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EDN Access--11.21.96 FIBER Vs COPPER: Sometimes it's not an easy choic

-November 21, 1996

EDN logoDesign FeaturesNovember 21, 1996


FIBER Vs COPPER: Sometimes it's not an easy choice

Bill Travis, Senior Technical Editor


Choosing between fiber-optic and copper interconnect systems is sometimes difficult,
entailing considerations of distance, cost, required bandwidth, and specialized expertise.

  You have two basic choices for configuring hard-wired, high-speed system-to-system communications: fiber-optic (FO) or copper-based interconnects. The choice between these competing technologies can be either clear-cut or blurry, depending on considerations of distance, bandwidth, and cost. Although a comprehensive treatment of the technologies, product types, and manufacturers would fill a tome at least 3 in. thick, a few technical aspects and a sampling of available hardware for FO- and copper-based interconnect systems can help ease your design choice.

  There is an ongoing debate between the FO and copper camps, with no clear winner in sight. Naturally, vendors that specialize in a particular technology have an ax to grind, so you must carefully listen to both sides. The arguments are along the following lines: "X is a lower cost technology than Y." The rebuttal: "Yes, on the face of it, but Y is coming down in cost; besides, you have to consider total system cost." The following section (based on material from References 1 through 5) presents some of the FO-vs-copper arguments:

Immunity to EMI, RFI, and lightning. Because fiber is a dielectric material, it's not susceptible to electrical interference. FO-product vendors also claim that fiber systems make secure communications easier. Interference immunity and lack of emissions are givens in FO systems and in the fiber medium itself. However, as Ed Sayre of Northeast Systems Associates (an independent consultant organization specializing in interconnect systems) points out, it's a different story at the ends of the fiber cables. FO transmission usually involves multiplexing signals into a serial data stream. Multiplexers and demultiplexers entail extremely high-speed switching, giving rise to emissions that you must provide shielding against. As for secure communications, FO cables emit no electromagnetic signals that you can detect, and it's more difficult to tap into FO cable than into copper. However, if you're adept, it is possible to tap into fiber cable and snoop.

Weight and size. Optical fiber generally has an outside diameter ranging from 0.125 to 1 mm. The outer diameter of a communications cable pair or coaxial cable can range from 1 to 10 mm. Because the specific gravity of glass is approximately one-fourth that of copper and the dimensions of fiber cables are smaller, the weight of a fiber-optic system is one-third to one-tenth that of a copper system. Note that this weight comparison applies to the cables only. For short-distance transmission, a fiber-optic system can actually weigh more than a copper system because the weight of the electronic modules at the cable ends is usually greater than that for copper-based systems.

Cost. Cost is a complex issue, dependent on many factors. Fiber costs less for long-distance applications because copper cables need many repeaters (amplifiers) in long hauls. A fiber-optic cable can easily transmit a signal as far as 80 km or more without the need for amplification. For distances of only a few meters, however, copper systems are less expensive and, it seems, always will be. For high bandwidths, the cost-per-meter distance of fiber itself is lower than that for copper. However, the cost of the electronics and elecro-optics at the cable ends can be substantial. You can compare the costs of the two technologies by determining a break-even distance. Years ago, the break-even distance was 10 km or more; today, it can be less than 100m because of drops in the cost of FO components and rises in the price of copper.

Ease of upgrading. FO proponents argue that installing fiber ensures performance as higher speed networks emerge in the future. Because current applications use only a fraction of the available bandwidth in a fiber system, a pre-existing fiber system can accommodate any conceivable future upgrade. Copper advocates argue that Category-5 unshielded-twisted-pair (UTP) systems have enough bandwidth to accommodate any imaginable network upgrade. They also assert that connecting fiber to the desktop computer is clearly overkill and too costly and has more speed than most users want or need. They do agree, however, that fiber is suitable for building backbones and for bridging long spans between buildings.

Ease of installation. The copper camp maintains that fiber is tricky stuff to work with and requires specialized expertise and equipment. Fiber advocates counter that, on the contrary, UTP is more difficult to install because of its susceptibility to EMI and RFI; you must take extreme care when installing it near electrical machinery, high-voltage systems, and other interference producers. The fiber proponents also assert that, at data rates above 100 Mbps, testing copper systems (especially for near-end crosstalk, or NEXT) is difficult and expensive. Testing optical systems, they argue, is easy using relatively inexpensive optical-loss sets. As usual, both arguments have merit, and the pros and cons of the two technologies are highly dependent on the circumstances. In most cases, wiring a fiber-optic system is more difficult and requires more specialized skills than connecting a copper-based system.

