Gigabit transmission complicates fiber selection

Ever since optical fiber has been used in local area networks (LANs), one of its most significant advantages over other cabling options has been its ability to handle ever-increasing amounts of data at higher and higher speeds. Until now, end-users have been told that the standard 62.5-micron multimode fiber used in a vast majority of lan applications has virtually unlimited bandwidth when used at the distances involved in the customer`s network.

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The installed base of 62.5-micron fiber justifies a look at a new version of this medium.

William Charuk,

Berk-Tek Inc.

Ever since optical fiber has been used in local area networks (LANs), one of its most significant advantages over other cabling options has been its ability to handle ever-increasing amounts of data at higher and higher speeds. Until now, end-users have been told that the standard 62.5-micron multimode fiber used in a vast majority of lan applications has virtually unlimited bandwidth when used at the distances involved in the customer`s network.

But, as protocols for ever-higher-speed applications have been developed and implemented, and as next-generation designs for lan equipment have begun to take shape, the optical characteristics of 62.5-micron fiber have come under scrutiny. The de facto industry standard for multimode fiber has been and continues to be Fiber Distributed Data Interface (fddi)-grade 62.5-micron multimode fiber. This fiber requires an attenuation of 3.75 decibels per kilometer and a modal bandwidth of 160 megahertz-kilometers at 850 nanometers, and an attenuation of 1.5 dB/km and a modal bandwidth of 500 MHz-km at 1300 nm (see "Understanding bandwidth," page 52).

Unfortunately, the incessant drive to higher-speed protocols has made it evident that the "virtually unlimited bandwidth" of today`s 62.5-micron multimode fiber may not be enough to meet the demands of the future.

Standards for the future

The initial goal of the 802.3z committee of the Institute of Electrical and Electronics Engineers (ieee--New York City) was to establish 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. As the model was developed to test the new standard, the bandwidth specification, rather than attenuation, emerged as the limiting factor in the system. The premise that the fiber had virtually unlimited bandwidth was no longer valid.

Today`s managers of information systems must prepare their networks for the future, which will demand more bandwidth not only in the network backbone but in horizontal cabling as well, as runs to the desktop lengthen and speeds increase.

Interestingly, the Gigabit Ethernet standard allows for the use of vertical-cavity surface-emitting lasers (vcsels) operating at 850 nm as the fiber-optic cable`s transmission source. vcsels, along with other short-wavelength laser alternatives, offer the promise of significantly improved manufacturing efficiencies compared to traditional 1300-nm laser sources. Thus, these 850-nm devices may replace the more traditional 1300-nm lasers as the most cost-effective transmission sources.

This technology lets end-users install a laser transmitter in their multimode systems at a very affordable price. It is also the reason why most network operators will continue to use the 850-nm window in their networks.

Fiber limitations

To determine whether the 300-meter objective targeted by the Gigabit Ethernet draft standard was achievable, the 802.3z committee ran tests on cables containing fddi-grade 62.5- micron multimode fiber. Surprisingly, the results did not support the objective. The maximum distance that 1.25-Gbit/sec signals could be sent in the worst-case scenario was determined to be 260 meters at 850 nm.

This past June, the ieee 802.3z Gigabit Ethernet standard was unanimously approved. 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. Also, a note in the specification states that "to support 300-meter link distances, the modal bandwidth would have to be 220 MHz-km for 62.5-micron fiber."

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.

The relationship between the Gigabit Ethernet length limitation and modal bandwidth can be extrapolated using the model generated during development of the standard. For the distances of concern in the lan environment, the relationship is almost linear.

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. The table summarizes the various standards and distance limitations currently imposed by them.

Is there an issue with standard 62.5-micron multimode fiber? Yes, and the issue is that the fiber that has been used for years with complete confidence by thousands of end-users will not support all future fiber applications. Data collected in two studies conducted in the early 1990s illustrates the magnitude of this problem and the dilemma we face.

