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Technology Watch
The optical world*
 
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Optical technologies are the driving force behind the bandwidth growth of the Internet. Indeed, the Internet as we know it would not be feasible without optical technologies and the related networking standards. This is made clear in “The Optical World”, a Technology Watch Report published in June 2011 by ITU’s Telecommunication Standardization Sector (ITU–T). By enabling bandwidth hungry applications for video, optical technologies have opened the way for new business models such as YouTube, allowing users to share video clips.

ITU–T standards in optical transport networks have played a leading role in transforming the Internet’s bandwidth capabilities. This work is led by ITU–T Study Group 15, which has developed a set of international standards (Recommendations) that defines the existing optical transport network (OTN) framework. ITU–T Study Group 15 is now working on future technologies (such as gigabit-capable and 10-gigabit-capable passive optical networks), anticipating unprecedented bandwidth demands from service providers and consumers.

Optics is the science of light and has a very long history. Optical networking, silicon photonics (light technology), nanotechnologies and non-linear optics are all areas where breakthroughs could lead to major changes in the way computers, networks and data centres are designed. The new ITU–T Technology Watch Report provides an overview of the optical world, and surveys standards and ongoing research that will lead to a new generation of Internet and computing devices. Some of the challenges the world faces in energy, health and the environment may be solved with the help of optics and photonics.

Why optical technology?

Today, the most widely used optical technology is optical fibre for high-speed interconnections, such as in server racks, connecting offices, buildings and metropolitan networks — and even continents via submarine cables. Optical technology is also used in CD-ROM drives, laser printers, photocopiers and scanners. But there are always some conventional electronic circuits or components in these devices because, to date, high costs have prevented optical components from finding their way into computers.

Now that optical technology is maturing, prices are falling and optical alternatives are starting to be incorporated into computer systems. Compared with copper wire and electronics, optical equipment has many advantages for use in communication networks and computers: less power consumed; higher speeds; more compact; much lighter; cheaper to manufacture; higher storage capacity; immune to electromagnetic interference; free from short circuits; lower operational costs for service providers; and reduced carbon footprint for telecommunication.

Optical technology to meet broadband demand

The gradual incorporation of optical technology into the world of traditional electronics is paving the way for the era of the optical world. Industry experts see optical technology as the most feasible means of solving the bandwidth limitation of electronics. Photons have low-loss transmission and provide large bandwidth, offering multiplexing capacity for communicating several channels simultaneously without interference. In terms of speed, data travel through an electronic network at megabits per second (Mbit/s), as compared to terabits per second (Tbit/s) in photonic networks. The power requirement for an electronic switching component is about one microwatt, whereas the power requirement for a photonic switching component is measured in nanowatts.

The ever-increasing need for computational speed, coupled with the rapid increase in global demand for Internet access, television and video services, and nextgeneration broadband, has focused interest on optical computing technology. Service providers in the telecommunication sector have transformed their offerings into a new business model for triple-play services (combining voice, data and video) that rely on sufficient bandwidth to carry large amounts of data. As telecommunication networks expand on a global scale, optical communication will continue to be a strategic enabler for nextgeneration Internet services.

Because of its advantages over electrical transmission, optical fibre is rapidly replacing copper wire in core networks in the developed world and is key for developing countries in bridging the digital divide. The high frequency of the optical carrier enables significantly more information to be transmitted over a single channel than is possible with a conventional radio or microwave systems.

Communication networks using optical fibre still need to convert the electrical signal into an optical one for transmission, and then back into electrical form at the receiving end. This means that the potential bandwidth of optical fibres is not being fully exploited. Future research and standardization work will focus on developing purely optical devices for communication networks.

Current and future standardization activities

 

Optical transport networks: From timedivision multiplexing (TDM) to packet

The ITU–T manual “Optical Transport Networks: from TDM to Packet”, launched in February 2011, is based on the standards issued by ITU–T on optical transport networks. It covers synchronous digital hierarchy (SDH), optical transport network (OTN) and Ethernet over transport (EoT) Recommendations, which are now used in telecommunication networks. The manual describes the problems associated with implementing OTN structures, and shows how ITU–T Recommendations address these challenges. It offers practical help and guidance to operators, suppliers and management in planning and implementing OTN. The manual is available at:
http://www.itu.int/publ/T-HDB-IMPL.08-2010/en.

 

Optical transport networks

Synchronous optical networking (SONET), adopted as the backbone of most fibre-optic telecommunication networks in the late 1990s, was originally designed for optical interfaces that used a single wavelength per fibre. As fibre-optic technology has advanced, it has become more economical to transmit multiple SONET signals over the same fibre. The optical transport network (OTN) architecture for this is specified in Recommendation ITU–T G.872.

OTN is the only transport layer in the industry that can carry a full 10/40/100 Gbit/s Ethernet signal from Internet Protocol (IP)/Ethernet switches and routers at full bandwidth. It is composed of a set of optical network elements connected by optical fibre links. The OTN framework is based on a set of Recommendations with ITU–T G.709 “Interfaces for the Optical Transport Network (OTN)” at the heart.

