Fibre Optic Cable: Unsung Hero
telecoms-mag.com
When all the dust has settled from the excitement surrounding high-speed data transmission, whether xDSL or wireless, will people once again remember that fibre is the thankless workhorse that enabled the revolution in the first place? Will fibre emerge as the successor to the broadband throne?
Malcolm Barnett
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There is an old adage about being ?penny wise and dollar poor.? Applying this to the fibre question is a bit more complicated, but it still holds water. Carriers that waste too much time on interim broadband access methods because they perceive fibre as being prohibitively expensive may be overlooking the inevitability of the fibre future.
It is safe to say that service provision via fibre will initially be limited in scope, and complementary to copper solutions for the short term, simply on the basis of cost to operators, and ultimately to what subscribers are willing to pay. But this equation must always be reconciled with the constant that service demands are increasingly driving innovation, not just the cost of technology. The fact remains that demand for information, Net-based services and applications continues to increase, so much so that the maximum available transport rates are doubling approximately every two years.
In time, optical fibre will be the backbone of the information superhighway, transporting voice, video and data to businesses, schools, hospitals and homes. Time is therefore the variable in the equation; it is universally agreed that the price of optical components and cables is falling more rapidly than that of copper cables. As the clock rate for microelectronics increases, the demands on interconnection technology become increasingly difficult to meet using conventional electrical solutions. As a result of this rapid growth, electronic functions in communication networks eventually will be replaced by photonic functions, which provide higher information-carrying capacity.
All-Optical Destiny
For years, the commercial use of optical fibres for voice and data transmission was merely a dream. The problem was one of light loss over distance. In 1970, three Corning, Inc. researchers, Robert Maurer, Donald Keck and Peter Schultz successfully solved the problem. They created the first optical waveguides -- glass fibres made from fused silica -- which maintained the strength of laser light signals over significant distances.
In the early 1980s, standard single-mode optical fibre became available for commercial use. This fibre was optimised for use in the 1310 nm operating window. This window had minimal chromatic dispersion, but not a minimal attenuation. In the mid-1980s, dispersion-shifted optical fibres were developed which shifted the minimum chromatic dispersion to the 1550 nm operating window where attenuation is lower. End users could then take advantage of both minimal chromatic dispersion and attenuation. With the development of the erbium-doped fibre amplifier (EDFA) in the early 1990s, end users began to move toward multiple wavelengths on a single optical fibre. Unfortunately, multiple wavelengths can lead to unwanted interactions in the presence of very low chromatic dispersion. To combat this, non-zero dispersion-shifted optical fibres were developed. These fibres continued to be optimised for use in the 1550 nm operating window; however, they had a small amount of chromatic dispersion to mitigate such non-linear effects as four wave mixing in dense wavelength division multiplexing (DWDM) systems.
The development of dispersion-shifted and non-zero dispersion-shifted optical fibres has thus enabled higher data rates over longer distances. The need for regeneration electronics has been decreased, lowering system cost and improving reliability.
With the advent of DWDM systems, customers are able to place more data on a single fibre without incurring the cost of installing new cable.
At the same time, the increase in demand for bandwidth has brought two reactions from the optical fibre industry: increased data rates, and increased fibre penetration. The move to non-zero dispersion-shifted fibres has allowed for huge amounts of data to be transmitted over a single fibre. Increases in both the speed of the data rates and the number of wavelengths in use have allowed single fibres to carry data in the range of 1 Tbps with 2 Tbps on the horizon.
Additionally, the amount of fibre demand has increased dramatically. Where cables containing 100 and 200 fibres used to be common, backbone lines containing fibre counts of 400 or 800 optical fibres are now beginning to be used. In addition to the higher fibre count backbones, fibre is being used closer to the end user. Ten years ago fibre was primarily used in the backbone networks, but today subscriber loops are commonly using optical fibre. As telephone and cable TV companies rebuild their existing systems, fibre is being used to replace copper even in the neighbourhood. Eventually, fibre will be used to the home itself in an effort to satisfy the increasing demand for bandwidth.
