cpet384_researchpaper_bartholme_spaulding by handongqp



         Fiber Optics
A survey of current and emerging technologies

                 CPET 384

      Brian Bartholme & Scott Spaulding

              Professor Steffen

              December 4, 2006


A survey of current and emerging trends related to fiber optic data communication. This

includes a history of fiber optics, a brief discussion of the current implementations of fiber, and a

more detailed analysis of the emerging fiber-to-the-premises trend, namely the system being

developed by Verizon Communications in the United States.

                        Fiber Optics: A survey of current and emerging technologies

    In data communications, the primary objective is always to send data faster and more

efficiently, while reducing errors and overhead as much as possible. This desire, and

increasingly, the need for higher speeds of transmission have resulted in deference to the

ultimate benchmark of speed: light. Fiber optic technology, exploiting the properties of light, is

able to transmit data farther and faster than electrical transmission. The current prevailing

conditions of increased fiber demand and practicality are driving fiber's deployment and reach.

The purpose of this paper is to delve into the topic of fiber optic technology, investigating the

history of fiber, the basic concepts, its current usage, and the future of fiber in data

communications, namely in the form of fiber-to-the premises.

                                           History of Fiber

     The first communication device that used light waves was developed and patented by

Alexander Graham Bell in 1880. Named the Photophone, the device used sunlight to carry

voice waves from a transmitting mirror to a receiving station. A microphone was connected to

the transmitting mirror which had sunlight shown onto it. As a person spoke into the

microphone, the sound waves vibrated the mirror which in turn vibrated the sunlight. The

vibrating sunlight was reflected to a receiving station. The receiving station used a newly

discovered element called selenium whose electrical resistance changed with light. The

modulating sunlight beam turned the electrical resistance into electricity which drove a small

speaker, thus enabling a person on the receiving end to be able to hear the person speaking at

the transmitting end.

    While this system worked, it was not at all practical. What was needed was a way to

channel the light through a medium just as electricity is channeled through wires. This idea

was demonstrated by focusing a light beam into a jet of water. The light would follow the

water as it curved downward. This phenomenon is known as Total Internal Reflection. The

light follows the flow of water because of the different densities of the water and the

surrounding air. Light travels more slowly in water than in air, so as the light reaches the

water/air boundary, it bends as it speeds up in the air. When the light traveling in the water

reaches a certain angle, it is reflected back into the water. At this point the water becomes a

light wave carrier. This principal was known and demonstrated in Bell’s era, but it was used

mostly as a science trick to amuse audiences. It wasn’t until almost 80 years later that this

principal was applied in the creation of fiber optic cable.

    One thing that was needed to create fiber optic communications was a usable light source.

Ordinary light is a mixture of many frequencies that dissipate in all directions, so it was never

useful as a dependable light source for communication. The development of the laser in 1958

changed everything. A laser is a narrow order beam vibrating at a single pure frequency.

Lasers vibrate at around a thousand trillion times per second, so they are able to carry trillions

of bits of data on a single light wave.

    With the development of the Laser a usable light source was now available. However, a

suitable medium was still needed to carry the light. Air was not practical as it would diffuse or

dissipate the light. Something pure and stable was needed. The answer came from the science

trick of the 1880’s: Total Internal Reflection. Instead of water, however, glass was used as the

medium. This development came about in 1956 through the efforts of University of Michigan

freshman Lawrence Curtis.

      Until that time, doctors were using gastroscopes to look into the stomachs of patients.

These hollow tubes with mirrors on them were painful to the patients and difficult for the

doctors to use. For his freshman physics project, Curtis decided to try to develop a more

humane gastroscope using glass fibers. Through experimentation he found that glass fibers

would transmit an image over a few feet. However, when he bundled the fibers together the

light was refracted out of them rather than remaining within the fibers. This problem was

again related to Total Internal Reflection. When the fibers were separate there was a different

density boundary because of the air space between the fibers. With the fibers pressed together

there was no air, so there was no boundary difference. With the boundaries all the same, the

light could not reflect. Instead, it refracted out and traveled from one fiber to the next. Curtis

then came up with the idea of surrounding the fiber with a layer of purer glass, called a

cladding. Like the air, the cladding would provide a lower density barrier which would allow

the light wave to reflect back into the fiber. It would also protect the inner fiber, the core, and

allow multiple fibers to be bundled together. This core/cladding fiber would preserve the Total

Internal Reflection principal. Curtis was able to create such a fiber by inserting a glass rod into

a hollow glass tube of different density and melting the two together. He was able to pull the

glass into fibers of 40 feet or more, which far exceeded the 2 foot fibers he had originally

created. By bundling the fibers together he was able to create a working endoscope which was

tested on a patient two weeks later. Within a decade, fiber optic endoscopes were a routine

part of medicine.

