Recent Trends in Fibre Optics Communication
Communication refers to information transmission and reception. The information being transmitted could be of various forms, either analog (voice, video, text) or digital form. In 1838 the telegraph was invented by Samuel Morse. This ushered in a new era in communications i.e. electrical communications.
Later systems also sent optical signals through the air, but clouds, rain, and other atmospheric disturbances can disrupt optical signals sent through the air.
Electric signals through wires avoid that problem. The wires used in electrical communication systems are usually made of copper.
The first coaxial cable system was introduced 1940 and it had the capability to transmit 300 voice channels.
The first microwave system was put into service in 1948 with a carrier frequency of 4GHz. Coaxial and microwave systems were operating at 100Mbit/s.
Optical fibre communication is the method of transmitting information by sending light through a optical fibre made of glass or plastic.
The light forms an electromagnetic carrier wave that is modulated to carry information.
A single fibre optic cable is as thin as a single human hair. Many cables can be bundled together to form one cable and thus increase the amount of data that can be transmitted drastically.
Light in a stream of water stays inside the water and bends with it. Daniel Colladon first described this "light fountain" or "light pipe" in an 1842 article.
Optical fibre was first successfully developed in 1970 by Corning Glass Works, with attenuation low enough for communication purposes (about 20dB/km), and at the same time GaAs semiconductor lasers were developed that were compact and therefore suitable for transmitting light through fibre optic cables for long distances.
On 22 April 1977, General Telephone and Electronics sent the first live telephone traffic through fibre optics at a 6 Mbit/s throughput in Long Beach, California.
The second generation of fibre-optic communication was developed for commercial use in the early 1980s, operated at 1.3 µm, and used InGaAsP semiconductor lasers. These early systems were initially limited by multi mode fibre dispersion, and in 1981 the single-mode fibre was revealed to greatly improve system performance. These systems operated at bit rates of up to 1.7 Gb/s with repeater spacing upto 50km.
The first transatlantic telephone cable to use optical fibre went into operation in 1988
Third-generation fibre-optic systems operated at 1.55 µm and had losses of about 0.2 dB/km. These systems operated at 2.5 Gbit/s with repeater spacing in excess of 100 km.
The fourth generation of fibre-optic communication systems used optical amplification to reduce the need for repeaters and wavelength-division multiplexing to increase data capacity. These two improvements caused a revolution that resulted in the doubling of system capacity every 6 months starting in 1992 until a bit rate of 10 Tb/s was reached by 2001. Recently, bit-rates of up to 14 Tbit/s have been reached over a single 160 km line using optical amplifiers.
What is an optical fibre, its various parts
An optical fibre is a flexible optically transparent fibre, usually made of glass or plastic, through which light can be transmitted.
The light in an optical fibre carries data in the same way that electrical signals do in copper cables.
Light is kept in the "core" of the optical fiber by total internal reflection.
Transmission properties are dictated by the structural characteristics of the fiber.
The propagation of light along a waveguide can be described in terms of a set guided electromagnetic waves called modes of the waveguide.
Parts of an optical fibre
Core: It the central region through which the light signal travels.
Cladding: It has a lower refractive index than the core. It is the outer material that surrounds the core.
Buffer Coating: Protects the fibre
Principle of Operation
Light pulses travel in an optical fibre as per the principle of total internal reflection.
Conditions for ‘total internal reflection’
Light should travel from denser medium to rarer medium
Angle of incidence should be greater than critical angle
Acceptance angle: The maximum angle in which external light rays may strike the air-glass interface and still propagate down the fiber
Types of Optical Fibre
Step Index Fibre
The refractive index of the core is slightly greater than that of the cladding. Step Index Fibres are of two types:
a) Single mode step index b) Multi-mode step index
Graded Index Fibre
The refractive index gradually decreases from core to cladding
Fibre losses, dispersion and non-linear effects
Material Absorption: Absorption by impurities in silica.
Rayleigh Scattering: Occurs due to varying refractive index.
Intermodal Dispersion: Caused due to varying velocities of different modes. Leads to pulse spreading.
Chromatic Dispersion: Consists of waveguide dispersion and material dispersion
Stimulated Raman Scattering
Stimulated Brillouin Scattering
Self Phase Modulation
Four Wave Mixing
Optical fibre communication system
Optical fibres can be used as a medium for telecommunication as they are flexible and can be bounded as cables.
Both multi-mode and single-mode fibres are used in communications, with multi-mode fibre used mostly for short distances and single mode fibre for longer distances.
Fibres are generally used in pairs, with each fibre carrying signals in one particular direction
The light signals propagating in the fibre can be modulated at rates as high as 40 Gb/s and each fibre can carry many independent channels, each by a different wavelength of light (wavelength-division-multiplex WDM).