Bandwidth. It's indisputable that FO has higher bandwidth than any alternative medium available. Single-mode fiber offers a bandwidth-distance product in the several tens of GHz·km; less expensive multimode fiber has a product ranging from 1 to 10 GHz·km. Using fiber-optics, it's also possible to use wavelength-division multiplexing (WDM), in which you use different carrier wavelengths to transmit information. WDM can increase the amount of transmitted information by as much as 10 times, compared with single-wavelength systems. The information-carrying capacity of fiber, in terms of cross-sectional area of the cable, is many times higher than that available from copper-based systems.

A few fiber fundamentals

Figure 1 shows the two basic types of optical fiber: single-mode (Figure 1a) and multimode (Figure 1b). The core of multimode fiber is much larger than that of single-mode fiber; it thus allows many rays (modes) of light to simultaneously travel the fiber. Single-mode fiber exhibits less dispersion caused by multiple rays and, thus, offers better light-pulse fidelity over longer distances. In single-mode fiber, the refractive index of the fiber core is a discrete step higher than that of the cladding; it is therefore called "step-index" fiber. In the "graded-index" multimode fiber in Figure 1b, the core's central refractive index, nA, is greater than that of the outside edge of the core, nB. The refractive-index profile from nA to nB is parabolic. The light rays do not reflect from the cladding in straight lines but, rather, follow a serpentine path because of the parabolically varying refractive index. (Note that single-mode fiber is more expensive, handles longer distances, and has higher bandwidth than multimode fiber.)  Losses in fiber-optic cable stem principally from scattering, in which light rays partially disperse (scatter) as they travel along the fiber, and absorption, in which impurities in the silica absorb some of the optical energy. Another source of losses is bending, which causes less-than-total reflection at the core-cladding boundary. Splices and cable connectors also contribute to fiber-optic losses. Cable losses in single-mode fibers are typically well below 1 dB/km.

Figure 2, derived from Reference 2, shows some of the splicing defects that can produce losses.

  In Figure 2a, a reflection loss occurs because of the different refractive indices of the fiber and the air between the end faces. The cure for this type of loss is to apply a liquid with the same refractive index as the fiber between the end faces or fuse the fibers together. Offset and tilt account for losses in Figures 2b and 2c. In Figure 2d, the separation (disregarding the reflection loss) of the two end faces produces losses. Deformed end faces lead to losses in Figures 2e and 2f. Losses accrue from differences in core diameter in Figure 2g. A gaggle of hairy equations enables you to calculate these splice-induced losses.

  Silica fiber exhibits losses per kilometer similar to those shown in

Figure 3. The three numbered dips, called "windows," are the low-loss regions the fiber-optics industry addresses. The first dip, at 850 nm, was the first to be exploited, because of the wide availability of LEDs and detectors at this wavelength. Low-cost multimode fiber makes extensive use of this window. The second window, at 1300 nm, is popular in single-mode applications because of its lower losses and dispersion. You use the 1550-nm third window only in systems needing low loss at long distances or in WDM systems. Both performance and cost increase as wavelength increases. Because of the availability of low-cost CD-type lasers, a fourth wavelength (780 nm) is currently gaining popularity.

  For short hauls, low-cost plastic fiber offers an alternative to glass. Its loss-vs-distance curve exhibits dips at 570 and 650 nm (green and red light, respectively). Only the 650-nm window is usable, because 570-nm detectors have extremely low efficiency. Plastic fiber is usable over distances to approximately 100m. It's easier to interconnect than glass fiber—you need only a wire stripper, scissors, 600-grit sandpaper, and some 3-mm lapping film. It suffers from brittleness at low temperatures, and you must not impose a bend radius smaller than 20 mm, because severe attenuation can result.