    The development of a fiber based endoscope was another giant step forward for fiber optics.

However, the problem of sending information over a large distance still remained. A fiber was

needed that could carry light waves farther than one kilometer, the distance that radio waves

could travel down coaxial wire. The fiber used for the endoscopes was not good enough for this

purpose, which prompted a search for a purer substance. The resolution to this problem came in

1970 from three men who worked for Corning Glass in upstate New York. Peter Schultz, Robert

Maurer, and Don Keck developed the technique of creating a fiber of fused silica: the purest

known form of glass. Fused silica is a very high temperature glass that is very difficult to melt

and draw. The trio was able to create a hollow cylinder of pure silica, the inside of which they

coated with a slightly less pure layer of silica. The coating was done initially by pulling the

smoke like substance through the cylinder with a vacuum cleaner. The cylinder was then heated

whereupon it collapsed on itself; the outside cylinder forming the cladding and the inside layer

forming the core. The melted glass was then drawn to form the actual fiber strand. Using laser

technology and the newly engineered form of fiber, the age of fiber optic cabling was born. By

the year 2000, an estimated 260 million kilometers of fiber optic cable had been installed,

enough to go to the moon and back 350 times.

                                           Current Technologies

     Present day usage of fiber is mostly attributable to mega-bandwidth carrying network

backbones, public utility companies, and government agencies. These entities have been using

fiber optics for several decades. Fiber is an excellent fit for these purposes, providing massive

amounts of bandwidth over long distances. The benefits of fiber, until just recently,

were mostly enjoyed only by those entities that could afford it or those that absolutely needed

the technology. However, decreasing prices and increases in demand are changing the

current landscape so as to extend fiber coverage to the general public: more specifically, right

to a customer's premises. This provides consumers and businesses with the same benefits

already enjoyed by large scale businesses. The entire concept of fiber to the premises is

redefining fiber's scope and examining the Fiber To The Premises (FTTP) process is a

worthwhile endeavor for understanding future developments.

                               Procession of data from satellite to the home

Satellite and super-head end

     Looking at the FTTP panorama, the Super Head-End (SHE) is equivalent to the mouth of a

large funnel, gathering massive amounts of data and channeling it down towards the end that is

the customer's premises. The process begins with a farm of satellites, taking in feeds from

essentially anywhere on the globe. The satellite signals are then processed by integrated

receiver/decoders (IRDs.) These devices take in the pure satellite signal, decode its signaling

scheme, extract the digital information contained within, and pass that data onto a digital

multiplexer. The multiplexer then combines the many signals and sends them out as one single

signal, and at a higher data rate.

Figure 1 - Conceptual breakdown of SHE and VHO components

     In the FTTP scenario, the multiplexer takes the television signals from the IRDs and

melds them together as one MPEG-2 video signal. This signal, which is still digital and not yet

optical, is sent to a Smart-stream Encryptor Modulator (SEM) using the Asynchronous Serial

Interface protocol. The SEM takes the multiplexer output and processes it to create an optical,

Gigabit-bandwidth signal. The signal is also modulated using a 64/256QAM

(quadrature amplitude modulation) scheme, and is then encrypted. After the digital signal

processing has been completed, the data is sent downstream to the final component of the super

head-end’s operational scope, the SONET multiplexer.

     Thus far, the video content has been pulled off of the satellites, converted into an optical

signal, modulated, and encrypted. The responsibility now lies with the SONET multiplexer to

make sure that this polished signal is distributed beyond the super head-end. Distributing the

signal necessitates being part of a network. As evidenced by its name, the SONET multiplexer

is a node on a SONET OC-192 wide area network transmitting at 10 Gigabits per second. This

high-speed, high-bandwidth, high distance-covering SONET network contains two Super

Head-Ends and multiple Video Hub Offices linked together by two separate networks, which

provide redundancy. The SONET network acts as a portal from which the Video Hub Offices,

the next major component of the FTTP scheme, can obtain national content signals for

incorporation into the final product that is distributed to the customer’s premises.