Recommendation ITU–T G.709 describes a means of communicating data over an optical network. Revised in June 2010, it now provides a mapping of a recent next-generation high-rate Ethernet standard from the Institute of Electrical and Electronics Engineers (IEEE) into OTN. Collaboration between ITU–T Study Group 15 and the IEEE P802.3ba 40 Gbit/s and 100 Gbit/s Ethernet Task Force has ensured that these new Ethernet rates are transportable over optical transport networks. A manual on OTN highlights the applications of ITU–T Recommendations in this field.

Optical communication systems reaching terabit speeds could become the norm in the future. A breakthrough in terms of data transfer speed in optical communications was announced in the May 2011 issue of the Nature Photonics journal: researchers at Karlsruhe Institute of Technology established a record by transmitting 26 Tbit/s using a single laser. Optical communications at terabit speed would be highly benefi cial to bandwidth-hungry applications such as cloud computing, three-dimensional television (3D TV), highdefi nition television (HDTV) and virtual reality. The breakthrough could pave the way for future standardization work in this area.

 

Optical fibres cables and systems

ITU–T has been active in the standardization of optical communication technology and the techniques for its optimal application within networks from the infancy of this industry. Since the first ITU–T Handbook related to optical fibres, “Optical Fibres for Telecommunications”, published in 1984, several others have been produced over the years to keep pace with developments in the field.

The latest manual, “Optical fibres, cables and systems”, was published in 2009. It aggregates the available information on ITU–T’s work, and is intended as a guide for technologists, middle-level management and regulators. ITU–T also conducts tutorials for developing countries based on the manual, to provide an in-depth insight into the application of its Recommendations in this field.

The manual aims to assist in the installation of optical fibre-based systems, explaining how ITU–T Recommendations address the practical issues associated with the application of this technology. The manual can be downloaded free of charge at:
http://www.itu.int/pub/T-HDB-OUT.10-2009-1/en.

 

Automatically switched optical network

Backbone networks must offer quality of service and flexibility. The purpose of the automatically switched optical network (ASON) is to automate resource and connection management within the network to preserve quality of service. This will be a huge challenge because, according to the annual Cisco Visual Networking Index, global IP traffic was an estimated 242.2 exabytes (1 exabyte = 1018 bytes = 1 billion gigabytes) in 2010 and is projected to increase to 965.5 exabytes by 2015.

The main issue in the area of standardization for ASON has been the optical control plane. Work to define the standards for the optical control plane is being investigated by international standards bodies, including ITU–T, which is working on ASON architecture.

Controlling quality of service is one of the essential elements for managed IP networks. ITU–T recently achieved significant results, in cooperation with IEEE, in standardizing the operations, administration and maintenance functionality for Ethernet networks. This lies at the core of IP packet transmission platforms and is an essential technology for next-generation networks. A recent manual highlights ITU–T Recommendations in this field.

Passive optical networks

A passive optical network (PON) extends from an operator’s central facility into individual homes, apartment buildings and businesses. One common application of PONs has been in providing broadband Internet access to homes for applications such as IP television.

The advantage of PONs is that they offer high reliability and high bandwidth at low cost. Some gigabitcapable PON products already include a power-saving mode, which reduces power consumption. The most recent standard, G.984 (GPON), represents a significant boost in total bandwidth and bandwidth efficiency, through the use of larger, variable-length packets. A GPON network delivers up to 2488 Mbit/s of downstream bandwidth, and 1244 Mbit/s upstream.

Optical exchanges

Peering of regular Internet traffic has led to switching-based Internet exchanges. Optical exchanges on the other hand will not require switching technology at peering locations. With the fall in cost of optical devices, they could become viable as components in Internet exchange points. An optical exchange is a peering location that allows traffic to pass from one provider to another in a connection-oriented manner. An example is NetherLight in Amsterdam.

The NetherLight Exchange Point is part of the Global Lambda Integrated Facility (GLIF) network. GLIF is an international organization that promotes optical networking using lambda switching, which can switch individual wavelengths of light onto separate paths for specific routing of information.GLIF uses optical multiplexing capabilities to provide the bandwidth needed for scientifi c research and collaboration on a global scale. Sites that form part of the GLIF network include CERNLight (Geneva), UKLight (London), MoscowLight (Moscow), and StarLight (Chicago).

Optical exchanges represent a new area of research for packet-based optical networks that could provide the backbone for grid computing.

Visible light communication

Visible light communication (VLC), using white light emitting diodes (LEDs), is emerging as a key technology for ubiquitous communication systems, because LEDs have the advantages of fast switching, long life expectancy, relatively low cost, and safety for the human body.

The signal would be generated in a room by slightly flickering all the lights in unison. People would not notice this, because the rate of modulation would be millions of times faster than a human eye can see. Since visible light cannot penetrate walls, there would be no interference from stray signals and reduced opportunity for outside hackers, thus making the system more secure than radio. In January 2010, Siemens researchers, in collaboration with the Heinrich Hertz Institute in Berlin, achieved a wireless data transfer rate of up to 500 Mbit/s using white LED light.

