M160 Internet Router Hardware Guide
PM=P
B–LL
PM=13dB–2km(1.0dB/km)–5(0.5dB)–2(0.5dB)–0.5dB[HOL]–1dB[CRM]
PM=13dB–2dB–2.5dB–1dB–0.5dB–1dB
PM=6dB
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 18 to
calculate link loss (LL) as the sum of fiber attenuation (8 km @ 0.5 dB/km,
or 4 dB) and loss for sevenconnectors (0.5 dB per connector, or 3.5 dB).
The power margin (PM) is calculated as follows:
PM=P
B–LL
PM=13dB–8km(0.5dB/km)–7(0.5dB)
PM=13dB–4dB–3.5dB
PM=5.5dB
In both examples, the calculatedpower margin is greater than zero,
indicating that the link has sufficient powerfor transmission and does
not exceed the maximum receiver input power.
Attenuating 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 differentoutput power levels and
tolerate differentinput power levels. Interfaces that have a longer reach can
transmit enough powerto 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 reachclass can also saturate one another.
To prevent saturation, you might need to attenuate power at the PIC receiver,
particularly if youknow that it has a shorter reach than the interface thatis
sending the signal. Determine the amount of attenuation needed by measuring
the power level at eachreceiver. Attenuate the power to bring it within the
allowable range; for short lengths of fiber, with fiber and connector los sc lose
tozero,anattenuatorof5to10dBshouldbesufficient.
Forspecificationsofminimumandmaximuminputlevel(receiver
sensitivity and receiver saturation)and minimum and maximum output
level (averagelaunch power) for the SONET/SDH PICs supported on the
M160 router, seethe M160 Internet Router PIC Guide.
Routing Engine Interface Cable and Wire Specifications
For management and serviceoperations, you connect the Routing Engine to
an external console or managementnetwork through ports on the Connector
74 Routing Engine Interface Cable and Wire Specifications