ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 193

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We are going to calculate the useful throughput associated with this ideal case
(Du
2
). As in the example above, we consider the use of short preambles for
1,500-byte data transmitted at a speed of 14 Mbit/s.
According to our preceding calculations, the data transmission time corre-
sponds to Tt
1
, i.e.:
Tt s
Data
bytes bit byte
14 Mbit/s
=
×
+≈
1534 8
1455 000167
,
..μ 0s
Since the duration of the ACK frame is 72 μs, its transmission time is equal to:
Tt s s s
ACK
=+ =72 1455 00002175μμ μ..
CIFS and RIFS are fixed value timers. However, this value varies from one tech-
nology to another. For HomePlug 1.0, the value is 35.84 μs for CIFS and 26 µs for
RIFS.
Therefore, the overall transmission time is equal to:
Tt Tt Tt
2
0001949=+ ++ CIFS RIFS s
ACKData
.
In our ideal case, the useful throughput is therefore equal to:
Du
Tt
2
2
1500 8
6157=
×
,
.
bytes bit byte
Mbit/s
We notice that the higher the overhead, the lower the useful throughput. Since a
single station transmits over the medium, this data rate corresponds to the maxi-
mum useful throughput.
Everything gets more complicated when the network consists of more than two
stations that simultaneously attempt transmissions over the medium. When a sta-
tion hears that the medium is busy after trying to get access to the medium or after
waiting for a CIFS, it defers its transmission. For this purpose, it triggers a timer cal-
culated using the back-off algorithm.
The additional waiting time and the random back-off timer obviously increase
the overhead as illustrated by Figure 8.19.
174 Installation
Figure 8.19 Maximal overhead when transmitting data
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