Tutorial – Optical Networks

What Is Optical Networking?

As the name suggests, optical networks form a class of networks where optical, rather than electronic, components are the building blocks of the network.  Compared to metallic cable, fiber optic systems offer greater bandwidths, lower attenuation, and no crosstalk or electrical interference.  Those advantages have led to the dramatic growth of fiber optic systems worldwide.  Today, nearly all long-haul telecommunications depend on the use of optical networks for their large capacity and robust performance.

Standards

Standards for fiber optic cable and other optical components have been developed over the last 20 years primarily by the American National Standards Institute (ANSI) and the International Telecommunications Union (ITU).   Standards for fiber optic transmission have been developed initially in North America under the name Synchronous Optical Network (SONET) and later by the ITU using the name Synchronous Digital Hierarchy (SDH).

Historical Milestones

• 1958: Discovery of laser
• Mid-60s: Demonstration of guided wave optics
• 1970: Production of low-loss fibers, which made long-distance optical transmission possible
• 1970: Invention of semiconductor laser diode, which made highly refined optical transceivers possible
• 70s-80s: Use of fiber in telephony: SONET/SDH standards from ITU
• Mid-80s: LANs/MANs: broadcast-and-select architectures
• 1988: First trans-Atlantic optical fiber laid
• Late-80s: Development of EDFA (optical amplifier), which greatly alleviated distance limitations
• Mid/late-90s: DWDM systems explode
• Late-90s: Intelligent Optical networks
• 20?? Soliton transmission with optical TDM

Optical Networking: Why?

The “traditional” networks consist, for the most part, of a collection of electronic switches interconnected by point-to-point optical fiber links, which can span local, metropolitan, or wide area networks.  To accommodate continually increasing demand for bandwidth and flexibility, such networks are being enhanced by adding more fibers and switches, increasing the bit rate per fiber, and upgrading the switches’ size, throughput and functionality.  Such enhancements eventually lead to very large and complex networks that are difficult and expensive to construct, operate and maintain.  Recent and emerging advances in optical technology promise revolutionary all-optical networks capable of providing improved economy, flexibility and robustness while still capable of making use of the large existing fiber base.

Principles and Operation  

An optical fiber is a cylindrical waveguide made of two transparent materials each with a different index of refraction.  The two materials, usually high-quality glass, are arranged concentrically to form an inner core and an outer cladding.  Different entry angles of the light source result in multiple modes of wave propagation.  Propagation can be restricted to a single mode by using a small-diameter core.

The choice between single-mode and multimode fiber depends on the desired repeater spacing or transmission rate; single mode is the preferred choice for long-haul or high data-rate systems.  The earliest form of multimode fiber was the step-index, where the core has a uniform index of refraction and the concentric cladding also has a uniform but lower index.  In this case the propagation velocity within the core is constant, so that rays traveling a longer path arrive behind rays traveling a shorter path, thus producing pulse spreading, or dispersion.  These dispersive effects may be remedied by constructing a fiber whose refractive index increases toward the axis, with a resulting refractive index profile that is parabolic.  With a graded-index fiber, rays that travel longer paths have greater velocity than rays traveling the shorter paths due to decreasing refractive index with radial distance.  The various modes then tend to have the same arrival time, such that dispersion is minimized and greater bandwidths become possible for multimode fibers.

Within the spectrum available in a fiber optic system, there are three low-loss windows, at wavelengths of approximately 850, 1300, and 1550 nm.  Early applications of fiber optics for communications applications were based on the short-wavelength band of roughly 800 to 860 nm.  Operation in the longer-wavelength bands, particularly at 1300 and 1550 nm, is attractive because of improved attenuation and dispersion characteristics at these wavelengths.  Typically today the shorter-wavelength band is used for short-haul, low data rate systems, and the longer-wavelength bands are applied to long-haul, high data rate systems.  Special fibers have been developed that shift the minimum dispersion to about 1550 nm to take advantage of lower attenuation as well as minimum dispersion.  These fibers are called dispersion-shifted fibers, and are important to single-mode fiber applications.

Low-data rate, short-haul fiber optic systems tend toward multimode cable, LED transmitters, and PIN diode receivers.  High-data rate, long-haul systems tend toward single-mode cable, laser diode transmitters, and avalanche photodiode receivers.   Latest generation fiber optic systems have introduced innovations that have significantly improved the bandwidth and repeater spacing possible.  Coherent detection via either homodyne or heterodyne techniques allows much greater bandwidths to be realized.  Several wavelengths can be transmitted simultaneously in wavelength-division multiplexing, analogous to frequency-division multiplexing used in telephony.  Optical amplifiers are now available that eliminate electronics and instead use specially doped fiber or semiconductor laser devices.  The use of optical amplifiers will allow a fiber optic system to be upgraded in bit rate without replacement of the repeaters.  Optical amplifiers have also been used to achieve ultra-long distances via soliton transmission, which is the transmission of an idealized pulse without loss of pulse shape.

Types of Optical Networks

Optical networks may be classified in several ways.  Opaque optical networks include optical-electronic-optical (OEO) conversion, while in all-optical networks each connection is totally optical (or transparent) except at the end nodes.

Optical networks may be single wavelength or multiple-wavelengths (WDM).  The use of SONET/SDH with a single carrier is a typical example of an opaque, single-wavelength optical network.

