WDM is a fiber optic technology that multiplexes multiple optical carrier signals onto a single optical fiber by using different wavelengths of laser light. This enables bidirectional communications over one fiber and increases network capacity. A WDM system uses a multiplexer to combine signals and a demultiplexer to separate them. CWDM and DWDM are two common types of WDM systems that differ in channel spacing and reach. CWDM uses wider spacing of 20nm between 1470-1610nm wavelengths, has a shorter reach of 100km, and is more cost-effective. DWDM more densely spaces narrow wavelengths, can reach thousands of kilometers with amplification, and supports higher speeds.

1. Introduction Of The WDM System
In fiber-optic communications, wavelength-division multiplexing (WDM) is a
technology which multiplexes a number of optical carrier signals into a
single optical fiber by using different wavelengths of laser light. This technique
enables bidirectional communications over one strand of fiber, as well as
multiplication of capacity. A WDM system (Figure 1) uses a multiplexer at the
transmitter to join the signals together, and a demultiplexer at the receiver to
split them apart. With the right type of fiber it is possible to have a device that
does both simultaneously, and can function as an optical add-drop multiplexer.
The concept was first published in 1978, and by 1980 WDM systems were
being realized in the laboratory. As a system concept, the ways of WDM
includes coarse wavelength-division multiplexing (CWDM) and dense
wavelength-division multiplexing (DWDM).
Figure 1: The WDM system
The CWDM System

2. In simple terms, CWDM equipment performs two functions: segregating the
light to ensure only the desired combination of wavelengths are used,
multiplexing and demultiplexing the signal across a single fiber link.
Typically CWDM solutions provide 8 wavelengths capability, separated by
20nm, from 1470nm to 1610nm, enabling the transport of 8 client interfaces
over the same fiber, as is shown in Figure 2. What’s more, CWDM has the
capability to transport up to 16 channels (wavelengths) in the spectrum grid
from 1270nm to 1610nm with a 20nm channel spacing. Each channel can
operate at either 2.5, 4 or 10Gbit/s. CWDM can not be amplified as most of
the channels are outside the operating window of the erbium doped fiber
amplifier (EDFA) used in Dense Wavelength Division Multiplexing (DWDM)
systems. This results in a shorter overall system reach of approximately 100
kilometers. However, due to the broader channel spacing in CWDM, cheaper
un-cooled lasers are used, giving a cost advantage over DWDM systems.
Figure 2:The CWDM system
CWDM proves to be the initial entry point for many organizations due to its
lower cost. Each CWDM wavelength typically supports up to 2.5Gbps and can
be expanded to 10Gbps support. This transfer rate is sufficient to support GbE,

3. Fast Ethernet or 1/2/4/8/10GFC,
STM-1/STM-4/STM-16/OC3/OC12/OC48, as well as other protocols.
CWDM is the technology of choice for cost efficiently transporting large
amounts of data traffic in telecoms or enterprise networks. Optical networking
and especially the use of CWDM technology has proven to be the most cost
efficient way of addressing this requirement.
In CWDM applications, a fiber pair (separate transmit and receive) is typically
used to serve multiple users by assigning a specific wavelength to each
subscriber. The process begins at the head end (HE) or hub, or central office
(CO), where individual signals at discrete wavelengths are multiplexed, or
combined, onto one fiber for downstream transmission. The multiplexing
function is accomplished by means of a passive CWDM multiplexer (Mux)
module employing a sequence of wavelength-specific filters. The filters are
connected in series to combine the various specific wavelengths onto a single
fiber for transmission to the field. In the outside plant a CWDM demultiplexer
(Demux) module, essentially a mirror of the Mux, is employed to pull off each
specific wavelength from the feeder fiber for distribution to individual FTTX
applications.
CWDM is suitable for use in metropolitan applications, also being used in
cable television networks, where different wavelengths are used for the
downstream and upstream signals. In these systems, the wavelengths used
are often widely separated, for example, the downstream signal might be at

4. 1310 nm while the upstream signal is at 1550nm. CWDM can also be used in
conjunction with a fiber switch and network interface device to combine
multiple fiber lines from the switch over one fiber. CWDM is optimized for a
cost conscience budgets in mind, with low-cost, small-powered laser
transmitters enabling deployments to closely match guaranteed revenue
streams.
The DWDM System
DWDM stands for Dense Wavelength Division Multiplexing. Here “dense”
means the wavelength channels are very narrow and close to each other.
DWDM uses the same transmission window but with denser channel spacing.
Channel plans vary, but a typical system would use 40 channels at 100 GHz
spacing or 80 channels with 50 GHz spacing.
DWDM works by combining and transmitting multiple signals simultaneously at
different wavelengths on the same fiber, as is shown in Figure 3. In effect, one
fiber is transformed into multiple virtual fibers. So, if you were to multiplex
eight OC -48 signals into one fiber, you would increase the carrying capacity
of that fiber from 2.5 Gb/s to 20 Gb/s. Currently, because of DWDM, single
fibers have been able to transmit data at speeds up to 400Gb/s.

5. Figure 3: The DWDM system
A basic DWDM system contains five main components: a DWDM terminal
multiplexer, an intermediate line repeater, an optical add-drop multiplexer
(OADM), a DWDM terminal demultiplexer and an Optical Supervisory Channel
(OSC). A DWDM terminal multiplexer contains a wavelength-converting
transponder for each data signal, an optical multiplexer and an optical amplifier
(EDFA). An intermediate line repeater is placed approximately every
80–100 km to compensate for the loss of optical power as the signal travels
along the fiber. An optical add-drop multiplexer is a remote amplification site
that amplifies the multi-wavelength signal that may have traversed up to
140 km or more before reaching the remote site. A DWDM terminal
demultiplexer consisting of an optical demultiplexer and one or more
wavelength-converting transponders separates the multi-wavelength optical
signal back into individual data signals and outputs them on separate fibers for
client-layer systems (such as SONET/SDH). An Optical Supervisory Channel

6. (OSC) is a data channel which uses an additional wavelength usually outside
the EDFA amplification band (at 1,510nm, 1,620nm, 1,310nm or another
proprietary wavelength).
DWDM is designed for long-haul transmission where wavelengths are packed
tightly together and do not suffer the effects of dispersion and attenuation.
When boosted by erbium doped fiber amplifiers (EDFAs)—a sort of
performance enhancer for high-speed communications—these systems can
work over thousands of kilometers. DWDM is widely used for the 1550nm band
so as to leverage the capabilities of EDFA. EDFAs are commonly used for the
1525nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L Band).
A key advantage to DWDM is that it’s protocol and bit rate independence.
DWDM-based networks can transmit data in IP, ATM, SONET/SDH, and
Ethernet, and handle bit rates between 100Mb/s and 2.5Gb/s. Therefore,
DWDM-based networks can carry different types of traffic at different speeds
over an optical channel. From a QOS standpoint, DWDM-based networks
create a lower cost way to quickly respond to customers’ bandwidth demands
and protocol changes.
Conclusion
WDM, as a multiplexing technology in optical field, can form a optic-layer
network called “all-optic network”, which will be the most advanced level of
optical communications. It will be the future trend of optical communications to
build a optical network layer based on WDM and OXC to eliminate the

7. bottleneck of photoelectric conversion with a pure all-optic network. As the first
and most important step of all-optic network communications, the application
and practice of WDM is very advantageous to developing the all-optic network
and pushing forward optical communications!
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