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Network configuration

3.2 Configuring optical networks

PROFIBUS Network Manual

50 System Manual, Edition 04/2009, C79000-G8976-C124-03

● With glass optical fibers, the number of splices (including repair splices)

● The length of the optical fiber (cable length)

● The link power margin on the link (for example for aging and temperature dependency of

the LEDs and photodiodes).

3.2.3 Optical power budget of a fiberoptic transmission system

Optical power budget

The transmitted optical power Pout and the received optical power Prec are specified in

dBm, the attenuation caused by connectors and the fiber is specified in dB.

dBm is a reference unit and describes the logarithmic ratio of the power level to the

reference power P0 = 1mW.

The following formula applies:

Px [in dBm] = 10*log(Px [in mW] / P0)

Table 3- 7 Examples

Transmit power Px Transmit power as logarithmic

power ratio Px to Po

10 mW + 10 dBm

1 mW 0 dBm

1 μW - 30 dBm

Transmitter

Depending on the fiber being used, the minimum and maximum optical power that can be

coupled into a fiber is specified. This power is reduced by the attenuation of the connected

transmission path resulting from the fiber itself (length, absorption, scattering, wavelength)

and the connectors used.

Receiver

The receiver is characterized by its optical sensitivity and its dynamic range. When

configuring an optical link, you should make sure that the power reaching the receiver does

not exceed its dynamic range. If the power falls below the minimum, this increases the bit

error rate (BER) due to the signaltonoise ratio of the receiver. If the maximum received

power is exceeded, saturation and overload effects increase pulse distortion and therefore

also increase the bit error rate.

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