Broadband Band-Aids
Coaxial cable systems were installed about 30 years ago as tree-and-branch systems with one-way amplifiers. That makes upstream signals difficult. Upgrading them for interactivity is proving very costly. Further, cable modem specifications are just now extending to integrated voice capabilities. As a shared medium, cable modem speeds decrease as more users go online.
xDSL requires a long foot copper drop from the curb to the house plus the copper from the terminations to the DLC and to the central office. These factors are conspiring to make it more difficult for operators to guarantee the kind of quality of service (QoS) expectations that are increasing as quickly as the demand for bandwidth itself. Moreover, shared bandwidth cable and xDSL engender security risks that fibre does not. With FTTH (fibre-to-the-home) for example, the subscriber is connected to the splitter, which is often miles from the termination, via a two-fibre (upstream/downstream) drop.
Nevertheless, as fibre moves closer to the end user and displaces copper, there are some adaptations that need to be achieved. Cable designs will need to evolve from being solely designed for backbone applications with higher fibre counts to being small size, low fibre count solutions needed in FTTH applications. Additionally, media conversion will be a big challenge. Today in the home or office, there are very few appliances capable of receiving an optical signal. The home television or telephone uses electrical signals. Where such a media conversion will take place is still in dispute. Should the fibre stop at the curb? the house? or the TV? Electronics manufacturers will have to provide solutions for changing the high-speed optical signal into a format useable by the average consumer. This may result in the creation of set-top boxes for the television, or a whole new breed of telephones for the home. The issue of powering such electronics is also unresolved. Should such electronics be powered off the electric grid, or should they be powered independently by the service provider?
For the horizontal run, from the telecoms or wiring closet to the work area, copper is still by far the most widely used transmission medium. Although the cost of installed, horizontal fibre optic cabling has come down, optical network interface cards for desktops and hubs are still expensive compared to copper, but so was the desktop calculator at one time.
Since the 1980s, the impact of fibre optics and the information superhighway with photonics, subsequently led to the improvement of devices that split, modify, amplify and manage photons in high-speed optical systems. Through photonic manipulation enabling cabling that interconnects hub locations, fibre optic cabling remains the most practical approach. While it may be tempting to utilise copper twisted pairs to support connections between hubs that are located in close proximity to each other, fibre optics present an approach that can be used consistently to allow for maximum flexibility as the network grows. Furthermore, the use of fibre to the individual network nodes is often discussed in the context of providing sufficient network bandwidth to the user to support applications such as the transfer of multimedia files and desktop videoconferencing.
Multiplying Capacity
Single transmission fibres have been considerably fortified via the development and implementation of multiplexing devices such as erbium-doped optical amplifiers (EDFAs) and dense wavelength division multiplexers (DWDMs). These act to solve bandwidth problems on the installed base so that carriers can readily accommodate high bandwidth desktop applications and services.
Multiplexing technology allows many data channels at different wavelengths to be bundled together, transmitted and subsequently unbundled at the end of the route. DWDM is capable of multiplexing at least 160 channels and is limited by the channel spacing, fibre dispersion and the availability of fibre amplifiers throughout the optical fibre spectrum. There is as yet is no perceptible limit to the number of channels that can be multiplexed; these figures are constantly changing on the R&D level. The most limiting factor right now for the high bit rate transmission is dispersion. Fibre non-linear effects and chromatic dispersion can be performance limiting factors because of the high output of optical amplifiers and the simultaneous transmission of multiple wavelengths. For installed conventional single-mode fibre, the high dispersion at the 1550 nm region limits the transmission distance at high bit rates. New fibre designs, such as non-zero dispersion-shifted fibre and non-zero dispersion shifted fibre having a large effected area, which are capable of controlling chromatic dispersion, reducing fibre nonlinear effects, and operating at high bit rates.
Corning believes that the drive to enhance the portion of the installed base that consists of standard unshifted single-mode fibre -- optimised for operations at 1310 nm -- has led to the large scale replacement of regenerators with EDFAs, which operate at 1550 nm. OC-48 WDM technology is well suited to this type of fibre and gives the carrier the flexibility to incrementally add wavelengths when needed.
