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Wavelength Division Multiplexing An
Alternative To Leased Fiber Lines
Fiber-optic systems are now the key solution to linking
high-speed LAN and WAN networks within and among buildings.
The number of high-speed networking applications using
fiber as the physical-layer backbone continues to grow,
such as:
- Fiber Distributed Data Interface (FDDI), running
at 100Mbps
- Synchronous Optical Network (SONET), running at
155Mbps and 622Mbps
- ESCON, running at 200Mbps
- Gigabit Ethernet, running at 1,000Mbps
- Fibre Channel, running at 1,062Mbps and below
- High-Performance Parallel Interface (HIPPI), running
at 1,200Mbps.
Optical fiber provides the backbone speed and distance
capability to support all these applications and more.
The Problem With Fiber
Mainframe systems capable of supporting ESCON are some
of the most fiber-intensive applications in wide-scale
deployment today. These enterprise systems generally
run on multimode 62.5/125 micron (µm, .001 millimeter)
fiber for connecting devices within 3 km or on single-mode
9/125 µm for distances of up to 20 km. Although these
fibers easily handle the 200Mbps data streams, each
channel or device interface requires dedicated use of
fiber connections. For applications such as on-line
remote transaction processing and disaster recovery
- which often run at full speed 24 hours per day, seven
days per week - the fibers are dedicated exclusively.
The net result is that an ESCON network requires from
a few pairs to a few hundred pairs of fibers among facilities
to deliver the necessary performance and connectivity.
The cost and availability of this fiber are key issues
in planning connectivity between facilities. Mainframe
applications continue to grow. Transaction processing
is expanding beyond current system capabilities, requiring
more devices and more connections. The new role of the
mainframe as an Internet server also has a dramatic
effect on device and fiber demand.
Wave Of The Future
Wavelength Division Multiplexing (WDM) is a new option
for adding applications or expanding existing applications
over currently available fiber links. WDM offers a cost
avoidance in that existing fiber can support multiple
applications over the same link without any performance
penalty. In addition, WDM technology requires fewer
links to support the performance needs of a new application.
Multiplexing techniques have a long history and are
now widely used in communications. Frequency Division
Multiplexing (FDM) is the standard technique used in
analog transmission systems, such as cable television.
Each communications channel uses a separate frequency.
Time Division Multiplexing (TDM) is standard for digital
transmission systems. Data communications and long-haul
telecommunications systems use TDM to combine multiple
low-speed channels in specific time slots of a higher-speed
channel.
A much newer technology, WDM is already a critical
tool for many high-speed communications systems. WDM
works on the same principles as FDM and TDM, except
the channel discriminator is wavelength instead of frequency
or time. Since light waves of different lengths do not
interfere with each other, multiple wavelength signals
can be transmitted through the same optical fiber without
error. By allowing multiple high-speed communications
applications to share the same fiber simultaneously,
WDM unlocks optical fiber's tremendous bandwidth capability:
more than one terabit per second.
The telecommunications industry is investing heavily
in WDM technologies. Communications service providers,
such as AT&T, MCI and US Sprint, are running out
of bandwidth capacity and are looking for more cost-effective,
time-sensitive alternatives to installing more dedicated
fiber lines. Industry standards are now being created
around high-speed WDM systems. Although these systems
present excellent capacity expansion alternatives for
central-office telecommunications, they don't meet the
speed and cost requirements of the data communications
industry.
New optical multiplexers employ WDM to increase existing
fiber capacity for data communications environments.
The WDM equipment converts each input data stream into
separate wavelengths (colors) and simultaneously transmits
these channels through the same optical fiber. Since
each wavelength is completely isolated from the others,
creating a discrete channel, and since the WDM unit
never processes the data, protocols can be mixed within
the same link. Essentially the unit creates "virtual
fibers" from one fiber. As a result, existing fiber
can be leveraged to add new applications within a metropolitan
area.
Many critical components combine to produce
this performance.
Remote wavelength conversion: The converter board houses
the critical elements to change the local channel into
the remote wavelength necessary for the long-distance
WDM signal. Each converter board receives the local
device signal into an electrooptic receiver. This signal
is directly forwarded to drive the remote single-mode
laser, which is specific for each channel. This process
is similar to conventional multimode-to-single-mode
conversion, except the single-mode laser is wavelength
specific. This example is a four-channel system - remote
channel 1 operates at 1300 nanometers (nm), while channels
2 through n are widely spread wavelengths around the
1550 nm band. The remote link can support distances
up to 16 km and greater. Where a single-mode long-distance
device is already running over a single-mode remote
fiber, removing the channel 1 converter allows the addition
of three new applications onto the existing fiber.
