Specifications

35Continuous Access and Data Replication Manager SAN Extensions Reference Guide
3
Wavelength Division
Multiplexing
Wavelength division multiplexing (WDM) is an optical technology used to add
connection capacity over an existing fiber optic network. It works by combining
and transmitting multiple optical signals simultaneously at different wavelengths
down a single fiber. In effect, one fiber is transformed into multiple virtual fibers.
This allows WDM to multiply the effective bandwidth capacity of the optical
fiber.
For example, by multiplexing eight 1-Gbps signals into a single fiber, the
data-carrying capacity of that fiber increases from 1 Gbps to 8 Gbps in aggregate.
The maximum data-carrying capacity that can be designed into a WDM system is
strongly dependent on the spacing between the wavelengths being used. For fused
silica fiber, the range of usable wavelengths lies within two spectral windows that
are each approximately 30 nm wide. One is nominally centered around 1300 nm
and the other around 1550 nm.
The spectral windows occur within the infrared spectrum where the optical
scattering and absorption losses associated with fused silica fiber are minimal and
transmission is maximized. Fiber optic systems in general are currently designed
to operate within these two transmission windows. WDM systems typically
combine multiple channels within either window where channel separations are
on the order of the window width divided by the number of channels. The
following is an example:
(30 nm window width) / (8 channels) = 3.75-nm channel
spacing
The most demanding of WDM configurations combine up to 128 channels within
a single transmission window. When adjacent wavelengths are separated in the
fiber by less than 1 nm it is considered dense-WDM (DWDM). When
wavelengths in the fiber are separated by more than 1 nm it is considered
coarse-WDM (CWDM).