This lecture seeks to introduce the technology behind the operation of fibre optic technologies.

7. Applications Layer: The layer of most interest to users as this is where services such as e-mail and file transfer are to be found. It is where the applications are stored. It provides access to the OSI environment for computer users.

6. Presentation Layer: Concerned with the representation of data i.e. the way that it is stored on the particular computer. End users may have different data representations; this layer converts both to a common language. Encryption and compression are carried out at this level.

5. Session Layer: Manages and terminates connections (sessions) between co-operating applications. It is the layer where logon procedures reside and verifies that users have the right to proceed to use higher levels in the protocol stack, password verification for instance.

4. Transport Layer: Allocates resources to the network layer, for example it can decide the quality of service that a particular connection will have and generally isolates upper layers from the physical network. It provides reliable end-to-end transfer of data, error recovery and flow control.

3. Network Layer: It provides the upper layers with independence from the data transmission and switching technologies that are used to interconnect systems. It is also responsible for establishing, maintaining and terminating connections.

2. Data Link Layer: The layer which is responsible for reliable transfer of information across the physical link and sends blocks of data (frames) with the necessary synchronisation, error and flow control. The hardware address of a Network Interface Card is dealt with in this layer. Medium Access Control (MAC) is also dealt with here. This deals with the method by which any node on the network can gain access to the shared bandwidth of the network - e.g. CSMA/ CD, Token Ring etc.

1. Physical Layer: This layer concerns the method of transmission of the unstructured bit stream across the network. It deals with voltage levels, data structures, encoding schemes such as Manchester Encoding, bit timing details and hardware details such as pin numbering, shape of plugs, specification of the cabling and frequency bands for unguided transmission lines such as microwave etc.

Carriers and the Technology

Without some sort of carrier there can be no WAN. The function of the carrier is to transport the data between two end points, regardless of the distance in between these end points.

In the early days of computer networking during the 1960s, the computers were linked using standard telephone lines which were twisted pair with perhaps some coaxial sections on the trunk routes. This technology gave the lowly data transfer rate of 56 kbits/ second. However, this was a connection between fairly low specification machines (compared to today) and the data that was to be transferred would not have been as large as that which we send today.

As time passed, the number of computers worldwide increased and the requirement for higher bandwidth connections increased too.

The figure above shows the frequency ranges that are occupied by various transmission technologies. As the frequency increases, the rate at which information may be transferred increases too.  It can be clearly seen that  optical fibres occupy the higher part of the spectrum at 10¹⁴ to  10¹⁵  Hz.

Sometimes the spectrum is referred to in terms of frequency (hertz), sometimes in terms of wavelength (metres).

To convert between these values we need to use a simple formula

The speed of light is 300,000,000 metres per second in vacuum. This is usually expressed mathematically as 3 x 10⁸ metres per second.

The wavelength is usually referred to in terms of micrometres (m = 10^-6) or nanometres (nm = 10^-9).

The frequency is usually referred to in terms of megahertz (Mhz = 10⁶) or gigahertz (GHz = 10⁹) or terahertz (THz = 10¹²).

Example:

The lower window for FO operation is around 850 nm. what is the frequency of this window?

The equation given above must be rearranged to put f on one side of the equation and the other 2 terms on the other side of the equation.



This is the same as 353 THz.

Try calculating the frequencies of the second and third windows used by fibre optic - see diagram.

Fibre Optic

It had been demonstrated during the 19th century that light could be carried using optically transparent materials and could therefore be used as a signalling method. It was not until 1970 that the first material, pure silica, was perfected for carrying light pulses over a long distance without too much attenuation.

The telecommunication companies viewed fibre optics with scepticism at first, but after a while decided to adopt fibre optic cabling as their backbone infrastructure.

The principle behind fibre optic communication is relatively simple. The light is shone into the core of the cable and is unable to escape due to the effect of Total Internal Reflection, TIR. This keeps the light within the fibre until it reaches the far end.

The purity or clarity of the glass is one factor that limits the distance that the light may travel before it required to be regenerated (strengthened).

The principle of reflection is fairly simple.  Light has been demonstrated to travel in straight lines. If a ray of light strikes a reflective surface, the angle to the normal of the incident ray is equal to the angle to the normal of the reflected ray.

The normal is a line that is drawn perpendicular to the reflective surface ie at right angles or 90 degrees. See the diagram below for an explanation of this phenomenon.


This is known as the law of reflection

The angle of incidence is equal to the angle of reflection.

