Installation and Reference Guide HP AdvanceStack 100Base-THub-12TX, Hub-12TXM, and Switch Modules 1997-06

HP Hub-12TX/Hub-12TXM Installation and Reference Guide

Installing the System 2-9

Distance Limit and Power Loss in Fiber Optics - When using fiber

optic cabling, the maximum length between two hubs can be up

to 2 kilometers. However, you must consider power loss when

calculating the actual length of cable that can be used with your

system. You can calculate power loss with the following formula:

m =

(p dB - i dB)

c dB/km

Variable Description

cable length (kilometers)

p dB power bud

i dB intervening devices

(e.g., patch cables and splices)

c dB/km

loss per kilometer of cable

Table 2.2 Calculating the Power Budget for Fiber Optic Devices

Note: To determine the power loss incurred by intervening devices and

specific cable type, inquire with the manufacturer. The power budget

depends on the gauge of cable as shown below.

Gauge of Fiber Cable

Power Budget

50/125 m

9.2 dB

62.5/125 m

13 dB

Table 2.3 Power Budget for Common Fiber Optic Devices

For a sample calculation, assume the following values:

• cable gauge - 62.5/125 mm, which means a 13 dB power budget,

• 2 patch panels along the path, each with 1.5 dB power loss,

• 1 splice with 1 dB power loss, and

• inherent power loss in the cable is 4 dB/km

The maximum cable length is therefore:

m =

13 dB - 4 dB

2.25 km

4 dB/km

Note: Even though your calculations for power loss may indicate a longer

permissible length based on signal strength (as seen in the preceding

example), we advise remaining within the recommended limits.