Interconnectives - Transmission Media

 Introduction.  For lack of a better word I have decided to refer to the physical or not-so-physical transmission media as interconnectives.  Remember, these connections are made via a large range of electromagnetic wave frequencies using "wires" or "wireless".  An electromagnetic wave is the physical form of energy described by the electromagnetic spectrum.  The spectrum starts at zero oscillations, rises through the range of frequencies that can be perceived by human senses through the various forms of light, up to the frequencies called X-rays and gamma rays.   The rates of frequency are measured in Hz - Hertz.  This is directly related to the wavelength, which can vary from billionths of a meter to several meters.  For whatever reasons, Hz are usually used for lower frequency bands and wavelengths are usually used when talking about higher frequency phenomena such as light.  The standard names and ranges/bands are given in the following diagram.

 Notice how the visible range is very very small.  We now need to quickly define a few terms which you have hopefully seen before. All signals consume some of their own energy to overcome the resistance inherent in the transmission media.  They also tend to disperse or radiate as they move. This reduction in signal strength is called attenuation. As signals travel they are subject to the influence of other influences such as noise, the ballast of fluorescent lamps and whatever.   The impact these other influences incur, unwanted modification of signals in transit, is called distortion. Another important characteristic of an electromagnetic wave/signal is how easily it is broken up or how frangible they are.  For example, radio waves can penetrate and be received through all but the densest materials. Directionality has to do with how well a signal can be focused towards a particular location.  This capability increases with the frequency Bandwidth is the maximum amount of data that can be carried over a specific transmission media. Now we need to state one basic principle. The lower the frequency, the lower the potential for carrying data because there are fewer state changes per unit time.  One bit of information can be transmitted per cycle. The following tables contain some of the most important signal characteristics for low and high frequencies.

 Low Frequencies persistent signal more durable more penetrating less frangible FCC regulated due to the above low bandwidth broad radiation

 High Frequencies more frangible high bandwidth potential for tightly focused transmission

 For example, light doesn't penetrate even the slimmest of opaque materials.  But it is very capable of transmitting data at high rates.  Radio waves tend to be rather omni-directional, but not very frangible.  We will make use of these concepts to compare different transmission media. Now we will start to describe different transmission media.  We will split this into two sections, one for wired, the other for wireless.

Wired Transmission Media

 We will survey the following wired transmission media Coaxial cable Twisted pair Fiber-optic cabling Coaxial Cable.  Coaxial cable, usually referred to as coax, is named because it has two concentric conductors separated by insulation.  The two conductors have the same axis.  The following diagram illustrates the standard configuration.

 Coax cables may look the same but they can have very different levels of impedance.   For example, the 10Base2 Ethernet coax cabling has 50 ohm impedance in a wire that's three-eighths of an inch in diameter.  This provides 10Mbps signal speed for up to 185 meters. Two of the biggest advantages of coax are that it can support high bandwidth communications over relatively long runs without repeaters or hubs.  Coax was the original transmission media specified for Ethernet.  Since then it has become almost completely supplanted by some form of twisted pair wiring.  The main reasons for this are coax is a relatively fragile cable that does not suffer kinks, severe bends or crushing pressure very well.  Any of these can inhibit performance significantly.  Other major reasons for this supplanting are due to coax's cost and size.  It is more expensive than twisted pair and it takes up much more space in cable ducts, raceways and whatever. Currently, coax is generally used only for bringing broadband cable TV signals to subscribers. Twisted Pair Wire.  Twisted pair wiring has been used to support voice communications for quite a long time.  It has become the pervasive media used for LANs.  Twisted pair wiring makes use of two relatively thin wires, 18 to 24 American Wire Gauge, from 0.016 to 0.035 inches in diameter.  The wires are coated and spiraled around each other.  The twisting helps cancel out electromagnetic interference.  The gauge or thickness of the wire is highly related to its performance.  Thicker wires translate it to greater capability to handle greater bandwidth.  Unfortunately, as gauge increases so does attenuation. Twisted pair wiring comes in a large variety that range from a single pair of voice grade wires to 600-pair trunk cables.  Some other features that can be modified to improve performance are increasing thickness increasing twist rate using a variety of twist rates in bundles of multiple pairs shielding the pairs with a metallic barrier For LANs, four pairs of twisted pair wires are usually bundled in a sheathing.  The following image represents a shielded bundling with more than four pairs.