Refraction of Light

The direction of a ray of light can also be changed when the light travels between one transparent medium and another e.g. air to water or air to glass. If you observe a pencil in a glass of water, it will appear to be bent or displaced due to the effect of refraction. The picture below illustrates this.

What is happening in scientific terms is that the rays of light crossing the boundaries between water and air, glass and air and water, glass and air have been bent from the straight lines that we assume they will take. This gives us the distorted image seen above.

The speed of light is different in different materials. The measure that we use c is for light travelling in a vacuum. The speed of light in water, glass, air or any other material than a vacuum is always less than c. As light enters a block of glass, it slows down, but speeds up again as it leaves. The diagram below illustrates this.

As the ray of light enters the block of glass, it is slowed. This causes the ray to be bent towards the normal. It can be seen that the angle b is less than the angle a.

As the ray of light leaves the block on the far side, it re-enters the air and speeds up once more. This means that as a ray of light leaves a dense medium such as the glass and re-enters the air (less dense) the angle of incidence b is less than the angle a.

This effect can be seen clearly  when you run a bath of water.  The water appears to be shallower than it really is and  the apparent positions of objects below the surface of the water  are not the same as their real positions. The diagram below illustrates this.

This leads us to the conclusion that light is bent away from the normal as it leaves a dense medium and enters a less dense medium.

Now we must take the conclusion above to its limits to explore the way in which fibre optic cable works. We shall now increase the angle at which the ray of light leaves the denser medium and explore the consequences. We shall use glass as the dense medium and air as the less dense medium.


In the diagram above, it can be seen that the ray of light is bent away from the normal as it leaves the glass. In other words, angle b is greater than angle b. Now we shall increase the angle of incidence a.

As we increase angle a, it can be seen that we will reach a point where the light leaving the glass travels parallel to the surface of the glass. At this point the angle b is 90 degrees.

When this condition is reached, the angle a is known as the critical angle for the denser medium, in this case glass.

Now we shall increase the angle of incidence a so that it is greater than the critical angle.

It can now be seen that as the angle of incidence increases beyond the critical angle for the material, the ray of light is reflected back inside the material and does not exit.

In this condition, i.e. above the critical angle, a = b

The ray of light has undergone total internal reflection and stays within the glass.

The principle that is displayed above is the underpinning principle of fibre optic communications.

To calculate the critical angle, it is necessary to divide the  refractive index of the less dense medium  (air) by the more dense medium (glass) and this gives us the sine of the critical angle. Once we know the sine of the critical angle, we can take the inverse sine of this figure to gain the correct angle.

The critical angle is often denoted as

Transmission Characteristics of Fibre Optics

The fibre carries a signal encoded beam of light by virtue of total internal reflection. TIR occurs at the interface of a transparent medium with another medium. If the transparent medium has a higher index of refraction than the surrounding medium, TIR occurs. This makes the optical fibre a waveguide for frequencies in the range 10¹⁴ to 10¹⁵ Hz, covering the visible and part of the IR spectrum.

The diagram above illustrates the principle of total internal reflection as used in fibre optic communications. The shallow angles of the light entering the FO cable are reflected totally and will not leave the cable until they reach the far end of the cable.

Frequency of Operation

Light travels best through fibres in three separate 'windows'. These windows are frequency ranges within which injected light is attenuated least. This attenuation, surprisingly, is due to light absorbtion by water molecules that are found within the glass, although the very latest technologies are managing to address this problem.

The diagram below depicts the three separate windows (plus a forth window at 1625 nm which is currently under development).

Image copyright Cisco Systems
The optical fibre spectrum

The attenuation can be seen in the graph below that plots attenuation against wavelength.

Image copyright Cisco Systems

 Optical loss versus wavelength

In the graph above, the Rayleigh scattering is due to imperfections in the fibre strand's density that occurred during the manufacturing cooling process. These imperfections absorb and scatter the light whilst in transit to the destination. This scattering of the light affects shorter wavelengths (higher frequency) more than longer wavelengths (lower frequency) and limits the use of wavelengths below 800 nm.

Above 1700 nm the glass itself absorbs the energy of the light so there is only a certain frequency range that is available for use for optical communications.

Conclusion

We have seen that the wavelength of light is related to its frequency by the equation

We have seen that light will travel in straight lines and is affected by reflective surfaces. When light hits a reflective surface, the angle of incidence is equal to the angle of reflection. These angles are measured with respect to the normal which is perpendicular to where the light hits the reflective surface.

We have also seen that light is bent as it enters another transparent medium. If the medium is more dense, the light is bent towards the normal. If the medium is less dense, the light is bent away from the normal.

