Specifications

Wavelength Division Multiplexing
37Continuous Access and Data Replication Manager SAN Extensions Reference Guide
WDM product variations
WDM vendors currently offer at least three variations of the products described in
this guide. WDM system architectures can be delineated in the most general sense
as being:
■ Passive (optically and with respect to transmission protocol)
■ Active with respect to signal amplification
■ Active with respect to protocol handling
Most WDM products on the market today fit uniquely into one of these categories,
or have been designed to combine attributes of each.
Passive systems
These WDM systems are transparent to transmission protocol and data rate. As
such, they establish open interfaces that give operators the flexibility to transport
Fibre Channel, SONET/SDH, asynchronous, ATM, Frame Relay, and other
protocols over the same fiber. A truly passive optical system also passes the
optical signal without any form of signal conditioning such as amplification or
attenuation (beyond the levels inherent to the system components).
Active signal amplification
Certain WDM products are offered with line amplifiers and attenuators. These
features are included primarily to facilitate interfacing via fiber optic links to other
telecommunications hardware. Line amplifiers allow the boosting of weak signals
received from peripheral network components, as well as boosting the signals
being transmitted that might otherwise fall below the threshold sensitivities of
receiving equipment.
Similarly, incoming and outgoing signals can be attenuated if they are sensed as
being above receiver saturation levels. These active systems typically monitor
power levels to ensure that operation is maintained within the power budget of the
hardware. Power monitoring capability is usually accomplished with
hardware/software control loops, which can add significantly to the cost of the
product.