ZyXEL Communications PLA-470 V2 - V3.0.5 Przewodnik Instalacji Pobierz pdf (Strona 194)

The transmission time (Tt3) becomes:

Tt T T Tt Tt

+++ + ++

Wait

CIFS

Backoff Data ACK

RIFS

Since the waiting time and the back-off timer are not fixed, it is difficult to

determine their values. However, we can consider that the sum of the waiting time

and back-off time is generally equivalent to the transmission time in the ideal case.

The back-off timer can be considered as zero compared with the waiting time. As

for the waiting time, it corresponds to the transmission time of another station.

Therefore, the useful throughput is equivalent to:

Tt T T Tt

Data Data

Wait Backoff

and is formulated as:

≈≈

Data

When the network consists of two stations, the useful throughput of each sta-

tion is almost equal to the maximum useful throughput divided by the number of

stations forming the network. This formula can be generalized for a PLC network

consisting of n stations transmitting at the same speed.

The useful throughput for each station is equivalent to:

≈

In addition, only the level 2 useful throughput was taken into account in our

preceding calculations. However, the MAC frame data correspond to an LLC frame

with a 4-byte header containing an IP packet with a 20-byte header. The IP packet

itself includes a TCP segment with a 24-byte header containing user data. There-

fore, we have a total of 48 additional overhead bytes. Data processing for the upper

layers (layers 3 and 4), which also generates overhead, was not taken into account.

To conclude, we can say that a PLC network never reaches the claimed capacity

on the physical medium. If data is transmitted at the speed of 14 Mbit/s, the number

of data bits for the user only represents approximately half of the raw capacity of

the electrical interface, i.e., 5 Mbit/s (625 Kb/s) in our example on average.

Table 8.9 summarizes the useful throughputs of various types of local area net-

works.

Compared with the transmission speed over the medium, the useful throughput

is much higher in Ethernet than in PLC.

Maximum PLC Actual Data Rate

After calculating PLC level 2 useful throughputs in the preceding section, we’ll go to

an upper level. For this purpose, we’ll use the Iperf traffic generator available at the

following address: http://dast.nlanr.net/Projects/Iperf/.

Network Data Rates 175

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