|The fact is that 10 Gigabit Ethernet
is on the drawing board and that 100 Gigabit Ethernet is on the
horizon. Events are moving fast in the world of fiber optic cabling.
Gigabit Ethernet has forced manufacturers to improve their modal
bandwidth at a wavelength of 850nm to 200 MHz-km in their multimode
cables in order to transmit up to 300 meters. This has now become
a de-facto minimum performance specification. The use of vertical
cavity surface emitting lasers (VCSELs), the renewed interest in
the higher modal bandwidth of 50/125 multimode cable and even higher
bandwidth singlemode cable is prompted these new higher speed options.
These are exciting times indeed.
In April of last year the EIA /TIA, after years of trying to
force fiber to fit the copper cabling mold, broke fiber out into
its own standard, 568-B.3. This new standard recognizes fiber's
uniqueness in network architectures, adds in new component designs,
such as the small form factor (SFF) connectors and high-bandwidth
fibers, including 50/125 fiber as an alternative to 62.5/125
Standards for the future
The Institute of Electrical and Electronics Engineers
(IEEE – New York City) standardized Gigabit Ethernet, the latest
high-speed solution to network overload, with the adoption of
IEEE 802.z in June 1998, marking a transition to laser-based networks.
Gigabit Ethernet responds to two fundamental requirements in fiber-to-the-desk
networking; industry-accepted standards and low costs. For example,
Gigabit Ethernet can operate with vertical-cavity surface-emitting
lasers (VCSELs), which cost about the same as light-emitting diodes
(LEDs) but offer superior performance and power. Because VCSELs
operate at the 850-nanometer wavelength, they are ideally suited
for multimode fiber; singlemode fiber is not designed to operate
at this wavelength. At the 1310-nm wavelength, singlemode fiber
represents the highest performance solution, but the necessary
electronics come at a significant system price premium.
Therefore, multimode fiber is the appropriate,
low-cost choice for Gigabit Ethernet systems. However, during
the development of the IEEE 802.3z standard, interaction between
lasers and mulitmode fibers produced unexpected bit-error rates
because lasers concentrate power in only a small percentage of
a fiber’s core, where even small centerline defects can dramatically
reduce laser performance, resulting in bandwidth-limiting effects.
As a result, the unanimously approved IEEE 802.3z standard would
not meet their objective of establishing a Gigabit Ethernet specification
that would allow for the transmission of 1.25-gigabit-per-second
signals over 300 meters of standard FDDI-grade fiber. The new
standard allows for 1.25 Gbit/sec transmission over distances
of 220 and 275 meters using the standard 160-and 200-MHz-km bandwidths,
respectively, in the 850-nm window.
Since many lengths within a LAN exceed both 220
and 275 meters, the use of 62.5-micron fiber at 850 nm in most
LAN applications, so prevalent in our market today, was in jeopardy.
Also, it became evident that some installations whose lengths
exceeded the draft standard might, should the installation prove
not to be a worst-case scenario, have problems if end-user equipment
was later upgraded.
Other pieces to the puzzle
Since the installed fiber base would
have to support other protocols as well, a brief examination of
other existing and potential applications must be considered.
The Asynchronous Transfer Mode (ATM) standard
of 622 megabits per second has been tested and is able to transmit
over standard 62.5-micron fiber a distance of 300 meters. However,
the next generation of the ATM standard will transmit at 2.5 Gbits/sec.
Current bandwidth specifications in the first window will limit
the transmission of these signals at 850 nm to possibly as short
a length as 100 meters.
Furthermore, the next-generation Fibre Channel
standard will call out a transmitting speed of 1.062 Gbits/sec,
potentially limiting link distance to less than 200 meters.
What is the correct fiber solution? As with
so many parts of the network, the best answer is: "It depends."
Relevant factors include:
- the distances involved in the network
- whether or not the fiber is installed as
- the current application
- the protocols the network will be required
to support in the future
In addition, the choice of fiber partly depends
on the type of fiber installed. Each fiber type has pros and
cons regarding its use in a network.
FDDI-grade, 62.5-micron fiber
If the distances are short (less than
200 meters), the bandwidth of this fiber will suffice for Gigabit
Ethernet and for most other near-term applications. The end-user,
therefore, does not need to change any designs or purchasing practices.
