Fiber- Optics

*** also see http://www.fiber-optics.info/ 

Laser light sources (transmitter), Fiber Optic cables, and optical readers (receiver) are the three components used for optical data communications.  These three components allow a unique blending of light and glass that transmits our words and thoughts, orders and memos, data and video anywhere in the world, immediately. Fiber is so tiny and light, compared to bulky copper cables.  Can you imagine the days when they laid copper cables like the one shown below, across hundreds of miles?  


Copper vs Fiber

Cable Type   

Pairs of Copper/Fiber   

Voice Channels   

Diameter (inches)   

Weight (lbs) per km

Copper   

900

21,000

3

16,000

Fiber   

12

290,000

0.5

450

A single optical fiber combined with fiber amplifiers can carry tens of millions of phone conversations, tens of thousands of television channels or transmit an entire encyclopedia in one-thousandth of a second.  In addition, recent cabling advances have produced bundles with 1728 fibers !!  Optical Fiber is now the answer to the never-ending quest for high-speed data transfer.  Theoretically, there is no limit to the bandwidth - since light travels virtually instantaneously from one point to another.  

Theoretically, one fiber, about the width of a human hair, is capable of carrying every call, made every day, in the United States. 

Frequency of Light vs other methods - light has an incredible frequency capability of 300 trillion Hz  !!!  Compare this with several billion Hz for Microwave, and 500 million Hz for RF.

Electrical Conversion - the Limiting Factor - no one has ever devised a "pure optical" system.  At the receive end of every fiber, there has to be a way to convert digital light pulses into data that can be stored, computed, massaged, and read by humans - and vice-versa for the transmit end.  Unfortunately, this means E/O (Electrical to Optical) electronic equipment is required.  So that's the limiting factor - electrical systems data throughput can't even come close to the data rates that optical systems can deliver. 

How much Fiber is in the Ground - by the end of 1998, there were more than 215 million kilometers of optical fiber installed for communications worldwide. Today there is so much installed fiber that a "fiber glut" has developed, mostly in the bog cities (rural areas still have a shortage of fiber).  The optical fibers transmit light pulses up to 13,000 miles, and are handling data rates that are doubling each year. Today, optical fibers are the best conduit for delivering an array of interactive services, using combinations of voice, data, and video.

*** see also the SONET section - since that is the de-facto standard used with most long-distance fiber-based networks

There are several advantage of fiber over copper:

SPEED: Fiber optic networks operate at high speeds - up into the gigabits
BANDWIDTH: large carrying capacity
DISTANCE: Signals can be transmitted further without needing to be "refreshed" or strengthened.
RESISTANCE: Greater resistance to electromagnetic noise such as radios, motors or other nearby cables.
MAINTENANCE: Fiber optic cables costs much less to maintain.

Distances

Depending on the equipment and application, fiber can be used to connect two devices whose distance of separation can range anywhere from one meter up to thousand of miles.  For very long distances - repeaters are used to reshape and amplify the signals. 

The type of transmit device (light emitter) and fiber (multimode or singlemode) dictates the wavelength and the distance.  There are two types of light emitters - Lighy Emitting Diodes (LED's) and Laser Diodes.  The electrical signal goes through the diode, creating light, which is then sent into the fiber for transmission.

 Speed Limitations - it's the Electronics

Unfortunately, we have no way of dealing directly with fiber on an optical level.  All the data communications equipment relies on electrical impulses, and the circuitry that converts those pulses to light is limited by the traditional electrical maximums.  R&D work is continuing on a pure optical amplifier/repeater, but so far it is still just that  .  .  .  R&D.  There are, however, PON's in existence (Passive Optical Network) that merely split the fiber into multiple streams along the way, without the use of electronics (see http://www.ponforum.org/ ).

Light and the conduits that carry it - optical fibers - are immune to magnetic and electrical interference.  But electricity and the conduits that carry it - wire and circuit board components - are subject to noise, crosstalk, and extranneous capacitance and inductance.  At extremely high frequencies, even a tiny 1/16"  wire is subject to all such interferences.  This affects even a small digital amplifier, when you add up all the tiny traces on circuit boards, metallic and semiconductor material in the chips, etc.

But despite the electrical limitations, fiber is a godsend.  Electronics equipment long ago outpaced all existing cabling.  Vendors built super-fast transmitters/receivers . . . but copper cable standards (and gold, silver, etc.) were unable to support those speeds.  But now, with the advent of optical fiber, electrical equipment is no longer limited by the medium. 

Full-Duplex

Normally, 2 strands of fiber are used for a connection, one for transmit, and one for receive.  This allows uninterrupted data flow in both directions simultaneously (Full Duplex).

But what if you had only one strand of fiber? Would that single strand of glass be completely worthless? No, by using a device called a "fiber singler" or "single fiber full duplex unit" you can actually send traffic in both directions over a single piece of glass. Traffic going in one direction is sent on one wavelength (e.g., 1310nm), while traffic going in the other direction is sent on a different wavelength (e.g., 1550nm).  But keep in mind that virtually all major telecommunications providers use full-duplex facilities.

Asynch Facilities  vs  Linear SONET vs SONET Rings

Asynch Facilities (Asynchronous) is an industry term that is a misnomer - it does not refer to asynchronous communications !!!.  It is a straight line point-to-point circuit.  It rides on a 45 Mbps T3 typically, (which many call synchronous but in reality, a T3 is plesiosynchronous, or "almost synchronous") or on a 155 Mbps OC3.  However, the term sometimes refers to any access or POP-to-POP circuit that it is not on a fiber ring.  Often confused with the term, Linear SONET - but keep in mind that asynch rides on copper, and Linear SONET rides on fiber.

Linear SONET - SONET that is not on a ring.  It is susceptible to failure since there is no way to restore a fiber cut, other than to send out a repair crew and wait.

SONET Rings - two or four fiber, bi-directional rings.  4-fiber rings with reverse protection switches are capable of near-instantaneous restoral - even with a full fiber cut !!