The process of communicating using fibre-optics involves the following steps: Creating the optical signal from an electrical signal, relaying the signal along the fibre, ensuring that the signal does not become too distorted or weak, receiving the optical signal and converting it into an electrical signal.
Basic Components of an Optical Fibre Communication System
a) Laser Diode
b) Light Emitting Diode
a) PIN diode
b) Avalanche photodiode
Wavelength Division Multiplexing
Signals with different wavelengths are combined, transmitted together, and separated again. That means more bandwidth—more data per second.
Dense WDM (DWDM) uses up to 100 wavelengths through a single fiber. Bandwidth up to 1 Tbps (1000 Gbps)
Advantages of optical fibre communication
Enormous Bandwidths: Optical fibres have enormous transmission bandwidths and high data rate. Using WDM, the information carrying capacity of optical fibres is enhanced to many orders of magnitude.
Low transmission loss: Due to the usage of ultra low loss fibres and the erbium doped silica fibres as optical amplifiers, one can achieve almost loss less transmission. Thus, repeater spacing is very large.
Immunity to cross talk: Optical fibres are free from any electromagnetic interference and radio frequency interference.
Electrical Isolation: The fibres are made from silica which is an electrical insulator. Therefore they do not pick up any electromagnetic wave or any high current lightening.
Signal security: The transmitted signal through the fibre does not radiate. Unlike in copper cables, a transmitted signal cannot be drawn from a fibre without tampering it. It is hack-proof.
Low cost and availability: Since the fibres are made of silica which is available in abundance. Hence, there is no shortage of material and optical fibers offer the potential for low cost communication.
Small size and weight: Optical fibres are light in weight. The size too is very small and so the space occupied by the fibre cable is negligible.
Reliability: Optical fibres do not undergo any chemical reaction or corrosion.
Disadvantages of optical fibre communication
Cost: It is the most expensive among all forms of guided media. Employment and maintenance of the equipment required to implement this technology is very costly. It is even more uneconomic when the bandwidth is not fully utilised.
Installation/maintenance: It is a relatively new technology. Installation and maintenance demands knowledge and expertise that is not yet available everywhere.
Unidirectional: Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed.
Coupling Losses: Coupling of fibres has to be done extremely carefully. There is practically no margin for error. Small mistakes can lead to a major loss of information.
Today’s optical fibre communication system
1. Fibre To The x (FTTx)
The Last Mile
The "last mile" is the final leg of delivering connectivity from a communications provider to a customer.
It is typically seen as an expensive challenge because "fanning out" wires and cables is a considerable physical undertaking.
As demand has escalated, particularly fuelled by the widespread adoption of the internet, the need for economical high-speed access by end-users has ballooned as well.
As requirements have changed, existing systems have proved to be inadequate. To date, although a number of approaches have been tried and used, no single clear solution to this problem has emerged.
Next Generation Access:
NGA is a term describing the upgradation of the existing telecommunication network by replacing some or all of the copper cable with optical fibre.
NGA is an important enabler for faster broadband internet access.
What is FTTx?
FTTH: Fibre to the Home
FTTB: Fibre to the Building/Basement
FTTC: Fibre to the Curb
FTTN: Fibre to the Node
It provides far faster connection speeds and carrying capacity than twisted pair conductors. Fibre has virtually unlimited bandwidth and hence is “future safe”.
New services such as VoIP, HDTV, interactive video are gaining popularity. A January 2009 study estimated that new technologies will drive Internet traffic up by 50 times the current rate within the next 10 years.
Fibre optic components are getting less expensive. Costs have decreased by about 75% since 2001.
Technologies such as Passive Optical Network (PON) and WDM have further reduced the cost of FTTH.
Point to Point Architecture
FTTH Architecture (contd.)
Active Optical Network
FTTH Architecture (contd.)
Passive Optical Network
Active vs Passive Optical Networks
Fibre optics uses light signals to transmit data. As this data moves across a fibre, there needs to be a way to separate it so that it gets to the proper destination.
An active optical system uses electrically powered switching equipment, such as a router or a switch aggregator, to manage signal distribution and direct signals to specific customers.
A passive optical network uses optical splitters to separate and collect optical signals as they move through the network. Powered equipment is required only at the source and receiving ends of the signal.
In some cases, FTTH systems may combine elements of both passive and active architectures to form a hybrid system.
Active vs Passive Optical Networks (contd.)
Passive optical networks, or PONs, are efficient, in that each fiber optic strand can serve up to 32 users. PONs have a low building cost relative to active optical networks along with lower maintenance costs.
PONs have less range than an active optical network, meaning subscribers must be geographically closer to the central source of the data. PONs also make it difficult to isolate a failure when they occur.
As the bandwidth in a PON is not dedicated to individual subscribers, data transmission speed may slow down during peak usage times.
AONs require at least one switch aggregator for every 48 subscribers. Because it requires power, an active optical network inherently is less reliable than a passive optical network.
