To Next Page

To Previous Page

Single-mode ber is so small in diameter that rays of light reect internally through one layer only.

Interfaces with single-mode opcs use lasers as light sources. Lasers generate a single wavelength of

light, which travels in a straight line through the single-mode ber. Compared to mulmode ber, single-

mode ber has a higher bandwidth and can carry signals for longer distances. It is consequently more

expensive.

For informaon about the maximum transmission distance and supported wavelength range for the

types of single-mode and mulmode ber-opc cables that are connected to the QFX Series, see the

Hardware Compability Tool. Exceeding the maximum transmission distances can result in signicant

signal loss, which causes unreliable transmission.

Aenuaon and Dispersion in Fiber-Opc Cable

An opcal data link funcons correctly provided that modulated light reaching the receiver has enough

power to be demodulated correctly.

Aenuaon

is the reducon in strength of the light signal during

transmission. Passive media components such as cables, cable splices, and connectors cause

aenuaon. Although aenuaon is signicantly lower for opcal ber than for other media, it sll

occurs in both mulmode and single-mode transmission. An ecient opcal data link must transmit

enough light to overcome aenuaon.

Dispersion

is the spreading of the signal over me. The following two types of dispersion can aect

signal transmission through an opcal data link:

• Chromac dispersion, which is the spreading of the signal over me caused by the dierent speeds

of light rays.

• Modal dispersion, which is the spreading of the signal over me caused by the dierent propagaon

modes in the ber.

For mulmode transmission, modal dispersion, rather than chromac dispersion or aenuaon, usually

limits the maximum bit rate and link length. For single-mode transmission, modal dispersion is not a

factor. However, at higher bit rates and over longer distances, chromac dispersion limits the maximum

link length.

An ecient opcal data link must have enough light to exceed the minimum power that the receiver

requires to operate within its specicaons. In addion, the total dispersion must be within the limits

specied for the type of link in the Telcordia Technologies document GR-253-CORE (Secon 4.3) and

Internaonal Telecommunicaons Union (ITU) document G.957.

When chromac dispersion is at the maximum allowed, its eect can be considered as a power penalty

in the power budget. The opcal power budget must allow for the sum of component aenuaon,

power penales (including those from dispersion), and a safety margin for unexpected losses.

Juniper QFX5200 User Manual

Questions and Answers: