Fiber Optic Networks

When we talk about optical networks, we are really talking about two generations of optical networks. In the first generation, optics was essentially used for transmission and simply to provide capacity. Optical fiber provided lower bit error rates and higher apacities than copper cables. All the switching and other intelligent network functions were handled by electronics. Examples of first-generation optical networks are SONET (synchronous optical network) and the essentially similar SDH (synchronous digital hierarchy) networks, which form the core of the telecommunications infrastructure in North America and in Europe and Asia, respectively, aswell as a variety of enterprise networks such as Fibre Channel.

Second-generation optical networks have routing, switching, and intelligence inthe optical layer. Before we discuss this generation of networks, we will first lookat the multiplexing techniques that provide the capacity needed to realize thesenetworks.

Basics  of Fiber Optic networks

Fiber optic technology is simply the use of light to transmit data. The general use of fiber optics did not begin until the 1970s. Robert Maurer of Corning Glass Works developed a fiber with a loss of 20 dB/km, promoting the commercial use of fiber. Since that time the use of fiber optics has increased dramatically. Advances in fiber technology, lower
production costs, and installation have all contributed to the wide use of fiber. The purpose of this paper is to provide an overview of fiber, its construction, and functionality. The heaviest use of fiber is in the telecommunications industry. Telephone companies initially used fiber to transport high volumes of voice traffic between central office locations. During the 1980s telephone companies began to deploy fiber throughout their networks. Fiber technology allows companies to “future proof” networks. We use the phrase “future proof” because fiber is theoretically unlimited in bandwidth. Bandwidth is a measurement of the data carrying capacity of the media (in this case, fiber). The greater the bandwidth, the more data or information that can be transmitted. Copper has a bandwidth and a distance limitation, making it less desirable.

Brief over view of fiber optic cable advantages 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.

Optical fiber is composed of several elements. The construction of a fiber optic cable consists of a core, cladding, coating buffer, strength member and outer jacket. The optic core is the light-carrying element at the center. The core is usually made up of a combination of silica and germania. The cladding surrounding the core is made of pure silica. The cladding has a slightly lower index of refraction than the core. The lower refractive index causes the light in the core to reflect off the cladding and stay within the core.

Single Mode cable is a single stand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission.  Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width. Synonyms mono-mode optical fiber, single-mode fiber, single-mode optical waveguide, uni-mode fiber. Single Modem fiber is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed – (single-mode on one single fiber) Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type. Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.

Multi-Mode cable has a little bit bigger diameter, with a common diameters in the 50-to-100 micron range for the light carry component (in the US the most common size is 62.5um). Most applications in which Multi-mode fiber is used, 2 fibers are used (WDM is not normally used on multi-mode fiber).  POF is a newer plastic-based cable which promises performance similar to glass cable on very short runs, but at a lower cost. Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS – Gigabit to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable’s core typically 850 or 1300nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabit and beyond.


What is the Fiber Optic Network

What is “fiber optic network design?” Fiber optic network design refers to the specialized processes leading to a successful installation and operation of a fiber optic network. It includes determining the type of communication system(s) which will be carried over the network, the geographic layout (premises, campus, outside plant (OSP, etc.), the transmission equipment required and the fiber network over which it will operate. Next we have to consider requirements for permits, easements, permissions and inspections. Once we get to that stage, we can consider actual component selection, placement, installation practices, testing, troubleshooting
and network equipment installation and startup. Finally, we have to consider documentation, maintenance and planning for restoration in event of an outage.

Design requires working with higher level network engineers usually from IT (information technology) departments and cable plant designers such as the architects and engineers overseeing a major project, as well as contractors involved with building the projects. Other groups like engineers or designers involved in aspects of project design such as security, CATV or industrial system designers or specialized designers like BICSI RCDDs for premises cabling may also be overseeing various parts of the project that involves the design and installation of fiber optic cable plants and systems. Designers should have an in-depth knowledge of fiber optic omponents and systems and installation processes as well as all applicable standards, codes and any other local regulations. They must also be familiar with most telecom technology (cabled or wireless), site surveys, local politics, codes and standards, and where to find experts in those fields when help is needed. Obviously, the fiber optic network designer must be familiar with electrical power systems, since the electronic hardware must be provided with high quality uninterruptible power at every location. And if they work for a contractor, estimating will be a very important issue, as that is where a profit or loss can be determined!
Those involved in fiber optic project design should already have some background in fiber optics, such as having completed a FOA CFOT certification course, and may have other training in the specialties of cable plant design such as electrical contracting apprenticeship, RCDD, SCTE or ISA training, etc. It’s also very important to know how to find in-depth information, mostly on the web, about products, standards, codes and, for the OSP networks, how to use online mapping services like Google Maps. Experience with CAD systems is a definite plus.

References for the fiber optic designer’s bookshelf include the FOA text, The Fiber Optic Technicians Manual, and the NECA/FOA-301 installation standard. When it comes to the NEC, I like Limited Energy Systems published by the NFPA. My own bookshelf has dozens of books on communications system design, but unfortunately, the fast pace of development in communications technologies means that many textbooks are hopelessly out of date unless it’s updated frequently. Better to rely on the web, especially the websites of well-established manufacturers. Getting trained specifically in fiber optic network design is becoming easier. This
material is covered in part in some advanced fiber optic courses offered by the FOAapproved schools and by large manufacturers who help you understand how to build networks using their products. The FOA has developed a curriculum to allow more of
our schools to offer a design specialty course and a new FOA design specialty certification. The bulk of the required material has been developed by a committee of experienced fiber installers and trainers working with the FOA.

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