If the angle of incidence in a dense medium is increased beyond a certain value, the light will no longer leave the dense medium and remains completely within it. This is known as total internal reflection. This is the basis of fibre optic technologies.

All materials have a property called the refractive index. This can be used to calculate the critical angle when two transparent materials share a common boundary.

Light travels in fibre optic media in three distinct windows.

Further Reading

Tannenbaum, Computer Networks:

Transmission media p82

Fibre optics vs. copper cable p92

Alcatel, France Telecom and Deutsche Telekom perform 
record optical transmission field trial and test ultimate limits 
of European fiber infrastructures                                                                                                

The goal of this research project is to investigate advanced ultra-high channel bit rate transmission technologies (170 Gbit/s or more per channel) and to demonstrate Terabit per second (1 000 Gigabits per second) network transmission capacity over installed fiber infrastructure. The trial announced today resulted in a point-to-point DWDM (Dense Wavelength Division Multiplexing) transmission of 1.28 Terabits per second over a 430 km standard (ITU-T G.652) single mode fiber link in France Telecom's network in the Marseille area.

Test results were achieved using eight densely packed optical transmission channels, each carrying a record bit rate of 170 Gbit/s (160 Gbit/s plus an overhead so as to allow bit error detection and correction). This channel bit rate is 16 times higher than the 10 Gbit/s channel bit rate of current standard DWDM transmission products. As an example, each 160 Gbit/s transmission channel could transport the content of 4 DVDs in about 1 second.

The TOPRATE field trial demonstrated that the severe sensitivity to transmission impairments associated with ultra-high channel bit rates can be managed over long distances and that traditional single-mode fiber - the standard transmission medium in the networks of France Telecom and Deutsche Telekom - is future proof with respect to a further increase of channel data rates to be used by the next generations of metro/core transmission equipment. This enables a cost-effective network upgrade solution for coping with the growing bandwidth demand and also results in the protection of the huge investment in the installed fiber bases - which were originally designed for much lower capacities. This pioneering field experiment also confirms Alcatel's leading research expertise in advanced optical transmission technologies.

Joëlle Gauthier, Alcatel's Vice President Research & Innovation commented that, " Research collaborations like TOPRATE are fruitful for Alcatel as they provide an opportunity to validate very advanced technologies, such as 160 Gbit/s transmission, with field trials in our customer's networks. It accelerates technology maturation and allows us to integrate customer's needs at a very early stage in the development cycle."
Pascal Viginier, Director of R&D Division of France Telecom, illustrates the motivation for France Telecom: "Increasing the bit rate per wavelength channel leads to potentially better equipment integration, higher capacity and lower network costs. So as to enable new high-bit-rate services combining voice, data and video, France Telecom needs to increase its network capacity at the lowest costs: this is why we analyze new disruptive technologies including 160 Gbit/s." One key issue of the trial was the compatibility of installed fiber infrastructure with such high channel bit rates over long distances: "Marseille field trial demonstrated the great potential of France Telecom fiber infrastructure, thus allowing smooth network growth for many years."

The joint work of France Telecom and Deutsche Telekom within this project also illustrates the cooperation between both operators in certain fields of research and development, which was officially announced in January 2004. "Toprate is a good example of our research collaboration with Deutsche Telekom" added Pascal Viginier.

About European research project TOPRATE:
The TOPRATE project (IST-2000-28657) is conducted within the European research program (5th Framework Program). The TOPRATE consortium includes Alcatel research facilities in Germany, which is project co-ordinator, Alcatel research facilities in France, France Telecom, Deutsche Telekom AG, the Heinrich-Hertz-Institute (Fraunhofer Gesellschaft, Germany), COM (Technical University, Denmark), VPISystems (Germany), Universidad Politecnica de Valencia (Spain). For more information about TOPRATE see following link: http://dbs.cordis.lu

About Alcatel
Alcatel provides communications solutions to telecommunication carriers, Internet service providers and enterprises for delivery of voice, data and video applications to their customers or employees. Alcatel brings its leading position in fixed and mobile broadband networks, applications and services, to help its partners and customers build a user-centric broadband world. With sales of EURO 12.5 billion in 2003, Alcatel operates in more than 130 countries.
For more information, visit Alcatel on the Internet: http://www.alcatel.com

Alcatel Press Contacts
Aurélie Boutin / HQ Tel :+ 33 (0)1 40 76 11 79 Aurelie.Boutin@alcatel.com
Mark Burnworth / HQ Tel :+ 33 (0)1 40 76 50 84 Mark.burnworth@alcatel.com

