This blog post is part of a series called “CommScope Definitions,”
in which we will explain common terms in communications network infrastructure.
Division Multiplexing (WDM) is a method of combining or separating multiple
wavelengths of light in or out of a single strand of fiber with each wavelength
of light carrying a different signal. The use of optical filters allows a
certain range of wavelengths and let another range of wavelengths pass through.
CommScope uses thin-film filter technology (TFF) to obtain this optical effect. Thin layers are stacked together. Consecutive
reflections on the interfaces between these layers create interference effects
that let light pass through for certain wavelengths and reflect others.
See Also: How Multiplexing Techniques
Enable Higher Speeds on Fiber Optic Cabling
WDM is a cost effective way to increase the capacity of a
network. There are two different types:
- Coarse Wave Division Multiplexing (CWDM) is standardized to have 18 different
wavelength channels with a spacing of 20 nanometers (nm) starting at 1270 nm
and ending at 1610 nm. Most systems use the eight channels in the upper band (eight
channels from 1470 nm to 1610 nm). The advantage of CWDM systems is that it is
always possible to upgrade at a later point in time to limit the installation
cost on day one. The wider channel
spacing places less stringent requirements on the lasers, which allows use of
less expensive lasers without temperature controllers.
- Dense Wave Division Multiplexing (DWDM) devices are mostly used in the core networks
to extend over very long distances and where more wavelengths are required
between sites. The 40 wavelength channels are distributed in the C-band from
1530 nm to 1570 nm. If required, DWDM can be “over-layed” on a CWDM
infrastructure to increase capacity.
Typically CWDM and DWDM have lower insertion losses compared to optical
splitters. This significantly increases the reach of the network from the
central office. Moreover, every customer has an assigned wavelength(s),
providing greater security and making eavesdropping difficult if not
There are a few different ways to
utilize WDMs in your network:
A mux (commonly known as a
multiplexer) combines multiple wavelength channels on a single fiber, and a
demux separates them again at the other end. A mux/demux set-up is especially
useful to increase the end-to-end
capacity of a deployed fiber. The mux is typically located in the central
office, and the demux unit located in either a cabinet or splice closure from
which point the fibers go to their destination in a star-shaped topology.
Instead of separating the
wavelengths at one end, it is also possible to add or drop individual
wavelengths at different points across the line while the other wavelengths
remain untouched. This is often the preferred choice when the topology is such
that the sites are grouped in a ring structure or when the distance between
them is long.
One fiber vs.
The upstream and downstream signals can be sent through the same fiber
at different wavelengths or on two separate fibers. Most CWDM systems are built
as two-fiber systems where one fiber is used for the upstream and the other
fiber is used for the downstream. In that case, each customer has two separate
fibers and one wavelength. If only one fiber is used, every customer will have
two separate wavelengths.
components can have a significant impact on the efficiency of communication
network roll-outs. The incorporation of passive WDM devices reduces the amount
of fibers in the network, decreasing both the footprint and investment cost of
network roll-outs. In existing networks, these components allow capacity
upgrades at a relatively low cost without additional construction works.