Video Hub Office (VHO)

    The Video Hub Offices (VHOs) are the mix masters of the process. They take in content

from a number of sources and multiplex them together to create one signal that is a combination

of services and content. The VHOs are attached to the SONET network, which enables them to

capture the national feeds being circulated on the ring from the SHE. VHOs also handle

localized content that includes broadcasts that are specific to the geographic area surrounding the

VHO. These signals would include local television channels and PEG channels (Public,

Education, Government). In addition they have the capability to distribute the Emergency Alert

System service (EAS) and Video on Demand (VOD), and can also facilitate ad insertion, which

incorporates commercials as part of the video broadcasts.

Figure 2 - Conceptual breakdown of VHO components

    The first task of the VHO is to extract the signal from the SONET network and de-multiplex

it. Next, the signal is sent through a Smart-stream Encryptor Modulator for decryption. The

output uses the Asynchronous Serial Interface protocol for transmission. At this point, the local

content, PEG content, and advertisements are added to the signal and it is once again sent

through a Smart-stream Encyptor Modulator, this time for encryption. This signal, along with

the VOD and EAS signals are then routed to the Video Serving Office (VSO).

    As mentioned, the EAS signal is received at the VHO and routed to the VSO, but first it is

decoded and then demodulated into an analog and a digital signal. This is to ensure that both

analog and digital types of receivers will pick up the emergency signal. The digital signal is

primarily used with digital televisions and set-top boxes. The analog signals are used by

standard televisions. As the signal enters a home, if a set-top box is used, the digital signal will

simply change the channel of the set-top box to that of the EAS system. If a box is not used, a

RF generator will cause the current channel to be preempted with the EAS signal. The VHO

portion of the FTTP process is concluded once the appropriate content is routed to the VSO,

bringing the video portion of FTTP one step closer to the end user.

Video Serving Office (VSO)

    The Video Serving Office (VSO) is the next step in the process of the signal generation. For

a given city or geographic area there will only be one VHO; however, there will be several

VSOs. The VSOs are usually placed where current telephone company Central Offices (COs)

are located. This is because COs are the origin of the phone and data (Internet) signals for a

customer. Since a CO services only a specific area of a city or region, there needs to be a VSO

for every CO so that all the customers in a fiber region will be able to receive service. This also

makes it easier to tie the signals together, and reduces facility and maintenance costs.

      The data signal is fed into a router, such as the Juniper ERX-1440, and is then sent to the

Optical Line Terminal (OLT). The Juniper ERX-1440 Broadband Services Router is a high

performance 40Gbps routing platform designed to support large central office deployments. Its

JUNOSe Operating System can handle the bandwidth demands of multiplay services and can

support a full suite of Internet routing protocols, including BGP-4, IS-IS, OSPF, and RIP. It is

capable of offering security services for Internet Protocol television (IPTV), and also supports

Gigabit Ethernet speeds.

    Along with the data stream, the telephone signal is also brought into the OLT which

combines the signals and converts them to an optical wave form. The Motorola AXS2200 is an

example of a leading edge OLT being used today. The light wave is then sent to a wave division

multiplexer (WDM). The video signal from the VHO, already a light wave, is also sent to the

WDM, but only after being run through an erbium doped fiber amplifier (EDFA) to strengthen

the signal. From here the signal leaves the VSO and heads toward the customer’s premises.

Figure 3 - Conceptual breakdown of VSO and customer premises components

Customer’s Premises

    Before a fiber reaches the customer's premises it is first routed to a distribution hub. These

hubs are placed throughout the VHO's territory and their task is to split the fiber into multiple

output fibers, which provides distribution to multiple customers from one single input fiber.

