Juniper SRX3400, Руководство по оборудованию, Страница: 213 / 232

Table 36: Estimated Values for Factors That Cause Link Loss
Estimated Link-Loss Value Link-Loss Factor
Single-mode—None
Multimode—0.5 dB
Higher-order mode losses
Single-mode—None
Multimode—None, if product of bandwidth and distance
is less than 500 MHz–km
Modal and chromatic dispersion
0.5 dB Connector
0.5 dB Splice
Single-mode—0.5 dB/km
Multimode—1 dB/km
Fiber attenuation
The following example uses the estimated values in Table 36 on page 199 to calculate
link loss ( LL ) for a 2 km-long multimode link with a power budget ( P
B
) 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 ( P
M
) is calculated as follows:
P
M
= P
B
– LL
P
M
= 13 dB – 2 km (1.0 dB/km) – 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB [HOL] – 1 dB [CRM]
P
M
= 13 dB – 2 dB – 2.5 dB – 1 dB – 0.5 dB – 1 dB
P
M
= 6 dB
The following sample calculation for an 8 km-long single-mode link with a power budget
(
P
B
) of 13 dB uses the estimated values from Table 36 on page 199 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 ( P
M
) is calculated as
follows:
P
M
= P
B
– LL
P
M
= 13 dB – 8 km (0.5 dB/km) – 7 (0.5 dB)
P
M
= 13 dB – 4 dB – 3.5 dB
199 Copyright © 2013, Juniper Networks, Inc.
Appendix D: Cable and Wire Guidelines for the SRX3400 Services Gateway Hardware