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An Ethernet Primer

Cabling Media

The two most common cabling media in use today for Ethernet networks are unshielded twisted-pair (UTP) cable and optical fiber.

Unshielded Twisted-Pair Cable (UTP)

Twisted-pair cable consists of multiple strands of cable (typically two or four) twisted around one another. This twisting reduces a type of interference known as crosstalk. Crosstalk happens when electrical signals leak out of a wire; twisting the wires around each other helps to cancel out the crosstalk.

Several types of twisted-pair cables exist. They're differentiated by the number of twists per inch that they have. Category 3 (or "Cat 3") has three twists per inch, and "Cat 5" has five twists per foot. The most frequently used type of cable today is Cat 5. Category 5 UTP is used for speeds of 100Mbps.

Fiber Optic Cable

Fiber optic cables speed pulses of light rather than electrons as with UTP. Therefore, when using fiber optic cable, you must first convert electrical signals to optical signals and then reverse that action at the other end.

The optical fibers themselves are glass filaments composed of three layers: the core, the cladding, and a protective layer known as the buffer. One of the basic principles of fiber optics depends on a property known as the index of refraction of light.

The index of refraction is a measurement of the speed of light in a given material. This can be described mathematically as n = c / v, where n is the index of refraction, c is the speed of light, and v is the speed of light in the material. The index of refraction for light traveling in a vacuum is 1. The index of refraction for light traveling in fiber optic cabling is approximately 1.46.

You might be asking yourself, "Why on earth would anyone want to know this?" Going back to the construction of fiber optic cables, the glass layer surrounding the glass core of the optical fiber is the cladding. The surrounding cladding is designed to have a lower index of refraction than the core. Relying on an optical property known as total internal reflection, the cladding's lower index of refraction keeps the light inside the glass core.

Two types of optical fiber are used in networking today: multimode and single-mode. To understand the difference, you must first understand what a mode is. A mode is defined as an electromagnetic field distribution that satisfies Maxwell's equations and the boundary conditions of an optical fiber.

Translated, you can think of a mode as a path that light pulses follow in a fiber. The number of modes is determined by the diameter of the core, the difference in the refractive indices of the core and the cladding, and the wavelength of the light pulse. The larger the diameter of the core is, the more modes are supported. The longer the wavelength of the light pulse is, the fewer modes are supported. 

Dispersion is the broadening of the light pulse, and is caused by the slightly different propagation time of each optical mode. Dispersion limits the spacing of light pulses and, as a result, total transmission bandwidth.

Multimode fiber

Multimode optical fiber has large core surrounded by a large-diameter cladding. The large diameter of the core also allows inexpensive coupling of the optical fibers with LED light sources. Because of the large diameter of the core, multimode fiber supports hundreds of modes. 

A mode, however, should not be thought of as a channel in the sense of broadband or CATV. When a light pulse is emitted into a fiber, the light wave is simultaneously split into all the modes. Because each mode has a slightly different length and, therefore, a different propagation time, the signal pulse is stretched. This effect is known as modal dispersion.

To compensate for modal dispersion, the core of multimode optical fiber is graded so that the index of refraction of the core gradually decreases from the center. Grading the index of refraction allows the speed of light to increase as the cladding is approached. The speed increase compensates for the different paths taken by different modes.

As already mentioned, multimode cores are quite large. The most common diameter is 62.5 microns, although 50 and 100 are also used. The cladding is also quite thick. The diameter of a multimode fiber is given as the core diameter/cladding diameter. Common diameters are 62.5/125, 50/125, and 100/140.

Above all the other advantages of optical fiber over UTP cabling, the one cited most often is distance. Multimode fiber can be used for distances up to 2000m for 10Mbps and 100Mbps Ethernet, and 550m for gigabit Ethernet.

Single-Mode Fiber

Single-mode fiber has a relatively small core, on the order of 8 to 10 microns. Unlike multimode fiber, single-mode fiber supports only one mode of propagation. It is this single mode of propagation, however, that minimizes modal dispersion.

With modal dispersion significantly reduced in single-mode fiber, another type of dispersion becomes a problem: material, or chromatic, dispersion. Material dispersion is caused by the different wavelengths of light sources such as LEDs and even lasers that produce a spectrum of light rather than a single wavelength.

Because of this and the narrow core of the optical fiber, expensive lasers must be used as light sources rather than LEDs. The lasers narrow the spectrum of wavelengths, minimizing material dispersion, but they also drive up the cost of single-mode fiber implementations. Single-mode fiber does not use graded cores. Instead, it has a step index construction in which the index of refraction changes in a single large increment.

The main reason to use single-mode fiber over multimode fiber is for increased distance. Distances of 5000m or more have been achieved for 10Mbps, 100Mbps, and even 1000Mbps Ethernet.

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