     Physically, a hub is an outdoor distribution enclosure, like the OmniReach Fiber

Distribution Hubs produced by ADC. These are located in the local neighborhoods or

geographic areas of the intended customer’s premises (see Figure 1.) The OmniReach hubs use

centralized splitting, as opposed to cascading splitting, because of the advantages centralized

splitting provides. These advantages include higher efficiency rates for the OLN cards, easier

testing and troubleshooting, minimization of signal loss, and a reduction of the overall number of

network components. Splitting within the hubs is carried out by Passive Optical Network splitter

modules taking a 1x32 approach. This means that one module will split a single signal into thirty

two individual fibers (see Figure 2.) These 32 fibers are routed to distribution terminals that are

placed within a few hundred feet of a customer’s premises. When a customer requests fiber

optic service, a “drop”, either aerial or buried, will connect the distribution terminal to the

Optical Network Terminal (ONT) that is placed on the customer’s residence. Hence the

acronym FTTP (Fiber To The Premises.)

    Fiber cables are laid from the VSO to the hub using either aerial or buried pathways. The

individual fibers are fused or spliced together to create a network of continuous glass from one

point to the other. At the VSO and the hub, the fibers are terminated using an Angled Polished

Connector (APC.) These connectors are a snap-in type of connector that provides for easy

connection and re-connection to other fibers (see Figure 3.) The APC snaps into a small coupler

that allows another APC to be snapped in on the other side. The end of the fiber in the APC is

polished and angled so that when two APCs are coupled together, the angles line up and mate in

order to form a continuous fiber strand (See Figure 4.) The angle of the connection helps to

eliminate reflection of the signal and “ghosting;” which is a projection of a faint signal past the

actual ending point of the fiber. This is similar to looking into a mirror and seeing several, faint

“ghost” images behind the original image.

    Like copper lines, there is a certain amount of loss of signal associated with fiber

connections and the distances of the networks. For each connector there is an average loss of 0.3

dB. Using singlemode fiber you can expect losses of about 0.5 dB per km for 1330 nm, to 0.4

dB per km for 1550 nm. Splice losses can range from almost nothing to several dB, but

generally, any loss over 0.3 db is unacceptable (see Figure 5.) The splitters used in the hubs

incur a loss of about 14 db. Video services are transmitted on the 1550 nm signal and this

wavelength is most susceptible to loss. Therefore, for a network that will be providing video

service, a total db loss from the VSO to the customer premises should not exceed about 20 db.

     The customer premises are like the end of the funnel of fiber optic information. The video,

data, and voice signals that have been channeled down the network are now separated into their

constituent electrical signals. This process begins with the signal from the hub's splitter being

sent on an individual fiber to an Optical Network Terminal (ONT) that is located on the outside

of a customer’s building. The ONT acts as a demodulator that separates the three signals.

Figure 4 - Actual ONT model in usage

    One ONT that is currently being used is the ONT1000V Optical Network Terminal

produced by Motorola. The major functions of the ONT1000V include Triplexer, PON MAC,

and telephony. The ONT is able to access the three wavelengths of the PON (1310 nm, 1490

nm, and 1550 nm) by utilizing the Triplexer. This contains an analog detector, data detector,

upstream laser, and wave division multiplexer. Data on the PON is handled by the PON MAC

within the ONT, which filters traffic intended for the ONT and disregards data intended for other

devices. The PON MAC also takes care of upstream data, making sure that it is formatted

correctly and synchronized. Analog voice traffic, also known as Plain Old Telephone Service

(POTS), is handled by the telephony functionality. Analog voice waveforms are converted into

digital data packets, enabling the ONT to provide telephone service over fiber. A digital signal

processor makes the conversions between analog and digital voice streams possible, while also

having the capability to detect dial tones and busy signals. The ONT100V is capable of handling

four separate telephone lines. Besides telephone jacks, the ONT also has a connection for a RJ-

45 Ethernet connection, for computer networking and Internet usage, and also a cable-out jack,

for connecting cable television to the customer's premises.


    The future of fiber is very promising and exciting. The developments and discoveries that

led to the harnessing of the power of light are as amazing as the technology itself. The

commercial use of fiber and its acceptance in high requirement situations have paved the way for

the next phase in the history of fiber, the FTTP movement. This deployment is redefining who

gets to use fiber and for what purposes. FTTP is intriguing to study, and the ways in which all of

the components come together is an engineering marvel. It certainly seems that the future will

be 'brighter' because of the rising popularity of fiber.


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                           Figure Caption

Figure 1. Hub

Figure 2. Four Splitters

Figure 3. Angled Polished Connector

Figure 4. Two APCs snapped in a coupler

Figure 5. Four fiber splices in a splice tray

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