 The two main types of twisted pair are shielded and unshielded.  Shielded twisted pair -STP uses an extra layer of foil or braided metallic wire directly below the sheathing.  While the extra shielding helps prevent extraneous noise from being inducted  into the wiring, it also increases self-impedance.  Certain radiation that would naturally occur without he shielding is bounced back through the signal and significantly increases attenuation and is usually more detrimental than beneficial. Unshielded twisted pair - UTP is much more common.  Four pair UTP makes use of eight wires or leads separated into groups of two.  One way these are often used is that one pair supports transmission and another pair supports reception.  The other wires aren't used in most LANs.  But for those that operate at speeds of 100Mbps or higher all four pairs are used. Twisted pair is a commodity and is very likely to have consistent performance regardless of the manufacturer.  UTP is classified by categories of performance by their function.  Manufacturers have to prove compliance with certain standards.  Originally, there were five benchmark categories numbered one through five.  Over time, the market has coalesced into two viable performance levels, Cat-3 and Cat-5.  Cat-6 and Cat-7 have been added but at present they are still very expensive.  Category 3 UTP offers 16MHz of bandwidth which translates to 10Mbps for up to 100 meters.  Category 5 can support up to 100MHz of bandwidth .  Depending on the distance limitations and LAN architecture, this can provide 100Mbps, 155Mbps or up to 256Mbps.  Unfortunately, as maximum speed increases, maximum distance decreases. Fiber Optic Cable.  Fiber optic cables can carry the higher frequencies of the electromagnetic spectrum.  There is a huge variety of fiber optic cabling and it seems the only universal characteristics are the center axis of the cable is made of high-purity optical media that's capable of reliably carrying light patterns over long distances the absence of either an electrical signal or pliable copper conductor means transmissions are relatively secure - virtually impossible to tap into when stressed sufficiently glass shatters thereby stopping the signal - which improves its security the optical media called the fiber is clad with a concentric protective layer of plastic The extent of attenuation over distance varies according to the wavelength of the light and to the composition of the fiber.  Fiber optic cabling is used in pairs, one for transmission, the other for reception. Fibers are usually described with a pair of numbers FiberDiameter/CladdingDiameter.  For example, one extremely common LAN fiber is known as 62.5/125 micron glass. Light can either radiate outwards, as in a candle, or be focused directionally as done in flashlights.  But even focused light is subject to some dispersion, think of flashlight beams as they get further from the source. There are two types of fiber optic transmission Multimode Single Mode Multimode transmissions are driven by a light emitting diode (LED).  LEDs sit at the lower end of the light spectrum do they have relatively lower bandwidth potential when compared to other light sources but they are relatively less expensive.  An LED is not a very concentrated light source and as such requires a fairly wide transmission path.  This rate of dispersion imposes practical limitations on the effective distance of LED driven fiber optic cable.  The following diagram represents this dispersion.

 Over distance, this dispersion results in some LED beam impacting the inside wall of the glass media.  When this happens, the impact is at a shallow angle and the light does not escape into the cladding.  The reflections puts the dispersed beams in a collision course with the remaining axial transmission.  The following diagram represents this.

 Thus, multimode beams are subject to attenuation due to these photonic collisions.  Also, the original signal will arrive before the multiple reflections. Multiple transmission modes can also be caused by improper terminations of the fiber optic cable and/or poor connections to the hardware.  Non-concentric terminations result in dispersion.  Even though some hardware takes advantage of multimodal dispersion it can get out of hand. While we have spent a lot of effort to present the things that can go wrong with multimodal transmissions, they still have some advantages, primarily the cabling and hardware that drive it are less expensive than for single mode.  It is also the case that it is easier to wok with when compared to single mode fiber optic cabling because it is several times larger. Single mode fiber makes use of an Injection Laser Diode - (ILD).  Lasers are well known for their highly concentrated beams.  This beam still disperses, but quite negligibly over distances associated with LANs.  Even at the outer limits of its effective range the beam still does not contact the inside walls of the fiber.  The transmission remains well aligned within the center axis of glass.  It reaches its destination in a single mode all at once.Single mode fibers are usually 5 or 10 microns with 125 micron classing.  The costs of the fiber and laser hardware make this more of a commercial grade infrastructure technology than a LAN technology.  There is tremendous available bandwidth.  It is usually used in commercial telephone networks.

Wireless Transmission Media