Straightforward Fiber Optic LAN Design

To those unfamiliar to fiber optics, the process of local area network (LAN) design may seem complicated and tricky. However, the evolution of fiber optic LANs and standards has significantly simplified this process. In fact, the design process is now straightforward. Not simple, mind you, but straightforward, as you will see. LAN design starts with your definition of three characteristics of the network: the end points of each link, a map of each fiber path between these end points, and the protocol to be run over the network. The endpoints of each link, which are the locations of the transmitter and receiver, define the link length. The map defines number of fiber to fiber connections. The length and number of connections will define the power loss between the transmitter and receiver. You will calculate and compare the power loss to the maximum loss allowed by the protocol. With most protocols, the link length and the protocol define the fiber to be used. More specifically, these two characteristics of the network define the fiber: core diameter, clad diameter, numerical aperture (NA), wavelength, bandwidth distance product [or dispersion rate] and the main characteristics of the transmitter-receiver pair. In addition to link length and protocol, you may need to define the maximum bit rate to run on the link. This characteristic is required for protocols that support multiple bit rates, such as asynchronous transfer mode (ATM) and Fiber Channel. Once you define the maximum bit rate, the fiber attenuation rate will either be defined or easily determined. The fiber attenuation rate will be defined in the case of networks designed to comply with TIA/EIA-568-A or -B. In all other cases, the fiber attenuation rate depends upon the competitive nature of the market. When any fiber characteristic is not defined by a protocol, the value you will use becomes determined by the cable manufacturers' desire to avoid their products appearing inferior to other products. Rarely, will cable manufacturers define attenuation rates to be worse [I.e., higher] than those values specified by competitors. For instance, almost all 62.5 µm cables have attenuation rates of 3.5 dB/km at 850 nm and 1.5 dB/km at 1300 nm. The protocol defines the major characteristics for the optoelectronics: wavelength, spectral width, optical power budget [or maximum allowable loss], bit rate and bit error rate (BER). The protocol, or the Building Wiring Standard, usually defines the connector to be used and its maximum loss rating in dB/pair. However, since the connector market is at the beginning of a major transition from legacy ST-compatible and SC products to small form factor [SFF] designs, the decision on connector type requires some evaluation to determine the connector best suited to the application. At this point in the process, you will have defined the important optical performance characteristics of the fiber, connectors, and optoelectronics. You continue the design process by defining the cable requirements. The cable has four major requirements, such as National Fire Protection Association(NFPA)/NEC rating, installation load, bend radius, and operating temperature range. The NEC rating, installation load, bend radius, and the environment and locations in which the cable is installed determine temperature range. The installation load may be influenced by how experienced the installers are: less experienced installers may require an increased installation load rating to avoid damaging the fibers. Most LANs that are restricted inside buildings have the same characteristics: tight-tube premises or distribution cable designs, OFNR or higher-rated backbone fiber cables with enclosures and patch panels at the ends, patch cords from patch panels to optoelectronics, and inner duct to segregate fiber cables from copper cables. LANs with cable between buildings have more potential choices and specifications than those restricted within buildings. Indoor cables may be spliced or patched to outdoor cables. Alternatively, indoor/outdoor cables may be used to reduce network cost. Indoor/outdoor cables come in at least three versions: cables with two jackets that require removal of the outer jacket to comply with the NEC; tight tube designs; and loose tube designs. Each of these versions has advantages and disadvantages, which you will need to evaluate to determine which best suits your specific application. In addition, the moisture resistance required for outdoor cables comes in five different forms: gel filled; gel filled and grease filled; gel filled with water-swellable tapes; water-swellable tapes without gel or grease. You will need to evaluate each of these options for each application on a case-by-case basis. Outdoor cables usually require up to 13 different specifications for resistance to environmental effects. These resistances include resistance to damage from moisture, rodents, crush loads, chemicals, high voltage, abrasion, and ionizing radiation. To this point, the process of FO LAN design is straightforward. Beyond this point, the subtleties complicate matters (e.g., the devil is in the details). The fiber specifications, as stated in the protocols and standards, are insufficient to prevent problems during implementation and operation of the network. Fiber characteristics such as core ovality (i.e., noncircularity) and core offset are required to ensure successful operation. The qualifiers in product data sheets become critical to choosing products appropriate to the application: values qualified with the words 'maximum', 'minimum,' 'typica,' or 'nominal' have different impacts on the reliability of the network. Finally, the analysis of network cost becomes critical to landing the job and to making a profit. Novices make the common mistake of choosing products based on product cost. Unfortunately, choosing products with this strategy does not guarantee that you will achieve the minimum total installed cost, which, in reality, is your true goal. Low-cost cable designs installed by high-labor-cost installers may have a higher installed cost than high-cost cable designs with low time requirements for end preparation. Inexpensive connectors that can only be installed slowly in our high-cost labor environment can have a higher installed cost than expensive connectors that can be installed rapidly. While the process is straightforward, the degree to which you address the details will determine the cost, profit, and reliability of the fiber optic LAN installation. For the last 11 years, we have been training people in how to create comprehensive specifications on fibers, cables, connectors, and electronics. These comprehensive specifications ensure low-cost network implementation and reliable network operation. PEARSON, a certified professional consultant, is president of Pearson Technologies Inc., and a veteran in fiber optic communications. He is a director of the Fiber Optic Association and director of certification of the same association. Pearson Technologies Inc., is a fiber optic consultant offering technical and sales training. Pearson can be reached at (800) 589-2549, (770) 591-8921 and

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