Overview of Structured Cabling - Optic Fiber

Optic Fiber Cabling System
Optical fiber truly is the ultimate communication medium. It is compact and easy to install. It has enormous bandwidth. It is completely immune to EMI and RFI. It doesn’t require special routing in the office. And it is becoming more affordable. Optical fiber is available in 2 fundamental forms –single mode and multi mode. Traditionally single mode has been used by the phone companies for long distance trunks, whilst Multimode has been used by companies for in-house communications.

Multimode or  Singlemode?
There are two general types of fiber available for use in data communications networks today—multimode and singlemode. These two fiber types multimode and singlemode. These two fiber types have several characteristics that differentiate them from each other. Before getting into the different types of fiber, let’s review a few facts that apply to optical fiber in general. All fiber is made up of three general parts— the core, cladding, and coating. The cladding layer is made of pure silica and has a lower  refractive index than the core. This difference in  refractive index is what allows the light to travel within the core. The core/ cladding interface acts like a mirror to keep the light from refracting out of the fiber. Optical fiber works because of this principle of total internal reflection.
Multimode
As the name suggests, multimode fiber is designed to support many modes or rays of light. Multimode fiber is available in several core sizes, each having characteristics that make it appropriate for different network needs. Fiber core size is measured in microns and multimode fiber is specified by its core size. Available today are 50mm, 62.5mm and 100mm multimode fibers. The most common fiber and the one recommended in ANSI X3T9.5 for FDDI (Fiber Distributed Data Interface) networks is 62.5mm fiber. The core size represents the nominal diameter of the core of the fiber. The cladding diameter of the fiber is the industry standard 125mm. diameter of the fiber is the industry standard 125mm.This fiber is often represented with this nomenclature: 62.5/125mm.
The properties of the 62.5/125mm fiber make it easy to work with while providing excellent optical characteristics. This multimode fiber is not very sensitive to stress like some other fiber designs, and is available with a variety of attenuation and bandwidth specifications.

Singlemode
Singlemode fiber has been used for long distance networks for some time and has become more popular in data communications over the last few years. As the name suggests, singlemode fiber is designed to support one mode of light. It is used with a very narrow laser source and can carry large amounts of information over long distances. Its core size is typically between 8.5 and 9.5mm and cladding diameters are the standard 125mm. With singlemode fiber, the mode field diameter (MFD) is specified in lieu of a core size require-ment. The MFD is slightly larger than the core and represents the actual diameter of the light mode propagating through the fiber. Singlemode fiber is chosen for its unlimited bandwidth and extremely low attenuation values. It is ideal for transmitting long distances, often up to 50 kilometers or more without requiring a repeater.

Attenuation

Attenuation is defined as the amount of light loss along a length of fiber. It is expressed as decibels (dB) and is usually normalized to a kilometer. Therefore, an attenuation value for multimode fiber might be 3.0dB/km. Attenuation is a logarithmic function and 3.0dB represents a 50% signal loss, in this case, over a kilometer of fiber. Glass fiber has transmission properties that vary with the wavelength of light. Some wavelengths are absorbed more while others are reflected. The two standard transmission wavelengths for multimode fiber are 850nm (nanometers) and 1300nm. LED sources that transmit at 850nm are fairly inexpensive and often meet the requirements of shorter data network installations. Typical fiber optic cable attenuation specifications at the 850nm wavelength (often referred to as the "850 window") would be between 3.0 and 4.0 dB/km. Specifications for the 1300nm window would often be between 1.0 and 1.5 dB/km.
Bandwidth
Fiber bandwidth is a specification that defines the transmission carrying capacity of the fiber. It is expressed in MHz*km and is typically one of the most difficult characteristics to understand. The bandwidth of a fiber is limited due to dispersion. Dispersion is the spreading and/or distortion of a light pulse as it travels from the transmitter end of the fiber to the receiver end.  

Fiber Standards & Specifications

ANSI / EIA / TIA
There are a large number of documents published under this combination of standards groups that define appropriate fiber, cable and electronics to be used in order to meet various network requirements. There are also a series of fiber optic test procedures (FOTPs) for all optical, environmental and physical testing that most suppliers use when qualifying their products.

ANSI / TIA / EIA-568A
ANSI/TIA/EIA-568A defines commercial building wiring standards for fiber optic cable, connectors, splices and network topology. Many system designers make these specifications and the tests a requirement when purchasing optical components.

FDDI
The Fiber Distributed Data Interface specification is often thought of as the name of a standard. FDDI is often used as a backbone network for lower speed networks like Ethernet or Token Ring. FDDI is a high speed fiber optic network consisting of dual counter-rotating rings and bypass switches. FDDI system specifications include a 100 Mbps data rate and a maximum distance between nodes of 2 km (multimode fiber). The fiber requirements are for 62.5/125 mm fiber size with a minimum bandwidth of 500 MHz*km at the 1300 nm operating

ISO / IEC
These international standards are similar in nature to the EIA/TIA documents often referred to in domestic network designs. In particular ISO/IEC 11801 is very similar to TIA/EIA-568A. However, there are differences that you should be aware of before you design or install a fiber based system to ISO/IEC 11801 standards. But, by and large, 568A and 11801 are harmonized to a large extent.



 
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