Infrared light could also be used in visible light communication. The Infrared Data Association (IrDA) has been developing technical standards for infrared wireless communication and has recently announced the GigaIR standard for very short range line-of-sight infrared communication links operating at 1 Gbit/s. Speeds of more than 1 Gbit/s have been obtained with infrared light, according to research carried out by Penn State University.

The Visible Light Communications Consortium (VLCC) and IEEE 802.15 Task Group 7 have been active in standardization work related to visible light communication. Major applications would be in private networks, home networks, sports broadcasts and live television interviews, and in optical beacons for collection of information in real time. In particular, applications could include use in restricted areas where secure access is important, the projection of high-definition images, and use in aircraft and medical facilities (because the optical system will not interfere with aircraft navigation or hospital systems). Visible light communication could also be used in global location positioning, to support intelligent transport systems.

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Photo credit: AFP
Apple’s Macbook Pro laptop that uses Intel’s Thunderbolt, a high-speed computer connection technology formerly codenamed Light Peak. This new technology is said to move digital films and other data “blazingly fast”

Optical data transfer inside the computer

New data transfer technology based on optical fibres will lead to dramatic advances in the performance and design of computers. An example is Thunderbolt (originally codenamed Light Peak), a technology that has been developed by Intel to give ordinary personal computers the ability to connect with other devices using high-speed fibre-optic cables at 10 Gbit/s — twenty times faster than a standard, copper-based USB 2.0 cable. An optic cable can drive a high-definition (HD) display or transfer an HD movie in seconds. Thunderbolt has been implemented in 2011, for example, in the new line of Apple’s MacBook Pro laptops.

Silicon photonics

The use of multi-core central processing units is one way of controlling power consumption in high-performance electronic circuits. And the low cost and high availability of silicon make it the material of choice for optoelectronic devices. Silicon photonics involves integrating optical and electronic circuits on silicon.

In August 2010, Intel announced the first complete photonic communication system developed from components fully integrated into silicon chips. Silicon photonic chips could, for instance, replace the electronic components between a computer memory and processor. They allow for the processor and memory to be further apart than is possible with copper wiring, and could change the way computer systems, laptops and data centres are designed.

The main challenge for silicon photonics is growing the laser on a silicon chip, because silicon is a poor laser material. In February 2011, however, researchers at the University of California announced that they had overcome this problem by taking advantage of the properties of nanostructures and by carefully controlling the growth process. This is the first time that researchers have grown lasers from high-performance materials directly on silicon.

Optical bus

IBM announced in 2010 that it had successfully developed an optical data bus on a printed circuit board that uses optical links for data transfer between the processor and other external components such as memory and input/output ports. By avoiding the signal-loss and cross-talk problems associated with copper, an optical bus would make supercomputers much faster. IBM is planning to use the optical bus in supercomputers for optical data transfer between printed circuit boards.

IBM is also conducting research aimed at developing parallel optical interconnects, referred to as optochips, which will enable ultra-fast data transfer between the components on the printed circuit board. The optochips are transceivers that convert the signal from electrical to optical and back again. IBM expects that within five years, the optochips will find their way into the supercomputer, replacing copper wiring as the connection between processors and memory.

All-optical computers

Light can be used to transmit, record and process information, and researchers are attempting to develop entirely new methods of computing that are not physically possible with electronics.

The main building block of conventional computers is the electronic transistor. In order to build an optical computer, an equivalent optical transistor is required. Over short distances, photons consume more energy than electrons in information processing and transmission. To overcome that challenge, researchers are exploring non-linear optical technology.

Another emerging field of research involves silicon nanowires, which can guide light like optical fibres. Because of their low power consumption and size, nanowires are expected to be used in areas such as implantable medical monitors that must be small and where fast processing is not needed.

Research on developing an all-optical computer is a multidisciplinary arena, involving topics such as biophotonics and nanophotonics. A new paradigm is needed for packaging all-optical components. Until this process is standardized so that the optical transistor equivalent can be mass produced, the optical computer will remain a dream.

The way forward

Standardization work in ITU is already engaged in the evolution towards an all-optical core network for Internet and other telecommunication services. A workshop is proposed to showcase the developments made and outline the future evolution of the optical network.

As optical devices become more commonplace, security will need to evolve into the optical domain, with firewalls and intrusion prevention systems. Along with the all-optical computer, a new area of study will be the energy-efficient optical network, because of its importance in reducing the carbon footprint of telecommunication and in protecting the environment.

 


* The ITU–T Technology Watch Function surveys the information and communication technology (ICT) landscape to capture new topics for standardization activities. Technology Watch Reports assess new technologies with regard to existing ITU–T and other standards, and the likely implications for future standardization. The Technology Watch Function is managed by the Policy and Technology Watch Division of ITU’s Telecommunication Standardization Bureau. The report “The Optical World” and other Technology Watch Reports are available at:
http://www.itu.int/ITU-T/techwatch

 

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