TRANSMISSION MEDIA

Transmission MediaTypes:1. Guided - signal is directed along a physical media. Examples: coaxial cable, twisted pair, fiber optic2. Unguided - provide a means of transmitting

electromagnetic signals but do not guide them.

Examples: air, vacuum, water, light wave propagation

Twisted PairComposed of 2 insulated wires twisted with each other to reduceelectromagnetic inference with other twisted wires. A thin-diameter wire may range from 22 to 26 guage or 0.016 to 0.036.A number of pairs (up to 300 pairs) are usually bundled togetheron a protective sheath.

Common Uses• Telephone• Private Branch Exchange (PBX)• Local Area Network (LAN)

Twisted Pair Types:1. Unshielded (UTP) - most common2. Shielded (STP) - used in noisy environments where the

shield protects against excessive electromagnetic interference.

Stranded and Solid Twisted PairStranded - most common and is very flexible for bending

around corners. Solid - has less attenuation and can span longer

distances, but is less flexible than stranded wire and cannot be repeatedly bent.

Twisted Pair CategoriesCategory Cable type Application 1 UTP Analog voice 2 UTP Digital voice 1 Mbps data 3 UTP, STP 16 Mbps data 4 UTP, STP 20 Mbps data 5 UTP, STP 100 Mbps data Level 6 UTP, STP 155 Mbps data Level 7 UTP, STP 1000 Mbps data

Twisted Pair Properties• exhibits more signal distortion per

meter thus used in limited distance, bandwidth, and data rate

• twisting reduces low frequency interference and crosstalk.

STP UTP

shield

Shielded and Unshielded Twisted PairsThe metal shield on the STP cable adds

protection against external interference.

main wire

insulation

ground

insulation

Coaxial CablePertains to several layers of materials surrounding a common axis, hence the term co-axis or coaxial. Contains an insulated solid or stranded copper wire surrounded by a solid or braided metallic shield, wrapped in a plastic cover. The outer jacket is usually made up of one of the following: plenum, teflon, or polyvinyl chloride.

Common Uses• carry long distance telephone signals• carry long distance television transmission• distribute TV signals to homes (cable TV)• render short-range connections like I/O devices on computers• local area networking (LAN)

Transmission Properties• central conductor carries current in one direction, the braided shield is used as the return path of that current, thus magnetic fields coming from the coax is self-cancelled that is why it is less susceptible to interference and crosstalk than TP.

Fiber OpticsA thin glass wire designed for light transmission, capable

of transmitting trillions of bits per second. An optical fiber is constructed of a transparent core made of pure silicon dioxide (SiO2), through which the light travels. This core is so transparent that you could see through a three-mile thick window made out of it. The core is surrounded by a cladding layer that reflects light, keeping it in the core. The cladding is surrounded by a plastic layer, a layer of kevlar fibers for strength and an outer sheath of plastic or Teflon.

Common Uses• backbone of long-haul telephone transmission• metropolitan trunks• local area network• Asynchronous Transfer Mode (ATM) & Fiber Distributed Data Interface (FDDI) are applications of optical fiber.

Transmission Properties• huge transmission capacity• light pulses are not affected by random radiation in the

environment as are electrical pulses.• error rate in transmitting light pulses is significantly

lower than electrical pulses. • allow longer distances to be spanned without repeaters

in between that regenerate fading signals. • more secure, because taps in the line can be detected• installation is streamlined due to the dramatically lower

weight of the material compared to copper cables.

Two primary types of fiber1. Multimode fiber

• light can enter the core at different angles, making it easier to connect the light source. • light rays bounce around within the core causing some distortion and providing less bandwidth •common for short distances • core diameter of from 50 to 100 microns.

2. Singlemode fiber• used for high-speed transmission over long distances• provides greater bandwidth than multimode• smaller core makes it more difficult to couple the

light source. • core diameter of less than 10 microns.

Multimode

Singlemode

cladding

core

Fiber StrandsThe fibers in this picture are being prepared for

splicing in a wiring closet. These few

strands can collectively transmit billions of bits

per second.

Fiber Versus Copper

Not only does optical fiber

provide enormous transmission

bandwidth, but it takes a lot less

room. The single strand of fiber in

the center is equivalent in

capacity to any one of the copper bundles in the

picture.