For longer runs, the installation of singlemode
fiber optic cables or a hybrid cable containing singlemode and
multimode fibers could be an answer. Singlemode fiber has an order
of magnitude more bandwidth capacity than multimode fiber, is
readily available, and is cost-effective. Also, this fiber type
is recognized by every standard as a potential solution for longer
Singlemode fiber’s high bandwidth is a major
advantage when considering its use in a network. But the cost
of connectivity products for singlemode fiber is higher due to
the requisite higher tolerances. In fact, the main disadvantage
of singlemode fiber systems is the cost of the associated electronics.
Because these systems employ traditional 1300-nm lasers as the
transmitter, they can cost up to five times more than multimode
systems, which employ light-emitting diodes. Connectorization
of the fiber is also more difficult and potentially more time-consuming.
Under the Gigabit Ethernet standard,
50-micron multimode fiber is an acceptable alternative. The major
advantage in using this fiber is that it is available with a modal
bandwidth of 500 MHz-km in both the 850- and 1300-nm windows,
so it supports gigabit transmission over 550-meter runs. In addition,
the cost of cables containing this fiber is typically less than
that of 62.5-micron fiber. Also, since the fiber has the same
tolerances as 62.5-micron glass, it is compatible with virtually
every field-installable and factory-terminated connector.
There are some disadvantages to using 50-micron
fiber, however. One issue is that it is more bend-sensitive than
62.5-micron fiber. The vast majority of installers are comfortable
with the handling characteristics of standard 62.5-micron multimode
fiber, and given the training required to learn new techniques,
the bend-sensitive characteristic of 50-micron fiber could lead
to increased attenuation in these systems if the fiber is improperly
While ease of use will undoubtedly improve
with experience (which will also occur with singlemode fiber)
problems could arise in future lightups that require maintenance.
This is another important area wher 50-micron fiber does not enjoy
the support of the other fiber types. For example, many contractors
have optical time-doain reflectometers (OTDRs) with multimode
and singlemode modules. If 50-micron fiber is added to the system,
another module for the OTDR will be required, increasing the contractor’s
In applications where all three fiber types
are used, the installer must take care to ensure that the correct
fiber is being worked on and that the correct equipment is being
used. Failure to do so could result in erroneous readings and
incorrect diagnosis of a fiber problem, leading to delays in repairs
and costly network outages.
While fiber manufacturers have stated that
both multimode fiber types are compatible, power penalties as
high as 4 dB can result if 50- and 62.5-micron types are connected.
If several different links are involved, a backbone extension’s
link-loss budget ma be exceeded, which clearly could be a major
problem with some large campus installations being upgraded in
handle Gigabit Ethernet.
62.5-micron fiber revisited
The standard-core-size fiber for taday’s LANs
and the one that has been in use for more than a decade is 62.5-micron.
The fiber itself and its compatible connectors and installation
practices are well-understood by most installers. Because this
fiber is compliant with all building specifications and standards,
many designers are more comfortable using it. Therein lies an
inherent advantage of 62.5-micron fiber.
However, this standard fiber has one major
disadvantage; the bandwidth in the 850-nm window is not adequate
to transmit gigabit signals to distances required for most LAN
installations. The "futureproofing" argument for 62.5-micron
fiber is no longer a valid proposition.
Another solution is a higher-bandwidth 62.5-micron
fiber in the 850-nm window. Up until now, most manufacturers of
fiber have been reluctant to offer this fiber type to the market
because of insufficient yield from manufacturing processes. A
number of cable manufacturers now offer 62.5-micron fiber capable
of longer distance transmission of gigabit signal while maintaining
existing performance for lower speed protocols. The link lengths
for 1000Base-SX (850-nm window) vary from 300 to over 500 meters
and all take advantage of the cost savings associated with VCSEL
The choice of the correct optical fiber to
be employed in the network has never been more complicated with
the advent of gigabit standards and with the addition of 50-micron
multimode fiber to he fiber mix. Singlemode fiber will always
be a viable option for any network should the end-user be willing
to pay additional money for the active electronics and connectorization.
The distance limitations imposed by the emerging
gigabit protocols on standard 62.5-micron fiber limit its applicability
in many installations. Therefore, those users concerned about
the ability of their structured-cabling infrastructure to support
gigabit applications in the future will need to look to enhanced
62.5- and 50-micron fiber solutions.
Given its advantages, its compatibility with
the installed base, its compliance with all current building specifications
and standards, and its familiarity to installers, enhanced 62.5-micron
fiber merits strong consideration as the fiber of choice for the
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