  An impressive array of fiber-optic products is available from a small army of suppliers. In perusing the specs for FO products, you would do well to emulate Vanna White and thoroughly learn all 26 letters of the alphabet. FO technology abounds in acronyms and initials. Some examples include ESCON (enterprise systems connection), FDDI (fibre-distributed data interface), NEXT, ATM (asynchronous-transfer mode), SONET (synchronous optical network), BER (bit-error rate), GRIN (gradient index), and, for connectors, FC (a threaded optical connector) and SC (subscription channel).

  Any FO system needs a means of changing electrical signals to light and vice versa. Transmitter/receiver pairs, or transceivers, are available from many suppliers. For example, the MGLM-1063 Gigabit Link Module from Methode Electronics is a full-duplex transmitter/receiver pair that operates at a data rate of 1.0625 Gbps. It handles a 20-bit-wide data bus with an 8B/10B encoding scheme, as specified by the Fibre Channel ANSI X3T11 Committee. The transceiver serializes the 20 bits to transmit through one of three fiber choices: 780- or 850-nm multimode or 1300-nm single mode. The receiver uses a PLL to recover the data clock and shifts the data into a deserializer. Using an industry-standard SC connector, the transceiver accommodates distances as long as 500m over 50/125-µm fiber.

  Another transceiver, from Amp, also uses the popular SC connector. The optical carrier unit specified by SONET, the OC-12, uses 1300-nm sources and detectors to provide 622-Mbps data transmission/reception. It's available in a single-mode version for long-distance and LAN-backbone applications and in a lower cost multimode version for short-run LANs. The multimode version uses low-cost, molded-optical-lens technology. The SC connector uses a standard 139-pin footprint; a 239 configuration that incorporates laser-control and clock-recovery circuits is also available.

  Speaking of packaging, Nortel (Northern Telecom), Lucent Technologies, and Hitachi Ltd have collaborated on a new package for accessing optoelectronic components. The eight-pin, mini-DIP (

Figure 4) measures only 13.2x7.6 mm and is pin-compatible with industry-standard 14-pin devices. The unit contains a laser diode for transmitting and a positive-intrinsic-negative (PIN) photodiode for reception. Lucent, for example, will offer a variety of products in the new package, including uncooled multiquantum-well (MQW) Fabry-Perot and uncontrolled distributed-feedback (DFB) lasers, along with complementary detectors.

  Transceivers are available for several specialized applications. Amp, for example, offers an ESCON transceiver in a 4x7-pin package. The 1300-nm device satisfies ESCON's 200-Mbps, 2-km transmission requirement. Yet another transceiver from Amp targets Ethernet and Fast Ethernet applications. It uses an inexpensive, 840-nm galium aluminum arsenide (GaAlAs) LED for transmitting over low-cost multimode fiber. You can use the transceiver either in analog mode or for ECL/TTL data communications to 100 Mbps. Another area of specialization is the 622-Mbps ATM. Siemens produces a single-mode transceiver with an industry-standard 1x9-pin footprint; it operates from one 5V supply and accommodates transmission distances to 15 km. A future version will extend the distance to 40 km.

  Specialized transceivers proliferate at Hewlett-Packard (HP). The HFBR-5320 ESCON transceiver, for example, uses 1300-nm multimode technology for 200-Mbps transmission to 2 km. It comes in an industry-standard 4x7-pin package. HP's single-mode FDX1125B targets FDDI/Fast Ethernet applications. Using SC connectors, the device comes in an industry-standard 1x9-pin footprint. Standard packages also house two other recent HP single-mode transceivers. The SDX1155B comes in a 1x9-pin package and operates at 155 Mbps to satisfy SONET/SDH/ATM requirements. The 2x9-pin CDX2622B operates at 622 Mbps and meets SONET OC1/3 SDH STM1 (S1.1) requirements.

  A new series of SONET-compatible transmitters and receivers is available from Mitsubishi Electronics America. The MF-2500DS Series includes three transmitters and two receivers. Transmitter options include an integrated electro-absorption modulator laser diode (EALD) for high optical power and a 1550-nm version you can use with an erbium-doped fiber amplifier (EDFA). The series is available for transmission distances that range from 15 to more than 100 km. To address "convergence" and "multimedia" applications, the company developed a return-path laser module designed for video and data return-path needs in CATV systems.

