Cisco 3825 Hardware Installation Guide - Page 192

ATM OC-3 Network Modules Chapter 11 Connecting ATM Network Modules Table 11-10 Link Loss Causes and Amounts Cause Amount of Loss Fiber attenuation SM: 0.5 dB/km MM: 1 dB/km Splice 0.5 dB Connector 0.5 dB Modal and chromatic dispersion Depends on fiber and wavelength1 Higher-order mode losses 0.5 dB Clock recovery 1 dB 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. Estimating the Power Margin The following example calculates a multimode power margin based on these values: • Power budget 10 dB (SONET worst-case specification for multimode fiber) • Link length 3 km • Four connectors • Three splices • Higher-order loss (HOL) • Clock recovery The power margin is: PM = PB - LL = 10 dB - [3 km x (1.0 dB/km) + 4 x (0.5 dB) + 3 x (0.5 dB) + 0.5 dB + 1 dB] = 2 dB The positive result means this link should have enough power for transmission. The product of bandwidth and distance is 155 MHz x 3 km = 465 MHz-km; this is within the dispersion limit of 500 MHz-km. Single-Mode Transmission Single-mode transmission is useful for longer distances, because there is a single transmission path within the fiber and modal dispersion does not occur. The maximum receive power for SML is -10 dBm, and the maximum transmit power is 0 dBm. The SML receiver can 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. The SMI receiver cannot be overloaded by the SMI transmitter and does not require a minimum fiber cable length or loss. 11-16 Cisco Network Modules Hardware Installation Guide OL-2485-20