In one study, AT&T surveyed 80 businesses to determine the distances involved in their lan installations. This study found that the maximum distance from the workstation to the telecommunications closet (TC) was 100 meters, as would be expected, based on the length limitations contained within TIA/EIA-568a and iso/iec-11801, the respective commercial building cabling standards of the Telecommunications Industry Association and Electronic Industries Alliance (TIA/EIA-- Arlington, VA) and the International Organization for Standardization and International Electrotechnical Commission (iso/iec--Geneva). The study also found that 95% of the runs from the TC to the main crossconnect were less than 300 meters in length and that the remaining 5% of these runs were less than 500 meters.

A second survey, conducted by Compaq Inc., questioned 107 businesses with more than 750 personal computers in use in a network environment. The survey shows that more than half of the installed base consists of horizontal runs longer than 175 meters and so will not be able to support all of the emerging standards, including Gigabit Ethernet, atm, and Fibre Channel.

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.

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 involved are short (less than 200 meters), the "virtually unlimited bandwidth" adage probably still holds true for standard fddi-grade, 62.5-micron multimode fiber. Therefore, at short-run distances, the end-user does not need to change any designs or purchasing practices. The bandwidth of this fiber will suffice for Gigabit Ethernet and for most other near-term applications.

Singlemode fiber

For longer runs, the installation of singlemode fiber-optic cables or a hybrid cable containing singlemode and multimode fibers could be the 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--and therefore, more costly.

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 where 50-micron fiber does not enjoy the support of the other fiber types. For example, many contractors have optical time-domain 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 may be exceeded, which clearly could be a major problem with some large campus installations being upgraded to handle Gigabit Ethernet.

Finally, as of this writing, 50-micron fiber is not included in the TIA/EIA-568a standard. For the network to be TIA/EIA-568a-standard-compliant today, 50-micron fiber is not an option.

62.5-micron fiber revisited

The standard-core-size fiber for today`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 new family of 62.5-micron fiber cable now available from our company, however, makes possible the longer-distance transmission of gigabit signals. When the parameters of the fiber used in these cables are inserted into the worst-case ieee 802.3z Gigabit Ethernet model, calculations show that the new fiber`s performance exceeds that of 50-micron fiber. Due to the larger numerical aperture of the 62.5-micron fiber, the power coupling loss is 4.7 dB less than that of 50-micron fiber, allowing for more attenuation headroom in the network for most applications. Furthermore, gigalite cables provide increased bandwidth for use with the vcsel sources required to support Gigabit Ethernet applications.

The cost of multimode cable in the network for gigabit applications is typically 10% of the cost of the overall network, including active electronics. With the use of enhanced fiber products, the cable cost would not change the cost of the network appreciably. In fact, with the savings achievable using vcsel technology, the overall cost of the network infrastructure should be less than the cost of a similar network that uses lasers at longer distances.

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 the 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.

Understanding bandwidth

Dispersion, or spreading of the light pulse as it travels down the fiber core, is the basic mechanism that affects optical-fiber bandwidth. To standardize optical-fiber bandwidth, the end-to-end bandwidth of a fiber is normalized to 1 kilometer. The Fiber Distributed Data Interface specification of 160 megahertz-kilometers at 850 nanometers and 500 MHz-km at 1300 nm actually means that if the fiber run is 500 meters long, the theoretical end-to-end modal bandwidth should then be 320 MHz (160 MHz-km/0.5 km) at 850 nm and 1000 MHz (500 MHz-km/0.5 km) at 1300 nm.

Fiber and cable manufacturers measure bandwidth using optical benches and typically call it "modal" bandwidth. The fiber-optic cable industry has done extensive testing to verify that the modal bandwidth of an optical fiber is not changed by the cabling process. In fact, in certain cases, modal bandwidth actually improves with cabling.

Unfortunately, measuring actual bandwidth in the field in installed systems with any degree of accuracy is not currently possible. Therefore, manufacturers must construct models and carry out applications testing to ensure that the worst-case scenarios for any installed system are addressed.

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Click here to enlarge image

The blue line represents the actual required bandwidth. For example, the modal bandwidth required for a signal to travel 500 meters is about 370 MHz-km.

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The graph shows distance limitations by protocol for three fiber types. gigalite 62.5-micron fiber cable makes possible the longer-distance transmission of gigabit signals.

William Charuk is business manager at Berk-Tek Inc.`s Fiber Optic Group (New Holland, PA).

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