  Honeywell recently announced its HFM 1222-331 token-ring fiber-optic receiver. Designed for 16-Mbps LAN applications, the device incorporates a PIN photodiode and associated preamplifier. It satisfies the requirements of the IEEE 802.5J signal standards. Honeywell also offers a new laser diode: the HFE 4080-321 vertical-cavity surface-emitting laser (VCSEL). The 850-nm laser operates from dc to more than 2 Gbps, with 5- to 15-mA drive current. Surface emission (in contrast with edge emission) makes lensing and light coupling easier.

  A series of specialized fiber-optic receivers and transmitters from Litton Poly-Scientific target the video market. The FO4700 AM receiver and FO4710 AM transmitter are available in 850- and 1300-nm versions. Using 62.5/125-mm multimode fiber, they handle 1V p-p analog signals over the range of 3 Hz to 20 MHz. The FO4717 FM receiver and FO4715 FM transmitter also come in 850- and 1300-nm versions and accommodate 1V p-p signals in an 8-MHz bandwidth. The company also offers a multilink RS-232C/422 and TTL data-transmission link and a transceiver that combines analog and digital video transmission.

  A wide variety of fiber-optic transmitters, receivers, amplifiers, and discrete components is also available from Lucent Technologies (formerly AT&T). Schweitzer Engineering Laboratories recently announced its SEL-2800 transceiver, designed to provide isolated EIA-232 transmissions over distances to 500m in harsh environments. Models 2848/2849/2850/2851 transceivers from SI Tech handle any conceivable Ethernet system. The company also offers a 24324 transmissive star that distributes optical signals to 24 workstations and an Ethernet repeater that extends link distances to 10 km. New models of optical stars are also available from Dymec Inc.

  For those who wish to transmit parallel data, Motorola offers its Optobus multichannel optical data link. It provides bidirectional, 4-Gbps aggregate data transfer of 10-bit-wide digital data. The raw data-transfer rate is 400 Mbps, over a distance as great as 300m. The system uses two 10-bit ribbon cables—one for transmit and the other for receive—and two transceiver modules. I/O is current-mode logic (CML), with 250-mV logic swings for interface with most positive emitter-coupled logic (PECL) ICs.

  Methode Electronics offers a multifiber connector system that comes in two-, eight-, and 12-fiber versions. The MP Series uses the same panel opening as single-fiber SC connectors. Its claim to fame is low cost—it uses injection-molded plastic, rather than expensive ceramic, ferrules. The connector is designed to accommodate fiber-optic ribbon cables, an emerging technology that's rapidly growing in popularity. And, on the topic of connectors, HP recently introduced a crimpless connector for 1-mm plastic fiber. Available in latching or nonlatching versions, the HFBR-453X Series requires no special tools to assemble. The connectors are designed to mate with HP's Versatile Link FO transmitter and receiver components that provide data rates from 40 kbps to 155 Mbps, over distances as great as 125m.

Copper's not dead

  The copper-based connector industry shows no indications that it's ready to roll over and die. New developments continue to push the edge of the envelope in data-transfer speed and bandwidth. Amp, for example, recently introduced the HSSDC (high-speed serial-data connector) family for applications requiring serial-data-transfer rates as high as 2.125 Gbps over distances of 30m or more. The system has approvals by the ANSI X3T11 Committee for Fibre Channel, and is under consideration by ANSI X3T10.1 for serial-storage architecture (SSA) and by IEEE 802.3 for Gigabit Ethernet. Plug-in connectors incorporate a small pc board that can provide compensation circuitry to optimize high-speed performance and maintain signal integrity.

  A Category-5 interconnection system from Thomas & Betts is capable of delivering data-transmission rates of 300 Mbps and higher over UTP cables. The ALL-LAN interconnection system recently received UL certification to TIA/EIA 568-A Category-5 standards. The system uses a 100V, four-pair configuration. Amp is also proficient in the design and manufacture of Category-5 cable and interconnection systems.

  VideoLan Technology has developed a way to use a premise's existing twisted-pair wiring to accommodate a fully integrated, corporate local- and wide-area video network. The VL2000 system uses the company's Metallic Fiber transmission and broadband switching technology to transmit NTSC (or PAL) video, plus data and full-spectrum audio in full-duplex mode. The transmission is not on the premise's LAN, but the VL2000 serves the PCs that operate on the LAN.