Passive Optical Networks
A PON is a network architecture in which unpowered optical splitters are used to enable a single optical fibre to serve multiple premises.
Downstream signals are broadcast to all premises sharing a single fibre.
Upstream signals are combined using a multiple access protocol, usually time division multiple access (TDMA).
Bandwidth sharing is the major reason why PON’s growth has been restricted. On a 155-Mbit/s PON link with four splits, each subscriber will receive 38.75 Mbps.
The different PON standards are:
APON (ATM PON)
BPON (Broadband PON)
GPON (Gigabite PON)
EPON (Ethernet PON)
Passive Optical Networks (contd.)
APON (Asynchronous Transfer Mode PON): This was the first Passive optical network standard. It was used primarily for business applications, and was based on the ATM protocol.
BPON (Broadband PON): It is a standard based on APON. It adds support for WDM and provides higher upstream bandwidth and dynamic upstream bandwidth allocation.
EPON (Ethernet PON): It uses ethernet instead of ATM for data encapsulation. EPONs offer higher bandwidth, lower costs, and broader service capabilities than APON.
GPON (Gigabit PON): It is an evolution of the BPON standard. GPON is a flexible option for providers because it is designed to handle both Ethernet and ATM traffic and it offers roughly twice the capacity of EPON.
Passive Optical Networks (contd.)
A Case Study: Verizon Fibre Optic Services
Verizon FiOS is a bundled home communications service, operating over a fibre optics communications network, that is offered in some areas of the United States of America. It provides internet, telephone and television services.
To serve a home, a single mode optical fibre extends from an optical line terminal at a FiOS central office out to the neighbourhoods where an optical splitter fans out the same signal on up to 32 fibers, thus serving up to 32 subscribers. At the subscriber's home, an optical network terminal transfers data onto the corresponding copper wiring for phone, video and Internet access.
Verizon initially installed slower BPONs but now only installs GPONs.
2. All Optical Networks
The advancements in the field of optical technology in recent years has made the All-Optical Network (AON) a viable option.
An AON transmits data streams by way of all-optical lightpaths established using wavelength division multiplexing (WDM). Data remains in the optical domain throughout transmission from source to destination.
The primary advantage of an AON is that data streams do not undergo optical-electrical-optical (OEO) conversion, which increases end-to-end latency.
DWDM fiber technology is likely to offer a single fiber containing hundreds of wavelength channels, each modulated at 10 Gb/s. Hence, links with a total capacity of tens of Tb/s may be attached to a single core router requiring switching capacity not available by present day electronics.
Right now, the optical switches have electrical core, where light pulses are converted back into electrical signals so that they can be routed towards their respective destinations.
This has a few advantages:
The switches handle smaller bandwidths than whole wavelengths and this works fine considering the current market requirements.
Easier network management, because standards are in place and resourcess are available. Optical equivalents are not, at present.
But, there are concerns that electrical cores won’t be able to cope with the explosion in the number of wavelengths in the networks (deployment of DWDM).
Optical Switching (contd.)
Optical Circuit Switching (OCS)
It is a two-way reservation technique. Lightpaths are established and taken down as needed.
Data transmission does not commence until the edge router receives acknowledgement of all resource reservations.
Its biggest advantage is that blocking at core routers is averted by delaying data transmission until the edge router receives acknowledgement of all resource reservations.
The main performance measure is the queueing delay at the edge routers. At peak usage hours, this delay can be very quite large.
The problem with circuit switching is that it is not efficient at handling bursty data traffic.
Sufficient bandwidth needs to be reserved to deal with the peak rate, and this bandwidth would be unused a lot of the time.
Optical Switching (contd.)
Optical Packet Switching (OPS)
The data stream is broken up into small packets of data. These packets are multiplexed together with packets from other data streams inside the network. The packets are switched inside the network based on their destination.
Packets are transmitted based on the store and forward philosophy. To facilitate switching, a packet header is added to each packet. The header carries addressing information like the destination address or the address of the next node in the path. each intermediate node stores an incoming packet, and then forwards it to the next node based on its header and a locally stored routing table. The header is sent at a slower speed and is processed electronically.
The lack of optical memory is a major obstacle. OPS networks are either difficult to realize, very bulky, or very expensive, even after a decade of research in this area.
Optical Switching (contd.)
Optical Burst Switching (OBS)
It is a technique that allows dynamic sub-wavelength switching of data.
It is a compromise between OCS and OPS.
Packets are aggregated into data bursts at the edge of the network to form the data payload.
The header of a burst is called a control packet, and it is sent beforehand to allocate transmission channels for the burst. It is transmitted in optical form in a separated wavelength channnel termed the control channel and it is processed electronically at each OBS router, whereas the data burst is transmitted in all optical form from one end to the other end of the network.
After the burst has passed a router, the router can accept new reservation requests.
