Juniper Networks M5 - Manual (Page 71)

Network Cable Specifications and Guidelines
The following example uses the estimated values in Table 12 to calculate link loss (LL) for a
2 km-long multimode link with a power budget (PB) of 13 dB:
•
Fiber attenuation for 2 km @ 1.0 dB/km= 2 dB
•
Loss for five connectors @ 0.5 dB per connector = 5(0.5 dB) = 2.5 dB
•
Loss for two splices @ 0.5 dB per splice =2(0.5 dB) = 1 dB
•
Higher-order loss = 0.5 dB
•
Clock recovery module = 1 dB
The power margin (PM) is calculated as follows:
PM = PB – LL
PM = 13 dB – 2 km (1.0 dB/km) – 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB [HOL] – 1 dB [CRM]
PM = 13 dB – 2 dB – 2.5 dB – 1 dB – 0.5 dB – 1 dB
PM = 6 dB
The following sample calculation for an 8 km-long single-mode link with a power budget (PB)
of 13 dB uses the estimated values from Table 12 to calculate link loss (LL) as the sum of
fiber attenuation (8 km @ 0.5 dB/km, or 4 dB) and loss for seven connectors (0.5 dB per
connector, or 3.5 dB). The power margin (PM) is calculated as follows:
PM = PB – LL
PM = 13 dB – 8 km (0.5 dB/km) – 7 (0.5 dB)
PM = 13 dB – 4 dB – 3.5 dB
PM = 5.5 dB
In both examples, the calculated power margin is greater than zero, indicating that the link
has sufficient power for transmission and does not exceed the maximum receiver input
power.
Attenuate to Prevent Saturation at SONET/SDH PICs
SONET/SDH interfaces in the different reach classes—short reach (SR), intermediate reach
(IR), and long reach (LR)—generate different output power levels and tolerate different input
power levels. Interfaces that have a longer reach can transmit enough power to saturate
the receivers on PICs that have a shorter reach. Specifically, LR interfaces can saturate IR
PICs, and both IR and LR interfaces can saturate SR PICs. Interfaces in the same reach class
can also potentially saturate one another.
To prevent saturation, you might need to attenuate power at the PIC receiver, particularly if
you know that it has a shorter reach than the interface that is sending the signal. Determine
the amount of attenuation needed by measuring the power level at each receiver. Attenuate
Prepare for Router Installation 51