Rip cord

Core tube

Each of the 12 ribbons has 24 fibers

Water-blocking tape

Inner sheath

Metal armor

High-densitypolyethylenejacket

Dielectric strength member

.9" diameter

288 Fibers in One CableWith the assistance of Antec Corporation, Lucent developed this record high-fiber-count, singlemode cable with 288 fibers. In 1996, Time Warner Cable in New York purchased 50 miles of it for transporting high-bandwidth video, voice and data to its Manhattan subscribers.

Fiber Optics GlossaryThe following terms are courtesy of Panduit Corporation, a leading manufacturer of wiring and network cabling products. For more information, visit www.panduit.com.

adapterA mechanical device designed to align fiber-optic connectors. It contains the split sleeve (interconnect sleeve) that holds the two ferrules together.adapter sleeveA mechanical fixture within the adapter body that aligns and holds two terminated fiber connectors. Adapter sleeve material is typically phosphor bronze, ceramic or polymer.

absorptionThe absorbing of light energy within an optical fiber due to natural impurities in the glass. Absorption and scattering are the main cause of attenuation (signal loss) in an optical fiber.acceptance angleThe angle, measured in degrees, at which the core of the fiber will accept light, measured from the fiber core axis (center of core). aramid yarnA member in optical fiber cable that provides support, protection and tensile strength. Also referred to as KEVLAR, which is a brand of aramid yarn.attenuationThe loss of signal strength (optical power) during transmission between two points. It expresses the total loss of an optical system, measured in decibels per kilometer (dB/km) at specific wavelengths.

Acceptance Cone

Acceptance Angle

Core

Cladding

backbone cablingThe interbuilding and intrabuilding cable connections between entrance facilities, equipment rooms and the telecommunications closets. It consists of the transmission media, main and intermediate cross-connects and terminations at these locations.bandwidthThe information-carrying capacity of an optical fiber. It is measured in MHz-km and GHz-km, as distance plays an important role.bufferThe protective layer that surrounds the fiber cladding. Fabrication techniques include tight or loose tube buffering.cable assemblyAn optical fiber cable that has connectors installed on one or both ends. When connectors are installed to only one end, it is referred to as a pigtail. When connectors are installed on both ends, it is known as a patch cord.

chromatic dispersionThe spreading of light pulses caused by the difference in refractive indices at different wavelengths.claddingThe material surrounding the core of an optical fiber. The cladding has a lower refractive index (faster speed) in order to keep the light in the core. The cladding and core make up an optical waveguide.cleaveThe process of scoring and breaking the optical fiber end in order to terminate a connector.coatingA protective layer applied over the fiber cladding during the drawing process to protect it from the environment.

Black polyurethane outer jacket

Strength members

Buffer Jacket

Silicone coating

Cladding (silica)

Core (silica) Optical fiber

connectorA mechanical device used on a fiber to provide a means for aligning, attaching and decouplng the fiber to a transmitter, receiver or other another fiber. Commonly used connections include 568SC (Duplex SC), ST, FDDI, FC, D4 and Biconic.coreThe central region of an optical fiber through which light is transmitted. It has a higher refractive index (slower speed) than the surrounding cladding.couplerA device that feeds the light from two or more fibers into the core of a single fiber.decibelA unit of measure used to express the relative strength of a signal.

dielectricA material such as a glass fiber, which is not metallic and is not conductive.dispersionThe cause of bandwidth limitations in a fiber. Dispersion causes the spreading or broadening of light pulses as they travel through a fiber. The three major types are modal dispersion, chromatic dispersion and waveguide dispersion.duplex cordA two-fiber cable used for bi-directional transmission.EMI (electromagnetic interference)The interference in signal transmission or reception resulting from radiation of electrical or magnetic fields. Optical fibers are not susceptible to EMI.

enclosureA cabinet used to organize and enclose cable terminations and splices for use within main equipment rooms, entrance facilities, main or intermediate cross-connects and telecommunications closets.epoxyA thermosetting resin used to secure the fiber with the connector ferrule.extrinsic lossThe loss that is induced in an optical transmission system by an external source. In a fiber-optic link, this can be caused by improper alignment of connectors or splices.ferruleA mechanically rigid fixture within a connector body that aligns and holds the fiber (core and cladding) within the connector. Ferrule materials are ceramic, plastic and stainless steel.

