Troubleshooting guide

8-12
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 8 Troubleshooting the Cisco Catalyst 6509
Troubleshooting Serial Lines
The difference between the power budget and the link loss (LL) is called the power margin (PM). If the
power margin is zero or positive, the link should work. If it is negative, the signal may not arrive with
enough power to operate the receiver.
Power loss over a fiber-optic link arises from the following causes:
• Attenuation caused by passive components (cables, cable splices, and connectors) is common to
both multimode and single-mode transmission.Attenuation is significantly lower for optical fiber
than for other media.
• The signal spreads in time because of differing speeds of the different wavelengths of light
(chromatic dispersion).
• In multimode fiber, the signal spreads in time because of the different propagation modes (modal
dispersion).
• Higher-order mode loss (HOL) results from light radiated into the fiber cladding.
• Clock recovery at the receiver consumes a small amount of power.
The power lost over the data link is the sum of all these losses. Table 8-3 gives an estimate of the amount
of loss attributable to each cause.
Note These are typical values; refer to the manufacturer for actual values.
Managing Receiver Overload
The maximum receive power for SML is –10 dBm, and the maximum transmit power is 0 dBm. The
SML receiver can therefore be overloaded when using short lengths of fiber. Overloading the receiver
does not damage it, but can cause unreliable operation. To prevent overloading an SML receiver, insert
a minimum 10-dB attenuator on the link between any SML transmitter and the receiver.
Note For the gigabit Ethernet line card, PR
max
is greater than or equal to PT
max
, so an attenuator is unnecessary.
Table 8-3 Link Loss Causes and Values
Cause Amount of Loss
Fiber attenuation 0.5 dB/km (SM, 1 dB/km (MM)
Splice 0.5 dB
Connector 0.5 dB
Modal and chromatic dispersion Depends on fiber and wavelength
1
1. Dispersion is usually negligible for single-mode fiber. For multimode
fiber, the product of bandwidth and distance should be less than 500
MHz-km.
Higher-order mode losses 0.5 dB
Clock recovery 1 dB