DC reach their maximum value defined by the HomePlug 1.0 stan-

dard, these values are maintained, even if the BPC is decremented.

As explained above, when the algorithm is used, the stations of the same net-

work have the same probability of accessing the medium. The only drawback with

this algorithm is that it doesn’t guarantee a minimum time. Therefore, it is difficult

to use within real-time applications such as voice or video.

TDMA and Medium Access in HomePlug AV

Since the CSMA/CA algorithm does not guarantee a minimum transmission time, the

HomePlug AV standard, a HomePlug 1.0 extension, implements an allocation of transmis-

sion time slots based on the TDMA (time division multiple access) medium access system.

This medium access system is used for a deterministic allocation of the transmission

times for each station. This allocation is managed by the CCo device, which coordinates

the various network stations’ access to the medium.

Figure 3.8 illustrates the time division of the time spaces in the TDMA multiplexing

technique. We notice that the time base of a transmitted frame is divided into TDMA blocks

corresponding to time spaces dedicated to communications between two stations. During

the TDMA1 block, for example, only stations 1 and 2 communicate between themselves.

This ensures the time organization of the communication over the PLC network.

Therefore, HomePlug AV specifies determined time periods corresponding to two peri-

ods of the 220 V/50 Hz electrical signal synchronized on signal zero crossings. These TDMA

time areas are divided into several determined and fixed time allocations. One of the time

allocations is reserved for CSMA/CA frames and frame exchanges complying with the

HomePlug 1.0 and HomePlug AV standards.

44 Functionality

Figure 3.8 Time division of TDMA time spaces for a PLC frame

1 2 ... 58 59 60 61 62 63 64 65 66 67 68 ... 348 349

Sin comentarios

ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 63