  Although intended as a daughtercard-to-daughtercard backplane system, a new high-speed interconnect system from Siemens also offers system-to-system interconnect. The SpeedPac system uses balanced "twinax" lines in a metal housing to allow data rates to 2.5 Gbps, with signal rise times as low as 50 psec. The twinax lines form completely balanced transmission lines from striplines in the daughtercard, through the connector, and into the backplane. The system uses differential signaling and, therefore, does not rely on a ground reference; the result is lower noise. (Look for an upcoming EDN article on low-voltage differential-signaling ICs.) The HDM and HDM Plus backplane-interconnect systems from Teradyne also offer high-speed system-to-system interconnect.

Making light of copper

  The high speed and interference immunity of light would be of little interest if you had no way to convert the optical information to usable electrical signals. That's why Micro Linear has developed a family of fiber-optic transceiver ICs. The ML4664 is a single-chip Ethernet transceiver. It is fully compliant with the 10Base-FL fiber-optic Ethernet standard. The chip changes copper-wire, twisted-pair Ethernet signals into fiber-optic Ethernet, and vice versa. The use of fiber optics extends the range of the network from 100m to 20 km. Another IC from Micro Linear, the ML6680, is a single-chip, fiber-optic, token-ring transceiver. It converts the twisted-pair, copper-wire output from a token-ring transceiver chip (IEEE 802.5q) into a fiber-optic waveform (IEEE 802.5j). The most recent product from Micro Linear is not a fiber-optic transceiver, but it does address Fast Ethernet systems. The ML6694 transceiver reduces the cost of Fast Ethernet connections by partitioning the analog circuits into the transceiver (the ML6694) and the digital circuits into the Fast Ethernet controller.

  Vitesse Semiconductor exploits GaAs technology to make optical detector/amplifiers (Reference 6). The 266-Mbps VSC7802, the 532-Mbps VSC7805, and the 1.063-Gbps VSC7810 detect, amplify, and condition 840-nm light signals. Vitesse also works in collaboration with WL Gore to develop fiber-optic transceivers (Reference 7). WL Gore specifies a Vitesse parallel-to-serial conversion chip for use with its MIA (media-interface-adapter) transceiver. The new design reduces the size of the transceiver to the industry-standard 139-pin footprint and allows the user to place the Vitesse chip on the pc card.

 Fiber-optic and copper technology both continue to evolve—FO in cost reductions and greater ease of use, copper in high-speed performance. Choosing one technology over the other involves careful consideration of many factors, including cost, performance needs, and available expertise.

