ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 191)

for data at these levels, which is calculated according to the overhead used for man-

aging and sending the transmission.

As we saw in Chapter 5, the data sent over this electrical interface corresponds

to a physical frame, or PLCP-PDU. This frame consists of a PLCP header comprised

of two fields and data originating from the MAC layer. As illustrated in Figure 8.17,

each part of the PLCP-PDU is sent at different speeds.

The PCLP-PDU header includes start and end delimiters. These headers are

transmitted at a speed of 1 Mbit/s in the case of the long preamble.

The second PLCP-PDU field corresponds to the MAC frame itself. This frame is

sent at rates that can be as high as 1 to 4.5, 9, or 14 Mbit/s as far as HomePlug 1.0 is

concerned. The PLC uses its data rate variation mechanism to transmit at different

rates according to the characteristics of the electrical environment.

The transfer time, which is equal to the propagation time increased by the trans-

mission time, must be known to calculate the level 2 useful throughput. Since the

electrical interface is used as the transmission medium, we can consider that the

propagation time is equal to zero, as the electron moving speed over an electrical

wiring is equivalent to the speed of light. The transmission time (Tt) therefore corre-

sponds to the time required for data sending.

By definition, the level 2 useful throughput (Du) corresponds to the volume of

transmitted payload divided by the overall transmission time, i.e.:

Data

Let us consider a HomePlug 1.0 network whose frames use a short preamble and

in which the transmission speed is 14 Mbit/s for all the stations. We are going to cal-

culate the useful throughput (Du

) of a PLCP-PDU when sending 1,500-byte data.

Since the payload size is known, the transmission time, which is equivalent to the

sum of the PLCP-PDU header transmission time and of the MAC data transmission

time, is still to be calculated.

The MAC frame data comprise a 34-byte header. Therefore, their size is 1,534

bytes. Their transmission time (Tt

MAC

) is given by the following formula:

172 Installation

Figure 8.17 Structure of a PLCP-PDU

1 2 ... 186 187 188 189 190 191 192 193 194 195 196 ... 348 349

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 191