ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Pobierz pdf (Strona 192)

MAC

bytes bit byte

14 Mbit/s

≈

1534 8

0000876

The 120-bit PLCP-PDU header is sent at a rate of 1 Mbit/s. Therefore, its trans-

mission time (Tt

PLCP-PDU

) is:

Tt s s s s

PLCP PDU−

=++≈72 15 72 1455μμμ μ..

The total transmission time (Tt

) is therefore equivalent to:

Tt Tt Tt

00010215=+ ≈

−MAC PLCP PDU

The useful throughput is equivalent to the volume of transmitted information,

i.e., 1,500 bytes (12,000 bits) divided by the transmission time, i.e., 1.021 ms,

which corresponds to 11.74 Mbits/s:

1500 8

1174=

≈

bytes bit byte

Mbit / s

However, this data rate does not correspond to the reality. In the PLC, the send-

ing of data must comply with some rules related to the CSMA/CA (carrier sense

multiple access/collision avoidance) access method. This method is based on certain

mechanisms detailed in Chapter 3 that generate a rather high overhead.

In the ideal case where a single station transmits over the medium, when the

station transmits data, it listens to the medium. If the medium is free, it defers its

transmission while it waits for a CIFS time. When the CIFS times out, and if the

medium is still free, it transmits its data. Once the data transmission is completed,

the station waits for an RIFS time to know whether its data have been acknowl-

edged. As illustrated in Figure 8.18, the minimum overhead generated by the trans-

missions of the CIFS and RIFS timers of the ACK and of the headers is far from

being negligible.

Network Data Rates 173

Figure 8.18 Minimum overhead when transmitting data

1 2 ... 187 188 189 190 191 192 193 194 195 196 197 ... 348 349

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ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Strona 192