Fiber Type and Performance
As fiber becomes more widely employed in premises applications, a system designer should
evaluate both multimode and single-mode optical fiber to ensure the system meets present
requirements and will handle future upgrades. Data rates will only increase as new applications are created. The system designer can allow for higher data rates by installing optical fiber instead of other media. Use of optical fiber maximizes the prospects of ensuring compatibility with all future applications.
There are six primary network applications in use today. Each on operates somewhat differently from the others and some are interrelated. The systems are: Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), Fibre Channel, Asynchronous Transfer Mode (ATM), and Synchronous Optical Network (SONET). Some of these are designed for data transmission only. Others can carry voice, data, and video signals simultaneously despite the huge difference in the transmission rates for these three types of signals. This chapter will cover the transmission requirements for each application.
Ethernet is a bus-based network application. It originally began with coaxial cable as the primary bus medium, but fiber replaced coax to extend the usable distance. Ethernet versions
using fiber are 10BaseF (10 Mb/s), 100BaseF (100 Mb/s), 1000BaseF (1000 Mb/s), 10 Gigabit (10,000 Mb/s), and Terrabit (1 Trillion Bits). It is used primarily for data transmission. The Ethernet system was standardized as IEEE 802.3.
Token Ring is a ring-based network application used for data transmission. It operates at either 4 Mb/s or 15 Mb/s. Token Ring uses a "token" to pass data between stations. Only the station that has the token can transmit data. It uses twisted wire pairs (shielded and unshielded) or optical fiber as the transmission medium. Token Ring is based on the IEEE 802.5 standard.
Fiber Distributed Data Interface (FDDI) is a dual ring (counter-rotating), token-based network application for data and digital video transmissions. It was designed to accommodate higher data rates over longer distances with increased reliability over previous applications. It operates at 100 Mb/s using two rings: one ring for the signal and one ring as a backup in case of a node or cable failure. FDDI was originally written for 62.5/125 optical fiber, but versions for single- mode optical fiber and lower cost components have also been developed. Due to its high speed, FDDI can be used in the backbone to connect other networks operating with Ethernet or Token Ring.
Fibre Channel is a network application specifying single-mode fiber, multimode fiber, coaxial cable, or twisted pair cable. The fiber type recommended by the Fibre Channel specification depends on distance and bit rate. The data range from 133 MBaud to 1063 MBaud. Fibre Channel is primarily a high performance serial link to support mainframe, super-computer, and peripheral equipment communications.
Asynchronous Transfer Mode (ATM) is designed to allow the efficient transmission of high data rates between networks. If a user multiplexes voice (low rate), data (medium rate), and video (high rate) signals over the same system, the system must be capable of handling the signal that requires the highest information rate (probably video). ATM efficiently uses the available band- width by packaging the inputs from voice, data, and video sources into a series of 53 byte packets (5 bytes for addressing, 48 bytes for information) for transmission and switching at a rate that is compatible with the connecting network. ATM can operate at different speeds using the same packet system and automatically adjusts to the network speed of the addressee. As system requirements change, so can the data rate to meet those requirements. The data rates range
from 52 Mb/s to 622 Mb/s.
Synchronous Optical Network (SONET) is an optical multiplexing hierarchy for the transmission of voice, data, and/or video over single-mode fiber. SONET uses a base rate of 51.84 Mb/s (STS-1) with higher data rates in multiples of the base rate. SONET is not a network application in and of itself, but rather a system for coordinating and integrating different applications and networks over wide areas. SONET takes an incoming multiplexed signal and reformats it to an electrical signal called a synchronous transport signal (STS). The electrical signal is then converted to an optical carrier (OC) signal. For example, and STS-1 electrical signal would be converted to an OC-1 optical signal. The OC signal has the same rate, format and functions as the STS signal. The SONET signal can assume the same format as another application such as ATM or FDDI. Many of the developing high data rate application are basing their transmission criteria on the SONET transmission scheme.
*The Above Information is courtesy of Corning Cable Systems.
Fiber Optic transmitters are characterized by the wavelength at which they emit light. The nominal emission wavelength is called the center wavelength, of the transmitter, although the transmitted signal is actually a collection of wavelengths around this nominal value. The center wavelength is primarily a function of the type and configuration of the materials used to fabricate the transmitter. It is usually expressed in nanometers (nm).
LEDs with center wavelengths at 850 nm or 1300 nm have been in wide use for many years, and the transmission specifications for multimode fiber are given at these two wavelengths. LD transmitters for single-mode systems operate at center wavelengths of 1310 nm or 1550 nm; thus single-mode fibers carry specifications for transmission at these two wavelengths. CD lasers operate at a center wavelength of 780 nm and VCSEL development is targeting the 850 nm wavelength.
To account for manufacturing variation, transmitter specifications usually include a tolerance for actual vs. specified center wavelength. For LEDs, this tolerance may be as high as +- 50 nm, while for LDs it would typically be+- 10nm or less.
As mentioned above, the total power produced by an optical transmitter is not confined to just the center wavelength. It is distributed over a range of wavelengths spread about the center wavelength. This range is quantified as the spectral width, measured in nm and it impacts the overall transmission capacity of a fiber optic link. Spectral Width is usually expressed as full-width half maximum (FWHM) value, or as a root-mean-square (RMS) value. Spectral widths vary from a few nanometers for LDs and VCSELs to over one hundred nanometers for LEDs.
As with transmitters, each piece of optical fiber transmission equipment contains a receiver. Nearly all types of receivers used in optical fiber systems incorporate a photo detector such as a photodiode to convert the incoming optical signal back to an electrical signal. The operating wavelength of the receiver should match that of a transmitter. A receiver designed for 1300 nm operation may not be suitable for use at 850 nm.
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