Ring networks operate like bus networks with the exception of a terminating computer. In this configuration, the computers in the ring link to a main communication cable. The network receives information via a "token" containing information requested by one or more computers on the network. The token passes around the ring until the requesting computer(s) have received the data. The token uses a packet of information that serves as an address for the computer that requested the information. The computer then "empties" the token, which continues to travel the ring until another computer requests information to be put into the token. Figure 5 illustrates this topology.
An advanced version of the ring network uses two communication cables sending information in both directions. Known as a counter-rotating ring, this creates a fault tolerant network that will redirect transmission in the other direction, should a node on the network detect a disruption. This network uses fiber optic transceiver, one controlling unit in set in "master" mode along with several nodes that have been set as "remote" units. The first remote data transceiver receives the transmission from the master unit and retransmits it to the next remote unit as well as transmitting it back to the master unit. An interruption in the signal line on the first ring is bypassed via the second ring, allowing the network to maintain integrity. Figure 6 illustrates this configuration as it might be used in a ITS installation.
Star Network
Star networks incorporate multiport star couplers in to achieve the topology. Once again, a main controlling computer or computer server interconnects with all the other computers in the network. As with the bus topology with a backbone, the failure of one computer node does not cause a failure in the network. Figure 4 illustrates a star network topology. Both the bus and the star network topologies use a central computer that controls the system inputs and outputs. Also called a server, this computer has external connections, to the Internet for example, as well as connections to the computer nodes in the network.
Bus Network
A bus network topology, also called a daisy-chain topology has each computer directly connected on a main communication line. One end has a controller, and the other end has a terminator. Any computer that wants to talk to the main computer must wait its turn for access to the transmission line. In a straight network topology, only one computer can communicate at a time. When a computer uses the network, the information is sent to the controller, which then sends the information down the line of computers until it reaches the terminating computer. Each computer in the line receives the same information. Figure 2 illustrates a bus network topology. A bus network with a backbone operates in the same fashion, but each computer has an individual connection to the network. A bus network with a backbone offers greater reliability than a simple bus topology. In a simple bus, if one computer in the network goes down, the network is broken. A backbone adds reliability in that the loss of one computer does not disrupt the entire network. Figure 3 illustrates this topology with a backbone.
Fiber Optic Network Topologies for ITS and Other Systems
All networks involve the same basic principle: information can be sent to, shared with, passed on, or bypassed within a number of computer stations (nodes) and a master computer (server). Network applications include LANs, MANs, WANs, SANs, intrabuilding and interbuilding communications, broadcast distribution, intelligent transportation systems (ITS), telecommunications, supervisory control and data acquisition (SCADA) networks, etc. In addition to its oft-cited advantages (i.e., bandwidth, durability, ease of installation, immunity to EMI/RFI and harsh environmental conditions, long-term economies, etc.), optical fiber better accommodates today's increasingly complex network. architectures than copper alternatives. Figure 1 illustrates the interconnection between these types of networks.Networks can be configured in a number of topologies. These include a bus, with or without a backbone, a star network, a ring network, which can be redundant and/or self-healing, or some combination of these. Each topology has its strengths and weaknesses, and some network types work better for one application while another application would use a different network type. Local, metropolitan, or wide area networks generally use a combination, or "mesh" topology.
Dispersion vs. Wavelength
Single-mode fiber dispersion varies with wavelength and is controlled by fiber design (see Figure 11). The wavelength at which dispersion equals zero is called the zero-dispersion wavelength (λ º ). This is the wavelength at which fiber has its maximum information-carrying capacity. For standard single-mode fibers, this is in the region of 1310 nm. The units for dispersion are also shown in the pic.
Dispersion
Dispersion is the time distortion of an optical signal that results from the time of flight differences of different components of that signal, typically resulting in pulse broadening (see Figure 10). In digital transmission, dispersion limits the maximum data rate, the maximum distance, or the information-carrying capacity of a single-mode fiber link. In analog transmission, dispersion can cause a waveform to become significantly distorted and can result in unacceptable levels of composite second-order distortion (CSO).
Attenuation
Attenuation is the reduction of signal strength or light power over the length of the light-carrying medium. Fiber attenuation is measured in decibels per kilometer (dB/km).
Optical fiber offers superior performance over other transmission media because it combines high bandwidth with low attenuation. This allows signals to be transmitted over longer distances while using fewer regenerators or amplifiers, thus reducing cost and improving signal reliability.
Attenuation of an optical signal varies as a function of wavelength (see Figure 9). Attenuation is very low, as compared to other transmission media (i.e., copper, coaxial cable, etc.), with a typical value of 0.35 dB/km at 1300 nm for standard single-mode fiber. Attenuation at 1550 nm is even lower, with a typical value of 0.25 dB/km. This gives an optical signal, transmitted through fiber, the ability to travel more than 100 km without regeneration or amplification.
Attenuation is caused by several different factors, but primarily scattering and absorption. The scattering of light from molecular level irregularities in the glass structure leads to the general shape of the attenuation curve (see Figure 9). Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. It is these water ions that cause the “water peak” region on the attenuation curve, typically around 1383 nm. The removal of water ions is of particular interest to fiber manufacturers as this “water peak” region has a broadening effect and contributes to attenuation loss for nearby wavelengths. Some manufacturers now offer low water peak single-mode fibers, which offer additional bandwidth and flexibility compared with standard single-mode fibers. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.
How to Choose Optical Fiber
Single-Mode Fiber Performance CharacteristicsThe key optical performance parameters for single-mode fibers are attenuation, dispersion, and mode-field diameter.
Optical fiber performance parameters can vary significantly among fibers from different manufacturers in ways that can affect your system's performance. It is important to understand how to specify the fiber that best meets system requirements.
Business Voice and Multimedia Demonstrations
Nortel's business voice and multimedia solutions, including Nortel's IP Powered Business solution, demonstrate how small and medium sized businesses (SMBs) and enterprises can stay connected while on-the-go with applications that increase productivity. The demonstration will highlight Nortel Mobile Extension, a fixed mobile convergence (FMC) application that allows business subscribers to turn their mobile phone into a business extension that uses a single number, single voicemail and call grab capabilities for both their desk phone and mobile phones.
Optical and WDM-PON Solution Presentations
Nortel's 40G/100G Adaptive Optical Engine is a plug, play and evolve technology that is deployable over any fiber, allowing operators to reduce engineering, eliminate equipment and upgrade quickly and cost-effectively from 10G to 40G - and ultimately, all the way to 100G. The demonstration will also showcase Nortel's WDM-PON based Ethernet Access solution that eliminates bottlenecks and enables the delivery of triple play and high performance business and residential applications such as video streaming and VoIP.
The Cable Show 2009: Nortel Showcases Optical Technology, Voice and Multimedia Applications for Consumer and Business
WASHINGTON - Nortel* [TSX: NT, OTC: NRTLQ] invites attendees of The Cable Show 2009 ** (April 1-3, 2009 in Washington) to visit with our executives and other experts to discuss how 40G/100G optical solutions as well as business and consumer voice and multimedia applications can advance the delivery of information, entertainment and communications, while creating new business opportunities for the cable industry.
As the worldwide leader in carrier VoIP and an expert in next-generation optical technologies, Nortel is in a unique position to provide cable and multi-service operators (MSOs) network solutions that can reduce operational costs, support quadruple the network traffic and help drive new revenues through innovative service offerings.
At The Cable Show, Nortel experts in carrier VoIP and applications and optical network technology will be available to show how:
- Business voice and multimedia solutions keep busy professionals connected while on-the-go
Optical Scattering Instrument Characterization:
Integrated light scatter instruments can be characterized with respect to their ability to measure microroughness on different length scales. A methodology and computer program has been developed which allows instrument manufacturers to determine the transfer functions for their instruments. See Spatial Frequency Response Function.
Characterization of light scattering methodologies, such as determining instrument signature functions, play an important role in our work. For example, the BRDF that an instrument measures for a perfectly flat and defectless surface is dominated by the Rayleigh scatter in the air within the field of view of the instrument. This Rayleigh-equivalent polarized BRDF has been calculated and experimentally verified.
Resources:
A laser-based goniometric optical scatter instrument (GOSI) is available for measuring the bidirectional reflectance distribution function (BRDF), its polarization counterpart (Mueller matrix BRDF), or other light scattering ellipsometry parameters, from a variety of samples or surfaces. Another instrument, the Scanning Optical Scatter Instrument, is being developed to yield the scattering distribution in multiple directions at once, with partial polarimetric capabilities. These facilities are housed in clean environments to maintain sample integrity. See Bidirectional Optical Scattering Facility for details. Other instruments exist within the division under Spectrophotometry.
Model Software:
SCATMECH: Polarized Light Scattering C++ Class Library -- A C++ object class library has been developed to distribute models for polarized light scattering from surfaces. It is the intent of this library to allow researchers in the light-scattering community to fully utilize the models described in the publications found below. Included in the library are also a number of classes that may be useful to anyone working with polarized light. The library is constructed so that it can easily be expanded to include new models.
MIST: Modeled Integrated Scatter Tool -- The MIST program has been developed to provide users with a general application to model an integrated scattering system. The program performs an integration of the bidirectional reflectance distribution function (BRDF) over solid angles specified by the user and allows the dependence of these integrals on model parameters to be investigated. The models are provided by the SCATMECH library of scattering codes.
Light Scattering Ellipsometry:
The polarization of scattered light can often indicate the source of that scattered light. Using Light Scattering Ellipsometry, whereby the polarization of light scattered into directions out of the plane of incidence is measured for a fixed incident polarization, scattering from microroughness, subsurface defects, and particulate contamination can be distinguished. Experimental measurements and theoretical modeling have been carried out to demonstrate this effect in a variety of systems:
- Roughness of a single material (silicon, glass, steel, and titanium nitride)
- Subsurface defects (fused silica, glass ceramic, and subsurface defects in silicon)
- Roughness of a dielectric layer (SiO2 and polymer films on silicon)
- Particles above a single interface (polystyrene, copper, and gold spheres on silicon)
- Particles above a thin film (polystyrene spheres on polystyrene films on silicon)
- Special-effect pigmented coatings (metallic and pearlescent flakes)
- Overlay structures
Placing the technique on a firm metrological basis, so that it is quantitatively accurate, is a high priority of the program. Polarized light scattering in the Stokes-Mueller representation is also studied.
Optical Scattering From Surfaces
We study how material properties, surface topography, and contaminants affect the distribution of light scattered from surfaces, with an aim toward
- Developing standard measurement methods and standard artifacts for use in industry, and
- Providing a basis for interpreting scattered light distributions so that industry can optimize their use of optical scatter methods.