Optical Switching (contd.)
Advantages and disadvantages of OBS
Advantage over OCS
In an OCS system, a lightpath must be set up from source to destination in the optical network. If the data transmission duration is short relative to the set up time, bandwidth may not be efficiently utilized in the OCS system. In comparison, OBS does not require end-to-end lightpath set up, and therefore may offer more efficient bandwidth utilization.
Advantages over OPS
Time spent waiting in buffers is reduced.
A core optical router in an OPS network would have to perform processing operations for every arriving packet, whereas in an OBS network the router performs processing operations for an arriving burst which contains several packets.
The biggest disadvantage of OBS is the high packet losses which are inevitable due to the lack of adequate optical buffering.
Optical Cross-Connect (OXC)
An optical cross-connect (OXC) is a device used by telecommunications carriers to switch high-speed optical signals in a fibre optic network.
Optical signals are converted to electronic signals. The electronic signals are then switched by an electronic switch module. Finally the switched electronic signals are converted back into optical signals.
Such an architecture prevents an OXC from performing with the same speed as an all-optical cross-connect. An advantage is that the optical signals are regenerated, so they leave the node free of dispersion and attenuation.
The disadvantage of an all optical cross-connect is that it does not provide wavelength conversion and signal regeneration.
A compromise between the two leads to what is called a translucent OXC. The switch stage consists of an optical switch module and an electronic switch module. Optical signals passing through the switch stage can be switched either by the optical switch module or the electronic switch module.
Optical Cross-Connect (OXC) (contd.)
OXCs with wavelength conversion
3. Plastic/Polymer Optical Fibre (POF)
POF is an optical fiber which is made out of plastic. Traditionally poly methyl meth-acrylate (PMMA) is the core material, and fluorinated polymers are the cladding material.
The large size makes coupling easy from sources and connectors.
Plastic/Polymer Optical Fibre (POF) (contd.)
The traditional PMMA fibres are commonly used for low-speed, short-distance (up to 100 meters) applications in digital home appliances, home networks, industrial networks and car networks.
With the rising demand for high-speed home networking, POF is seen as a possible option for next-generation Gigabit/s links inside the house.
There is now a greatly increased interest in POF, given its mechanical flexibility to go with its ease of installation and low cost.
From an optical standpoint, conventional POF is much lower in performance than glass fiber. Its attenuation is much higher and its bandwidth is limited by its large numerical aperture and step-index profile.
Recent developments have led to low NA POFs that offers higher bandwidth and graded-index POF (GI-POF) that combines the higher bandwidth of graded-index fiber with the low cost of POF.
Some Statistical Data
The global consumption of fiber optic components in communication networks exploded from only $2.5 million in 1975 to $15.8 billion in 2000. Continued growth to $739 billion in 2025 is forecast.
Wavelength-division multiplexing (WDM) and parallel channel integration will combine to achieve ten-terabit interconnection fiber links, terminated in single small transmit and receive modules, well before 2050.
FTTH in Japan was first introduced in 1999by Nippon Telegraph and Telephone but it did not become a large player until 2001. Currently, many people are switching from DSL to FTTH. In September 2008, it was reported that for the first time, the number of FTTH connections (13.08 million connections) eclipsed that of DSL (12.29 million connections) and became the biggest means of broadband connection in Japan. Average real-world speed of FTTH is 66 Mbit/s in the whole of Japan, and 78 Mbit/s in Tokyo
In the United States, the largest fiber to the premises (FTTP) deployment to date is Verizon's FiOS. Initial FTTP offering was based on Broadband PON technology. Verizon has already upgraded to Gigabit PON, a faster optical access technology capable of providing 1GB/sec speeds to consumers.
Some Statistical data (contd.)
On April, 07 2009, the Federal Government announced a $43 billion plan to deploy FTTH to 93% of Australian households under a the National Broadband Network. Construction has commenced on the network with live speeds of up to 1000 Mbit/s via fiber
BT Openreach started a pilot at Ebbsfleet in Kent, Highams Park in London and Milton Keynes offering speeds of up to 100 Mbit/s, and have plans to make FTTP available to 2.5 million homes and businesses by 2012.
FTTB services are currently supplied in Hyderabad by Beam Telecom, offering a variety of plans for home users up to 6 Mbit/s, "power users" up to 20 Mbit/s and enterprises up to 30 Mbit/s. Beam Telecom have also launched fristever FTTH Solution in Hyderabad in three major townships by end of 2010, they have planned to complete FTTH setup in 20 upcoming townships by the end of 2011. FTTH services are due to be launched in 2011 by Hayai Broadband, allowing speeds of 100+ Mbit/s to the Internet and 1000+ Mbit/s (1 Gbit/s) within its own network. The coverage area will include most suburbs in Mumbai. BSNL has launched an FTTH service in Jaipur in late 2010.