Cabling Infrastructure
Expand fiber capacity with WDM
The technology provides flexibility to cost effectively increase the capacity of existing infrastructure.
by Steven Olen
Wave-division multiplexing
(WDM) technology allows independent and
simultaneous data streams to be transmitted
over a single fiber-optic connection using
different wavelengths, or colors of light.
These wavelengths are combined (multiplexed)
at the source end, and then separated
(demultiplexed) via color filters at the
destination end. The primary benefit is that
the bandwidth capacity on the fiber is
significantly increased, allowing for more
information to be sent over the existing
media.
Each color represents a specific CWDM wavelength. Two UTP gigabit ports from the gigabit switch at the corporate office are converted to fiber using media converters equipped with small form factor pluggables with two CWDM wavelengths.
There are two popular
implementations of WDM technology: dense
wavelength-division multiplexing (DWDM) and coarse
wavelength-division multiplexing (CWDM).
Both use multiple
wavelengths to carry independent signals.
They differ in terms of the spacing of the
wavelengths, the number of channels
supported and the cost.
DWDM has been widely
implemented in telecom long-haul optical
networking for many years. Its use in
enterprise and campus networks, however, is
typically cost prohibitive. Furthermore, the
large number of wavelengths offered is often
excessive and not required.
CWDM systems do not
require amplifiers, and they use
less-expensive, non-stabilized lasers in
conjunction with broadband filters to
provide a wider spacing that supports up to
18 wavelengths. CWDM costs are lower because
the lasers require less precision and
consume less power than their DWDM
counterparts. The maximum achievable
distance between nodes, however, is less
with CWDM.
When fiber infrastructure
is limited, network managers typically have
three options for implementing these new
applications. New fiber-optic cabling can be
installed for each new application, which
can be costly and time consuming. Protocol
converters can be used to converge the
different applications into time-division
multiplexing or Ethernet, but this can
require significant investment in equipment
and training. The third option is to use
CWDM technology.
FLEXIBILITY FOR EXPANSION
CWDM technology provides
the flexibility to cost effectively increase
the capacity of existing fiber
infrastructure. As a result, it eliminates
the costly options of laying new fiber or
installing expensive and complex equipment.
Bandwidth is increased because each
wavelength carries data independently from
one another, allowing the network to
securely mix speeds and protocols for
different applications and end-users.
The heart of a CWDM
network is a device called the CWDM
multiplexer (MUX). The MUX combines unique
wavelengths from different communications
sources onto a single fiber-optic line. At
the other end of the fiber line, another MUX
device is used to separate (de-MUX) the
individual wavelengths, and deliver them to
their destinations. CWDM MUXs are commonly
available in four- and eight-channel models.
An optical add/drop MUX
(OADM) can be used to add or drop a specific
wavelength along the route of the
fiber-optic line. This is useful in a large
campus network that operates over a single
shared fiber and needs to drop individual
links (wavelengths) at different locations.
In order to connect
communication devices into the MUX, the
optics from each device must be converted to
specific CWDM wavelengths. This can be
achieved with small form factor pluggable
(SFP) optical transceivers installed in the
communications equipment. SFP transceivers
are interchangeable fiber interfaces that
provide a cost-effective way of adapting
existing equipment to support the wide range
of wavelengths needed when implementing a
CWDM solution. The transceiver converts the
optical signal to the appropriate CWDM
wavelength, thereby providing each device
with a direct connection into the MUX.
Connecting SFP ports to
the CWDM MUX is straightforward. If the
communication device has a fixed fiber
connector with a standard wavelength, its
wavelength can be converted to a unique CWDM
wavelength using an inexpensive wavelength
media converter.
This holds true for
nearly all types of communications
equipment, including TDM devices, video
servers and serial controllers. Even if the
device has a metallic connector, it can be
directly converted to CWDM fiber by using a
copper-to-fiber media converter.
In one application of
CWDM (see illustration), a simple
point-to-point LAN network can be upgraded
to a multiprotocol, multidrop application.
The campus LAN has a single-mode, dual-fiber
link between two copper gigabit switches
using media converters. The fiber link
connects the corporate offices to the
manufacturing plant.
DOUBLE CAPACITY NEEDED
The existing gigabit
fiber is currently at capacity, and due to
company growth, the network manager now
needs to double the capacity to the
manufacturing plant. Additionally, about
halfway between these two buildings a new
call center has been added, which requires
DS-3 connectivity to a PBX system.
To support the new
requirements, the existing fiber is used to
create a CWDM network. Two unshielded twisted pair
(UTP) gigabit ports from the gigabit switch
at the corporate office are converted to
fiber using media converters equipped with
SFPs with two unique CWDM wavelengths. Both
media converters are installed in a
high-density rack in order to save space,
power and cost.
In addition, in order to
connect the DS-3 between the corporate
building and the new call center, its copper
connection (coax) is converted via another
media converter to another unique optical
wavelength. All three wavelengths then are
connected with fiber jumpers to a
four-channel CWDM MUX module mounted in the
same high-density rack with the media
converters. The CWDM MUX combines all the
wavelengths and sends them on a common CWDM
point-to-point fiber.
At the call center
building, an OADM is used to split off the
wavelength that is carrying the DS-3
service. A standalone media converter
changes the signal from optical back to its
native coax interface. The remaining
wavelengths continue to the manufacturing
plant. There, a standalone CWDM MUX
separates the wavelengths, and the fibers
for the wavelengths are connected to
SFP-enabled media converters that provide
the UTP connections to the Gigabit Ethernet
switch.
A number of factors
affect the design and selection of equipment
when deploying a CWDM-based network; overall
optical loss is perhaps the most important.
Many factors can result in optical signal
loss, including length and type of the
fiber, wavelengths used, number of
connectors, splices, patch panels and OADMs.
Detailed calculations should be performed
for each fiber-optic link to ensure the
proper optical devices are specified and
that the total loss does not exceed the
optical power budget.
Not all fiber is suitable
for use across the full CWDM spectrum, so
understanding the type of fiber available is
important, as well as its characteristics
before starting network design. Consider
relevant information about the length of the
fiber, attenuation characteristics and the
location of connectors and splices. If new
fiber must be installed, the ITU-T G.652D
standard should be considered for CWDM
network designs to provide the greatest
network flexibility for adding wavelengths.
Steven Olen is director of technical marketing for
Omnitron Systems, Irvine, Calif.
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