Regardless of cable design, it is the optical fibre that determines whether a DWDM system is possible. Today, the non-zero dispersion-shifted fibres are best suited for DWDM systems requiring high data rates over long distances. However, if the data rate required is somewhat lower, and distances are shorter, the standard single-mode fibre is still an acceptable fibre for supporting DWDM. In the outside plant environment, stranded loose tube cable and central tube cables are best suited for dealing with the environmental and mechanical rigors. Depending on the application, the customer may find a lower fibre count cable with individual fibres to be the best solution, or perhaps a high fibre count ribbon cable is better suited.
Older Fibre and DWDM
Some problems may exist in very old vintage optical fibres (circa early 1980s) when it comes to using DWDM systems. Additionally, systems using dispersion-shifted optical fibre may also have trouble with DWDM. These dispersion-shifted systems, however, are taking on new life through the use of wavelengths above or below the 1550 nm operating window. When operated beyond the 1550 nm window, dispersion-shifted fibres begin to operate similarly to non-zero dispersion-shifted fibres and have a new application with respect to DWDM systems.
Dispersion compensation modules can be added to a system to correct for the higher chromatic dispersion typically seen in dispersion-unshifted fibres at the 1550 nm operating window. Limiting the data rate may allow older fibre types to be used with newer systems as well. Using dispersion-shifted fibre above the 1550 nm operating window can make that fibre useful for DWDM. A number of solutions are available from electronics and photonics manufacturers depending on the nature of the existing plant and the new application desired.
One of the greatest challenges facing optical fibre is misunderstanding. Many customers, familiar with copper products and installation, do not completely understand the issues involved with optical fibre.
Optical fibres are housed inside hollow, cylindrical tubes, called buffer tubes. The inside diameter of the buffer tubes is much larger than the outside diameter of the optical fibre. This difference in diameter allows free movement of the optical fibre within the buffer tube so that the optical fibres are decoupled from the rest of the cable. A jelly like substance fills the tube around the optical fibres, preventing moisture from entering and allowing the optical fibre to ?float? within the buffer tube. It also eliminates the possibility of water freezing in the vicinity of the optical fibre, which could lead to an increase in attenuation or fibre breakage. With an average tensile breaking strength of 600,000 pounds per square inch, fibre exceeds the strength requirements of all of today?s communications applications (Figure 1).
In addition, glass is an extremely stable material, as shown by rigorous environmental testing. Similarly, data gathered in the field over several years has helped the industry establish fibre standards that suggest a very long service life. Optical fibre has indeed been shown to have virtually unlimited information-carrying capacity and exhibits little susceptibility to severe weather or interference from outside electromagnetic forces. Fibre?s immunity to adverse conditions such as moisture ingress, corrosion and fatigue make it possible to project its useful life out to 20 years or more.
Let There be Light
The increase in demand for bandwidth has brought two reactions from the optical fibre industry: increased data rates and increased fibre penetration. The move to non-zero dispersion-shifted fibres has allowed for huge amounts of data to be transmitted over a single fibre. Increases in both the speed of the data rates and the number of wavelengths in use have allowed single fibres to carry data in the range of 1 Tbps with 2 Tbps now looking possible.
Additionally, the amount of fibre demand has increased dramatically. Where cables containing 100 and 200 fibres used to be common, backbone lines containing fibre counts of 400 or 800 optical fibres are now beginning to be used. In addition to the higher fibre count backbones, fibre is being used closer to the end user. Ten years ago fibre was primarily used in the backbone networks, but today subscriber loops are commonly using optical fibre. As telephone and cable TV companies rebuild their existing systems, fibre is being used to replace copper even in the neighbourhood. Eventually, fibre will be used to the home itself in an effort to satisfy the increasing demand for bandwidth, thereby claiming its rightful legacy as a hero of the broadband revolution.
Malcom Barnett is senior vice president, Sales and Marketing Europe, Corning Cable Systems. |
