Connections +
Feature

Focus on… Engineering & design: An Optical Networking Primer

The advent of optical networks has spawned a new language. Here's a look at a few of the more common terms associated with "dancing the light fantastic".


May 1, 2002  


Print this page

In 1880, Alexander Graham Bell invented the photophone, which transmitted voice signals via beams of light. This device failed because of too many disruptions of the light beam: when the weather was cloudy, it didn’t work.

Today, technologies that generate and transmit light (commonly referred to as photonics), have altered the telecommunications infrastructure. The older network, through which communications were transmitted electrically over copper wires, has increasingly given way to systems built of optical components and fibers.

Most current telecommunications systems are not completely optical. They rely on optoelectronic devices that convert electrical energy to light energy that is transmitted over a fiber-optic line. It is then converted back to electrical energy by another optoelectronic device. The advent of fully optical networks may be several years away.

Yet, as is frequent in technological innovation, the advent of optical networks has resulted in new terms and phrases. Following is an explanation of some of the more common terms (and a few of the older ones) that have resulted from optical networking…

GENERATING THE SIGNAL: WHAT IS A LASER?

The acronym laser stands for “light amplification by stimulated emission of radiation.” Lasers produce narrow, finely focused beams of light. In a beam of such energy, all of the waves have the same phase and frequency. A laser beam is very pure, with the light approaching a single wavelength.

A laser consists of a chamber, or “cavity,” in which atoms, ions or molecules of a solid, liquid or gas are amplified or “pumped” with electricity or light. Mirrors at each end of the cavity allow energy to reflect back and forth and resonate, continually increasing in intensity. The energy emerges from the cavity as a continuous beam or as a series of pulses of light. The type of the material in the cavity of a laser determines the wavelength of the output.

Lasers create the light pulses that travel through optical fibers. In optical networks, the lasers are semiconductors that are about the size of a grain of salt. They are typically known as laser diodes, semiconductor lasers or laser chips.

Optical networks can use light sources other than laser beams for transmission of information along optical fibers. The LED or light emitting diode is a semiconductor device that emits light when electricity passes through it. It differs from a laser in that it produces waves that do not have the same phase and frequency.

ORGANIZING THE SIGNAL: WHAT IS DWDM?

DWDM or “dense wave division multiplexing” also is called “WDM” or “wave division multiplexing.” This technology allows two or more optical signals having different wavelengths to be transmitted simultaneously in the same direction over one optical fiber. Because optical fibers can carry many wavelengths of light simultaneously (without interaction between each of them), DWDM greatly increases the amount of information that a fiber can carry.

DWDM combines up to 160 wavelengths of light per strand of fiber. The technology breaks white light into multiple colours (wavelengths). Each signal travels within its own wavelength, modulated by the information (e.g., data, voice, video) it is carrying. The signals are then separated by wavelength, or demultiplexed, at the end of the fiber.

CARRYING THE SIGNAL: WHAT IS FIBER OPTIC CABLE?

Fiber optic cable, designed to transmit light, is a thin plastic tube that consists of several layers of materials. At its heart is the core, which in telecommunications applications is a hair-thin strand of pure silica glass or multiple strands that are bundled together. In some specialized industrial and automotive control applications, the core may be made of plastic.

The cladding, which also is made of glass, surrounds the core. The core and cladding are bonded together during the glass extrusion process and are inseparable. The core has a slightly higher refractive index than the cladding, which causes the cladding to act as a mirror. When light is guided down the core, it reflects off of the cladding and back into the core. A plastic buffer coating surrounds the cladding and protects it from moisture or damage. A strength member covers the buffer and prevents stretching problems when the fiber is pulled through conduits. An outer covering, called a jacket, surrounds the strength member.

In addition to the increased capacity (or bandwidth) of optical fiber networks, communicating by light over optical fibers offers advantages over communicating by electricity over copper wires. Photons travel through optical fibers several times faster than electrons travel through copper wires, thus reducing transmission times. Light, unlike electricity, is not affected by electromagnetic or RF interference. The error rate of information transmitted via light is significantly lower than that transmitted by electricity. Information can be sent over longer distances without the need to retransmit signals and can be transmitted more securely because taps in fiber lines can be detected. Fiber cable carries no current and, therefore, poses no danger, unlike live electrical wires. Finally, fiber cable weighs dramatically less than copper cable.

