Cisco NM-1A-OC3MM-EP= Datasheet download pdf (Page 16)

Connecting Cisco ATM Network Modules to the Network

ATM OC-3 Network Modules

Connecting Cisco ATM Network Modules to the Network

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.

The following example of a single-mode power margin assumes these values:

• Power budget 16 dB (SONET worst-case specification for SMI)

• Two buildings 8 kilometers apart

• Connections through a patch panel in an intervening building with a total of 12 connectors

PM = PB – LL

= 16 dB – 8 km x (0.5 dB/km) – 12 x (0.5 dB)

= 6 dB

The positive value means this link should have enough power for transmission.

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.

Table 9 Link Loss Causes and Amounts

Cause Amount of Loss

1 ... 15 16 17 18

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Cisco NM-1A-OC3MM-EP= Datasheet Page 16