Passive WDM: The heart of the unit, the WDM passively
(no power required) combines and separates the specific
wavelengths on one remote fiber. The WDM uses an interference
film technology developed more than 20 years ago by
the optics and photography industry for reflective and
antireflective lenses. Now dramatically refined, the
technology allows wavelength-selective reflection through
roughly 100 layers of the film with nanometer precision.
The WDM combiner uses the interference technology with
fused optical fiber couplers to combine all wavelengths
onto one remote "transmit" fiber. The WDM splitter is
a duplicate device that operates in the opposite direction.
It receives the combined wavelengths on the single remote
"receive" fiber and splits them onto separate fibers.
Local signal regeneration: Once the passive WDM element
separates the remote signals, each signal is routed
back to the converter board, which reconverts it to
the original 1300 nm wavelength. The output power to
the local devices is between -14 dBm (decibel relative
to 1 milliwatt) and -21 dBm and matches the protocol
input on the other side of the WDM link.
Practical Benefits
Compared with dedicated fiber alternatives, WDM technology
offers many key benefits, including:
- Leveraging of existing fiber capacity
- Lower cost
- Elimination of long-distance single-mode converters
- Faster access to new channels
- Protocol independence.
Leverage: WDM can leverage existing fiber to provide
new fully operational channels immediately. For example,
a four-channel WDM system can create three new application
paths for every fiber pair. The economic advantages
for distances beyond 4 km are significant. The ability
to leverage fibers also benefits private fiber installations
on a campus. Although high-fiber-count cables can be
less expensive to install in short-distance runs (less
than 2 km) than WDM equipment, the long lead time for
fiber cable and installation crews can exceed several
months.
Lower cost: WDM provides a more economical solution
for high-speed data communications applications. Its
cost advantages come from two main points:
1) WDM equipment is often less expensive than private
cable and leased-line alternatives for distances longer
than 2 km.
2) WDM equipment provides a granular or incremental
growth solution for adding new applications among facilities.
WDM equipment is added as needed, as opposed to the
installation of additional private cable, which requires
a substantial up-front investment.
The cost of installing dedicated cable varies significantly
by location, accessibility, right of way and total length.
Regardless of these variables, the cost to add fiber
capacity always includes a large initial investment.
This investment must be justified on the basis of immediate
applications and longer-term projections of application
growth to amortize the cost of the new cable over all
these applications. However, a WDM solution is a more
granular choice for adding capacity, since it can be
justified by only a few applications and capacity can
be increased at any time. WDM thus incurs added capacity
costs only as needed and eliminates the need for guesswork
in future growth projections.
Most WAN applications do not provide the option of
installing private fiber cable. Instead, a private fiber
service can be leased on a per-pair basis. Although
a much more granular solution than the large initial
capital outlay of purchased and installed cable, leased
fiber services are generally expensive; the cost can
vary from $100 to $1,000 per fiber per mile per month,
depending on region, availability, distance and number
of fiber pairs needed. Service providers often demand
a 10-year to 20-year commitment for leased fiber, limiting
the flexibility for running a business (for example,
data center relocation). WDM minimizes the impact of
such commitment because fewer fibers are required and
the equipment is re-deployable at any place and time.
No long-distance converters: WDM technology uses single-mode
remote lasers to create the separate wavelengths for
the WDM system. This feature incorporates a multimode-to-single-mode
conversion process required to interface the local input
(usually multimode) with most long-distance fiber communications
systems, saving the space and cost of converters or
long-distance laser cards.
Faster access to new channels: As the base of installed
fibers fills up and most service providers move toward
specialization, dedicated fiber is becoming harder to
obtain in most metropolitan areas. Even when fiber can
be procured, it often takes four to 12 months to have
complete point-to-point service connected.
Protocol independence: WDM systems create completely
independent, fully transparent paths over each fiber.
This allows the combination of multiple application
protocols over the same fiber without any issues of
latency, speed, proprietorship, software setup, etc.
A multi-channel WDM link will behave as multiple "virtual"
fiber pairs, letting users mix and reconfigure protocols
as needed.
Summary
WDM systems present a new alternative for network connectivity
in the enterprise. They offer cost advantages, flexibility
and quick response to application growth. Business managers
can show reduced costs and improved investment returns.
Data communications managers have the flexibility to
add a variety of applications and reconfigure devices
as needed immediately, with no penalty on performance.
Capacity planners can be more accurate and ensure availability
of resources when needed. In new LAN and WAN applications,
WDM systems are an excellent choice for network connectivity.
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Wave Division Multiplexers