Westermo ODW-720-F2 - Manual (Page 31)

31 6651-2235
Calculating system processing delay
Data exchange between commuinication devives via an ODW-720 fibre optic link,
will be delayed due to the length of the optical fibre and the signal processing within the
ODW-720. The signal processing delay is dependent on the data rate, and the fibre delay
is dependent on the total length of the optical fibre. The additional time resulting from
the optical fibre and ODW-720 is the Overall system delay.
Delay @ < 1.Mbit/s
Optical fibre length delay (typical) 5 µs/km
Signal processing, electrical to fibre (max) 1 tbit + 1 µs
Signal processing, fibre to electrical (max) 0.3 µs
Signal processing, fibre to fibre (max)
1.06 µs
Note: tbit = 1 / Baud rate (Baud rate in bit/s)
Example: Calculate the maximum processing delay in a network comprising of 10 com-
munication devices and 10 ODW-720 units, using a data rate of 115.2 kbit/s, with a total
fibre length of 40 km. A data exchange between the two communication devices located
at either end of the network.
1. Fibre: The total optical fibre length delay.
40 x 5 µs = 200 µs
2. Optical repeaters: The optical repeater delay x Number of optical repeaters
(excluding the ODW-720 units connected to the communication devices that are
exchanging data).
(10 – 2) x 1.06 µs = 8.48 µs.
3. Converter electrical to fibre: Signal processing delay inside the ODW-720 that is
sending data.
1 tbit + 1 µs = 1/115200 + 1 µs = 9.68 µs
4. Converter fibre to electrical: Signal processing delay inside the ODW-720 that is
receiving data.
0.3 µs
5. The system delay is calculated by summing the delays in item 1 to 4 above
and multiplying the result by two:
2 x (200 µs + 8.48 µs + 9.68 µs + 0.3 µs) = 437 µs