For free information…
When you contact any of the following manufacturers directly, please let them know you read about their products in EDN. Note: Unless otherwise noted, all Web sites are preceded by http://.
Amp Inc
Harrisburg, PA
Fiber optic: (717) 986-5160
Copper: (800) 522-6752
Anadigics Inc
Warren, NJ
(908) 668-5000
Aries Electronics
Milton Keynes, UK
(+44) 1980-26007
Augat Inc
Attleboro, MA
(508) 222-2202
Berg Electronics
Etters, PA
(717) 938-7222
Broadcom Corp
Irvine, CA
(714) 450-8700
Cinch Connector Division
Elk Grove Village, IL
(708) 981-6000
Circuit Assembly Corp
Irvine, CA
(714) 855-7887
Comlinear Corp
Fort Collins, CO
(800) 272-9959
Contech Research Inc
Attleboro, MA
(508) 226-4800
Dymec Inc
Chelmsford, MA
(508) 256-0025
EG&G Optoelectronics Canada
Vaudreuil, PQ, Canada
(514) 424-3300
Force Inc
Christiansburg, VA
(800) 732-5252
Fotec Inc
Medford, MA
(617) 396-6155
www.std.com/fotec
Framatome Connectors
USA Inc
Norwalk, CT
(203) 838-4444
Hadco Corp
Salem, NH
(603) 898-8010
Hamamatsu Corp
Bridgewater, NJ
(908) 231-0960
Harting Electronik Inc
Hoffman Estates, IL
(708) 519-7700
Hewlett-Packard
Palo Alto, CA
(800) 537-7715, ext 1654
www.hp.com
Hirose Electric Inc
Simi Valley, CA
(805) 522-7958
Hitachi America Ltd
Brisbane, CA
(800) 285-1601, ext 13
www.hitachi.com
Honeywell Micro
Switch Division
Freeport, IL
(800) 367-6786
ITT Cannon
Santa Ana, CA
(714) 557-4700
Circle No. 358
Johanson Manufacturing Corp
Boonton, NJ
(201) 334-2676
Litton Poly-Scientific
Blacksburg, VA
(800) 336-2112
www.litton-ps.com
Lucent Technologies Inc
Breinigsville, PA
(610) 391-2511
www.lucent.com
Methode Electronics
Chicago, IL
(800) 323-6858
Micro Linear Corp
San Jose, CA
(408) 433-5200
e-mail info@ulinear.com
Mitsubishi Electronics America
Sunnyvale, CA
(408) 774-3189
Molex Inc
Lisle, IL
(708) 969-4550
Motorola Inc
Phoenix, AZ
(800) 441-2447
Nortel (Northern Telecom)
Nashville, TN
(615) 734-4000
www.nortel.com
North East Systems Associates
Stow, MA
(508) 897-8787
www.nesa.com
Oki Semiconductor
Sunnyvale, CA
(408) 720-1900
Packard Hughes Interconnect
Irvine, CA
(714) 660-5772
Panduit Corp
Tinley Park, IL
(708) 532-1800, ext 324
Rambus Inc
Mountain View, CA
(415) 903-3800
RIFOCS Corp
Camarillo, CA
(805) 389-9800
Robinson Nugent Inc
New Albany, IN
(812) 945-0211
Samtec Inc
New Albany, IN
(812) 944-6733
Schweitzer Engineering Laboratories
Pullman, WA
(509) 332-1890
Seiko Instruments USA Inc
Torrance, CA
(310) 517-8113
www.seiko-usa-ecd.com
Siemens Electromechanical Components
Santa Clara, CA
(408) 982-1980
SI Tech
Geneva, IL
(630) 232-8640
Teradyne Inc
Nashua, NH
(603) 791-3000
www.teradyne.com
Thomas & Betts
Memphis, TN
(800) 247-9378
Vero Electronics Inc
Hamden, CT
(203) 288-8001
VideoLan Technologies
Louisville, KY
(502) 895-4858
Vitesse Semiconductor Corp
Camarillo, CA
(805) 388-3700
Winchester Electronics
Watertown, CT
(203) 945-5000
WL Gore & Associates Inc
Newark, DE
(302) 738-4880
Woven Electronics
Mauldin, SC
(864) 963-5131
Looking ahead
  For the foreseeable future, fiber-optic and copper-based systems will continue to vie for market share. As fiber-optic prices fall, the break-even distance steadily decreases. The "tricky-to-use" stigma for fiber optics will also wane, as FO-product manufacturers make their products more and more user-friendly. For example, low-cost plastic fiber is easy to apply; you need only cut it with scissors and do some minimal polishing.  As fiber-optic acceptance grows, we can expect semiconductor manufacturers to develop more highly integrated interface chips for optical applications. Many of these chips will use GaAs technology, because it's a natural for integrating the optical elements. Copper, too, offers great opportunities for the IC makers. The highest-speed systems will use differential signaling, giving rise to a generation of low-voltage differential-signaling (LVDS) chips.

References

  1. Goff, David, Fiber Optic Reference Guide, Focal Press, imprint of Butterworth-Heinemann, 1996.

  2. Murata, Hiroshi, Handbook of Optical Fibers and Cables, Second Edition, Marcel Dekker Inc, 1996.

  3. Sterling, Donald, Premises Cabling, Delmar Publishers, 1996.

  4. "Fiber vs Copper in High-Data-Rate Applications," Telecommunications Industry Association, 1994.

  5. "Optical Fiber Gains Market Share in Desktop LAN Applications," Telecommunications Industry Association, 1994.

  6. Schweber, Bill, "Optical detector/amplifier for Fibre Channel runs at 1.063 Gbps," EDN, April 27, 1995, pg 24.

  7. Quinnell, Richard A, "$200 module runs at 1.06 Gbps over copper," EDN, June 8, 1995, pg 22.



 You can reach Senior Technical Editor Bill Travis at (617) 558-4471, fax (617) 558-4470, e-mail b.travis@cahners.com


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