Fiber Optics

.. olor. The 1300nm and 1550nm transmitters emit light only in the infrared spectrum. The difference in performance of the various wavelengths is beyond the scope of this paper. What is important is an awareness of the wavelengths and that the equipment on both ends of the fiber needs to be matched. The final characteristic of transmitters is the output power.

This is a measure of the optical energy (intensity) launched into the fiber. It is measured in dBm. A typical value for multi mode transmitters used in Ethernet is -15dBm. Single mode transmitters have a wide range in power depending on the application. Receiver Specifications With a knowledge of transmitters, what happens at the other end of the cable is important.

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The light pulses are terminated and detected with a receiver. Receivers have three basic considerations. These are: 1.wavelength 2.mode (single vs. multi) 3.sensitivity Sensitivity is the counterpart to power for transmitters. It is a measurement of how much light is required to accurately detect and decode the data in light stream.

It is expressed in dBm and is a negative number. The smaller the number (remember -40 is smaller than -30) the better the receiver. Typical values range from -30dBm to -40dBm. Receive sensitivity and transmitter power are used to calculate the optical power budget available for the cable. This calculation is: Power Budget = Transmitter Power – Receiver Sensitivity, Using the typical values given for multi mode Ethernet above, the power budget would be: 15dBm = -15dBm – (-30dBm) The optical power budget must be greater then all of the cable plant losses (such as attenuation, losses due to splices and connectors, etc.) for the installation to work properly.

Connector Types Figure A. – SC Connector Figure B. – ST Connector Many different connector styles have found their way into fiber optic networking. The SC connector (Figure A) has recently been standardized by ANSI TIA/EIA-568A for use in structured wiring installations. Many single mode applications are now only available in the SC style.

The ST connector (Figure B) has been the connector of choice for these environments, and continues to be widely used. FDDI uses the MIC connector which is a duplex connector. It is physically larger then the SC connector, and the SC connector is gaining acceptance in the FDDI marketplace. Fiber Advantages Fiber provides several advantages to Ethernet and Fast Ethernet networks. The most common advantage and therefore use of fiber is to overcome the distance limitations of coaxial and twisted pair copper topologies.

Ethernet being run on coax (10Base2) has a maximum distance limitation of 185m, and Ethernet being run on twisted pair (10BaseT and 100BaseTX) has a limitation of 100m. Fiber can greatly extend these distances with multi-mode fiber providing 2000m and single-mode fiber supporting 5km in half duplex environments, and much more (depending on transmitter strength and receiver sensitivity) in full duplex installations. Ethernet running at 10Mbps has a limitation of 4 repeaters, providing some leniency in the solutions available for distance, however, Fast Ethernet only allows for 2 repeaters and only 5m of cable between them. As Fast Ethernet becomes more ubiquitous, the need for fiber optic cabling will grow as well. When distance is an issue, fiber provides what may be the only solution.

Even when using coaxial cable or twisted pair (shielded or unshielded), some electrical noise may be emitted by the cable. This is especially true as connectors and ground connections age or weaken. In some environments (medical for example), the potential risk associated with this is just not acceptable, and costs of alternative cable routings too high. Because fiber optic cabling uses light pulses to send the signal, there is NO radiated noise. This makes it perfectly safe to install this cabling in any sensitive environment. Optical fiber adds additional security protection as well. There are no emissions to pick up and decode, and it is not feasible to tap into it for the purposes of eavesdropping.

This makes fiber optic cabling ideal for secure network installations. Another problem that is common when using copper cabling is other electrical noise getting into the desired electrical networking signal. This can be a problem in noisy manufacturing environments or other heavy industrial applications. The use of optical fiber provides a signal that will be completely unaffected by this noise. In some instances, fiber provides the advantage that it can withstand more tension during the cable pulling.

It is also smaller in size then twisted pair cables and therefore takes up less room. Compared to Category 5 UTP, most duplex fiber optical cable can also endure a tighter bend radius while maintaining specified performance. Fiber Challenges Fiber optical cabling is not a cure-all however, there are some challenges to be resolved. The first (and probably the best known), is the cost of termination. Because of the need for perfect connections, splices and connections must be carefully cut and then polished to preserve the optical characteristics.

The connectors must also maintain a very high level of precision to guarantee alignment of the fibers. The second problem that is encountered when installing fiber cabling is that legacy equipment does not support fiber connections. Very few desktop computers have a fiber network interface, and some critical network equipment does not offer a fiber interface. In Ethernet, the size of the collision domain can effect the use of fiber. In a half duplex (shared media) environment, no 2 devices can be separated by more then 512 bit times.

While the transmission of a signal is faster through fiber than copper, only about 11% faster and not enough to make a significant difference. This limitation means that there are times when the signal quality and fiber are sufficient to carry the signal but the distance and network design rule out it’s use. Fiber Solutions Fortunately, the problems are not without solutions. As fiber deployment increases, the economy of scale for the manufacturers is driving costs down. Also, much work is being done to further reduce these costs, Plastic Optical Fiber is an example of one such development.

The need to connect to legacy equipment and infrastructure also has a solution. By using copper to fiber media converters, fiber can be connected to almost any legacy environment. Equipment equipped with an AUI port can also make use of fiber transceivers as well. Media converters are devices (usually small enough in size to fit in the palm of your hand) which take in signals from one media type and send it out on another media type. For those instances when collision domain restrictions preclude the use of fiber, a 2 port bridging device (such as Transition Networks Bridging Media Converter) with 10/100-Base-T(X) on one port and fiber on the other can be used. Bridges by definition break collision domains, and when connected to a server, workstation, or another bridge can operate in Full Duplex mode. In this mode, there are no limitations imposed by collision domains, and the distance attainable is solely a function of the fiber cable; and transmitters and receivers. Summary Fiber optic cabling is rapidly becoming the most viable choice for data networking infrastructure.

With the cost of cable, connectors, installation, and equipment becoming competitive with traditional copper solutions, fiber should be given serious consideration. Transition Networks’ complete line of fiber connectivity products are specifically designed to ease this migration to fiber. Once installed, fiber optic cabling will future proof your cabling infrastructure, providing support for even the fastest most demanding protocols. Work Cited www.idon’ Science.