Finally, optical networks may be passive or active.   A passive optical network (PON) is an all-optical network that utilizes only passive optical components, e.g., fibers, directional couplers, star couplers, wavelength routers, wavelength multiplexers, and filters.  The intended applications are fiber-in-the-loop (local loop) and fiber-to-the-home (FTTH). The optical signaling formats in PONs can employ wavelength-division multiplexing (WDM), subcarrier multiplexing, time-division multiplexing (TDM) or any combination of these. An active all-optical network (AON) enables each of a large number of optical WDM channels (wavelengths) to propagate from source to destination over long distances and high bit rates without optical-to-electronic format conversion within the network.

Optical Network Architecture

There are two standard optical architectures, linear and ring, both of which can provide network protection and restoration of services.  SONET rings are the most widely deployed architecture.  They can be thought of linear networks folded back to create a loop or ring.  But unlike linear architectures, rings are designed to guarantee automatic restoration of services when cable or nodes fails, by use of loops around the failed component. Because of this automatic protection against failures, these rings are called self-healing.  There are several SONET ring architectures that depend on the number of fibers, transmission direction, and level of switching protection.

Originally developed in the United States, the SONET standard was adopted by the ITU-T but renamed as the Synchronous Digital Hierarchy (SDH).  These standards provide a complete set of specifications to allow national and international connections at various levels.  Optical interfaces are defined that provide a universal fiber interface and permit mid-span interconnection of different vendor equipment.  A standardized signal structure allows any existing hierarchical rates (for example, DS-1, DS-3, E-1, and E-3) to be accommodated.  Overhead within the SONET signals facilitate synchronization, add and drop multiplexing, electronic switching, performance monitoring and network management of the composite and tributary signals.  The SONET hierarchy is built on synchronous multiplexing of a basic SONET rate of 51.84 Mb/s, so that higher SONET rates are simply N x 51.84 Mb/s.  The basic signal structure provides sufficient flexibility to carry a variety of lower-level rates within the 51.84 Mb/s signal.

Optical Networking vis-à-vis Other Technologies

• Size and Weight: Since individual optic fibers are typically only 125 μm in diameter, a multiple fiber cable can be made that is much smaller than corresponding metallic cables.

• Bandwidth: Fiber optic cables have bandwidths that can be orders of magnitude greater than metallic cable. Low data rate systems can be easily upgraded to higher rate systems without the need to replace the fibers. Upgrading can be achieved by changing light sources (LED to laser), improving the modulation technique, improving the receiver, or using wavelength division multiplexing.

• Repeater spacing: With low-loss fiber optic cable, the distance between repeaters can be significantly greater than in metallic cable systems. Moreover, losses in optical fibers are independent of bandwidth, whereas with coaxial or twisted pair cable the losses increase with bandwidth. Thus this advantage in repeater spacing increases with the system’s bandwidth.

• Electrical isolation:  Fiber optic cable is electrically nonconducting, which eliminates all electrical problems that now beset metallic cable.  Fiber optic systems are immune to power surges, lightning induced currents, ground loops, and short circuits. Fibers are not susceptible to electromagnetic interference from power lines, radio signals, adjacent cable systems, or other electromagnetic sources.

• Crosstalk: Because there is no optical coupling from one fiber to another within a cable, fiber optic systems are free from crosstalk. In metallic cable systems, by contrast, crosstalk is a common problem and is often the limiting factor in performance.

• Environment: Properly designed fiber optic systems are relatively unaffected by adverse temperature and moisture conditions and therefore have application to underwater cable. For metallic cable, however, moisture is a constant problem particularly in underground (buried) applications, resulting in short circuits, increased attenuation, corrosion, and increased crosstalk.

• Reliability: The reliability of optical fibers, optical drivers, and optical receivers has reached the point where the limiting factor is usually the associated electronics circuitry.

• Cost: The numerous advantages listed here for fiber optic systems have resulted in dramatic growth in their application with attendant reductions in cost due to technological improvements and sales volume.

• Frequency allocations: Fiber (and metallic) cable systems do not require frequency allocations from an already crowded frequency spectrum.  Moreover, cable systems do not have the terrain clearance, multipath fading, and interference problems common to radio systems.

Business Implications and Applications

Today fiber optic systems are much more cost effective than metallic cable, satellite, and radio for long haul, high bit rate applications.  Fiber optic cable is also expected eventually to overtake metallic cable in short haul applications, includ¬ing metro facilities and local networks. One final cost factor in favor of fiber optics is the choice of material, namly silicon, which of course is one of the earth’s most abundant elements, versus copper, which may someday be in short supply, or the radio spectrum, which is already in short supply.

How to Learn More about It

Courses

• SONET/SDH: Principles and Design  is an intensive, two-day course that covers everything needed to understand and deploy this important technology, namely, the basic principles; system architecture, components, and operation; end-to-end network design process; and applications.

• DWDM: Dense Wavelength Division Multiplexing Principles and Design, the sequel to the above course, covers the technology, architecture, and applications of DWDM. This comprehensive two-day course provides all of the knowledge and skills necessary to design and implement DWDM systems.

Books

• Digital Transmission Systems, Third Edition, by David R. Smith.  Kluwer Academic Publishers, 2004.  Good introduction to optical communications, including components, standards, and networks.  Good description of SONET and SDH.

• Optical Networks by Ramaswami and Sivarajan: A Practical Perspective, Second Edition.  Morgan Kaufmann Publishing, 2002.  A practical textbook that offers   details on all aspects of optical components and networks.

• Deploying Optical Networking Components by Gilbert Held.  McGraw-Hill.

• Fiber-Optic Communications Systems by Govind E. Agrawal.  Wiley & Sons.

Web Resources

• ITU (http://www.itu.int) ITU-T:  Telecom sector of the International Telecommunications Union, the United Nations treaty agency that sets telecommunications standards.