On the downside, fiber is more difficult to splice and thus, to install and repair. Its inability to carry current has forced network designers to adopt new strategies to power remote equipment when replacing copper with fiber.

REGENERATING THE SIGNAL: WHAT IS AN OPTICAL REPEATER?

A repeater or regenerator is a device that amplifies or regenerates a signal in order to extend the distance it may accurately be transmitted. This is necessary because a signal weakens or erodes as it travels further from its source. In an optical system, a repeater receives the signal, cleans it up by removing noise and pulse deterioration, and then amplifies and retransmits it. A signal may travel through one or more repeaters.

There are two types of optical repeaters: Optical-Electrical-Optical (OEO) and Optical-Optical-Optical (OOO). OEO repeaters convert photons into electrical signals before regenerating and retransmitting them optically. OOO repeaters are known as fiber amplifiers.

A fiber or optical amplifier is a device that boosts a light signal in an optical network without first converting it to an electrical signal. In other words, it acts on the light directly. Common kinds of fiber amplifiers include erbium-doped fiber amplifiers (also known as erbium amplifiers or EDFA), Raman fiber amplifiers and silicon optical amplifiers (SOA).

SORTING THE SIGNALS: WHAT IS AN OPTICAL SWITCH?

A switch is a device for making, breaking or changing the connections in or among communications pathways. An optical switch, also known as a “photonic switch” or “optical cross-connect,” performs this function with light as it travels through an optical network.

The two broad categories of optical switches are hybrid optical switches with electrical cores (OEO switches) and all-optical (OOO) switches. OEO switches convert light pulses into electrical signals to switch them between fibers, then convert them back to light. OEO switches are subject to the speed limitations of electrical switching.

All-optical switches maintain signals as light from input to output which increases network speed. The prevailing all-optical technology is micro-electro-mechanical systems or MEMS, which switch light by using tiny mirrors to reflect a light beam from one fiber to another.

TOWARDS THE FUTURE: WHAT IS WIRELESS OPTICAL NETWORKING?

Wireless networking has emerged as an efficient means for delivering data, voice and video. In general, fiber optic cabling or traditional copper plant is used for long-range transport. In office local area networks (LANs), wireless networking based on conventional radio frequency (RF) technology is now commonplace, due to the proliferation of laptop computers and handheld devices.

To link buildings and campuses together, wireless optical networking is emerging as an alternative solution to RF and conventional copper or optical fiber-base
d links. Instead of sending beams of light along a glass fiber, wireless optical networking sends laser beams over the air. This is known as atmospheric laser transmission, free space optics (FSO) or free space photonics” (FSP).

RF wireless technology offers longer-range transmission capabilities than FSO, but FSO provides much greater bandwidth capacity. Additionally, RF-based networks require significantly greater capital investment because spectrum licenses must be purchased.

FSO functions by optically aligning two or more laser transceivers with a clear line of sight between them. A transceiver both transmits and receives signals, so there is full duplex (bi-directional) capability.

One of the great advantages of FSO is that it eliminates the lengthy regulatory process and associated costs of obtaining permits for and digging trenches and laying fiber-optic cable. In addition, an FSO system can be set up in a matter of hours and can operate over a distance of several kilometres.

However, there are several problems associated with FSO systems. The leading concern is that atmospheric conditions can impact performance. While rain, snow, dust and smog can block light transmission and disrupt service, fog presents the greatest problem. Dense fog disrupts and dissipates laser signals because its small, dense moisture particles act like billions of tiny prisms. While technological advances have helped to minimize this concern, weather issues can limit the distance between transceivers.

Another problem is that laser beams may misalign when buildings move because of solar and wind loading or small earthquakes, although systems with auto-alignment capabilities resolve this issue. Additionally, very small pockets of turbulent air may disrupt transmission, but the use of multiple transmitters and receivers solves this problem.

Wireless optical networking offers an exciting opportunity to increase communications options and decrease connectivity bottlenecks in dense urban areas. The potential combination of generally reliable performance, low-costs, bandwidth scalability and rapid and flexible deployment, points to a bright — and light-filled — future.

Joseph Kershenbaum, B.A., M.B.A., J.D. (joseph@kershenbaum.com), is a technology executive who has advised communications manufacturers and service providers in the telecommunications and Internet industries.

Edward Kershenbaum, B.S., M.B.A. (ed@kershenbaum.com), formerly with SBC Communications Inc., is currently CIO of WorkplaceIQ Ltd.


Print this page

Related