About Deutsche Telekom
Deutsche Telekom is Europe's largest communications company and one of the largest communications carriers worldwide based on 2003 revenues of 55.8 billion Euro. The company is currently active in four business units: fixed line, mobile, IP services for mass markets and integrated IT and TC solutions. From 2005 on the group will be active in the strategic areas Broadband/Fixed-network, Mobile Communications and Business Customers. The business unit T-Com offers its customers a complete range of fixed-line voice telephony products and services with about 58 million access lines (customer figures as of June 2004). The company is a leading provider of high-speed digital access lines, with currently almost 5 million asymmetric digital subscriber line (T-DSL) services sold and almost 22 million channels using the information transfer standard ISDN (Integrated Services Digital Network). Through T-Mobile, Deutsche Telekom's mobile telephony subsidiary, and through other majority and minority shareholdings, Deutsche Telekom group today serves worldwide far above 71 million mobile telephony customers worldwide. T-Mobile International with its USA subsidiary T-Mobile USA is the first transatlantic operator utilizing the GSM digital wireless technology standard. T-Online is one of Europe's largest Internet service providers with about 13.3 million subscribers. T-Systems is Europe's largest provider of comprehensive IT and telecommunication solution and services to business customers in more than 20 countries. For more information about Deutsche Telekom, visit www.telekom.de/international.

About France Telecom
France Telecom is one of the world's leading telecommunications carriers, with 121.5 million customers on the five continents (220 countries and territories) and consolidated operating revenues of 46.1 billion euros for 2003 (23.2 billion euros for 1st semester 2004). Through its major international brands, including Orange, Wanadoo, Equant and GlobeCast, France Telecom provides businesses, consumers and other carriers with a complete portfolio of solutions that spans local, long-distance and international telephony, wireless, Internet, multimedia, data, broadcast and cable TV services.
France Telecom is the second-largest wireless operator and Internet access provider in Europe, and a world leader in telecommunications solutions for multinational corporations. France Telecom (NYSE: FTE) is listed on the Paris and New York stock exchanges.

NEW WORLD RECORD FOR INTERNET PERFORMANCE SET

Caltech and CERN top new performance threshold by sending 859GB at more 
than 6.6 Gbps across nearly 16,000 km

Ann Arbor, Mich.  September 1, 2004  An international team has broken
their own record and set a new Internet2(R) Land Speed Record by 
transferring 859 gigabytes of data in less than 17 minutes across nearly 
16,000 kilometers of networks at a rate of 6.63 gigabits per second, 
about 10,000 times faster than a typical home broadband connection. The 
record was set by a team consisting of members from the California 
Institute of Technology (Caltech) and CERN using the same IPv4 protocols 
deployed throughout the global Internet.

The Internet2 Land Speed Record (I2-LSR) is an open and ongoing 
competition for the highest-bandwidth, end-to-end networks, with judging 
based on the speed of transfer multiplied by the distance traveled. 
Because of delays due to the speed of light and other factors, data 
transfer over the Internet becomes more challenging as speed, or 
distance, or both increase. With a mark of more than 104.5 
petabit-meters per second, this record is the first time the 100 
"petabump" performance threshold has been broken.

The record was set with the support of Microsoft Research 
(research.microsoft.com), S2io (www.s2io.com), Intel (www.intel.com), 
Cisco Systems (www.cisco.com), AMD (www.amd.com), Newisys 
(www.newisys.com), the U.S. National Science Foundation (www.nsf.gov), 
the U.S. Department of Energy (www.doe.gov), the European Union through 
the DataTAG project (www.datatag.org), and the Corporation for Education 
Network Initiatives in California (www.cenic.org).

More information can be found at: 
http://ultralight.caltech.edu/lsr_06252004/

Details of past winning entries, complete rules, submission guidelines, 
and additional details are available at: http://lsr.internet2.edu/

ABILENE NETWORK UPGRADE TO 10 GBPS COMPLETE

New OC-192 circuits provide Abilene participants leading-edge networking
capability

WASHINGTON, D.C. - February 4, 2004 - Abilene, the most advanced research
and education network in the United States, today announced the completion
of its upgrade from 2.5 Gigabits per second (Gbps) to 10 Gbps.  The
Internet2(R) backbone network upgrade quadruples the capacity to more than
15,000 times faster than a typical home broadband connection.  Abilene
partners, Indiana University, Juniper Networks and Qwest Communications,
provided the equipment and services to successfully implement the network
upgrade.

Web Resources

Abilene Networks

Cisco Networks DWDM Technology

An excellent resource: World Network Maps

An Atlas of Cyberspaces

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