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.

Fiber limitations

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 an extension
• the current application
• the protocols the network will be required to support in the future

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.

Singlemode fiber

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 runs.

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.

50-micron fiber

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 handled.

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 capital outlay.

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 transmitter technology.

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 future.

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