Ferrule

Fiber

fiber

fiberA thin filament of glass optical waveguide consisting of a core (inner region) and a cladding (outer region) and a protective coating.fiber opticsInformation transmitted through optical fibers in the form of light.fusion spliceThe joining of two fiber ends by applying enough heat to fuse or melt the ends together to form a continuous single fiber.graded indexA fiber designed to be slower in the center of the core and faster toward the outside. The refractive index of the outer area is lower than the center, which allows the light waves to travel faster, and the difference in refractive index bends them inward. This type of fiber reduces modal dispersion and provides high bandwidth capabilities. Contrast with "step index" in this list of terms.

index of refractionSame as "refractive index."insertion loss or injection lossThe loss of light that results when two fibers are joined at a connection point.intrinsic lossThe loss due to inherent traits within the fiber; for example, absorption (light energy is absorbed in the glass) and splice loss (mismatched numerical aperture).laser diodeAn optoelectronic device that produces light with a narrow range of wavelengths. It is always used for singlemode fiber and certain high-bandwidth multimode fiber such as used with Gigabit Ethernet.

LED (light emitting diode)An optoelectronic device that produces light with a wide range of wavelengths. LEDs are typically used with lower-bandwidth multimode fiber.loose tubeThe protective tube surrounding one or more fibers. This is usually found in cables used for outdoor installations.macrobendingThe loss due to large scale bending (extrinsic loss). Bending causes imperfect guiding of light which will exceed the critical angle of reflection. Macrobending loss can be reversed once the bend is corrected.mechanical spliceJoining two fiber ends together by a temporary or permanent mechanical method in order to maintain continuous signal transmission.

microbendingThe loss of light due to small distrotions in the fiber, not usually visible to the naked eye.micron (¦m)One micro-meter or one millionth of a meter. Used to express the geometric dimension of fibers.modal dispersionThe spreading of light pulses along the length of the fiber caused by differential optical path lengths in a multimode fiber.modeThe light path through the fiber.multimodeAn optical fiber in which light travels in multiple modes. It typically has a 62.5 ¦m core within a 125 ¦m cladding.

Cladding

250 µm

62.5 µm 125 µm

Coating

multiplexCombining two or more signals into a single bit stream that can be individually recovered.nanometerA unit of measurement equal to one billionth of a meter.numerical aperture (NA)A number that expresses the light gathering capability of a fiber. It is the ratio of the refractive index of the core to the cladding.OFNR (Optical Fiber Non-conductive Riser)A type of optical fiber.OTDR (Optical Time Domain Reflectometer)An instrument that measures optical transmission characteristics by sending a short pulse of light down a fiber and observing backscattered light. Used to measure fiber attenuation and evaluate optical transmission at splices and connectors.

OFNP (Optical Fiber Non-conductive Plenum)A type of optical fiber.optical waveguideA system that can carry light. An optical fiber.photodiodeA device that receives optical power and changes it to electrical power.PC (Physical Contacting)Refers to the type of fiber-optic connector that makes actual contact of two terminated fiber ends, keeping signal losses to a minimum.patch cordA specific length of optical fiber cable with terminated connectors on each end. Used for connecting patch panels or optoelectronic devices.pigtailA specific length of fiber-optic cable with a terminated connector on one end. The bare end will usually be spliced to feeder cable.

polishing paperAlso known as lapping film, it is a paper with a fine grit used to remove any imperfections in the fiber end surface that may exist after cleaving. Fiber ends terminated within a connector are polished flush with the end of the ferrule.polishing puckA device used to hold the connector during the polishing of the fiber.receiver (RX)An optoelectronic device that converts optical signals into electrical signals.reflectionThe process that occurs when a light ray traveling in one material hits a different material and reflects back into the original material without loss of light.

Cladding(low refractive index)

Fiberjacket

light

Silica Glass Core(high refractive index)

refractionThe bending of light rays as they pass through a transmission medium of one refractive index into a medium with a different refractive index.refractive indexThe ratio of the velocity of light in a vacuum to the velocity of light in a specific material. Using 1.0 as the base reference, the higher the number, the slower the speed of the lightwaves.riserA pathway for indoor cables that pass between floors.scatteringA property of glass that causes light to deflect from the fiber and contribute to losses (intrinsic attenuation).singlemodeAn optical fiber in which the signal travels in one mode (path). It typically has an 8-10 ¦m core within a 125 ¦m cladding.

Coating

250 µm

8-10 µm

125 µm

Cladding

spliceA method for joining two optical fiber ends. Fusion splicing and mechanical splicing are the two types.splice closureA container used to hold and protect splice trays.splice trayA container used to hold, organize and protect spliced fibers.split sleeveThe part of a fiber-optic adapter that aligns the ferrules of two terminated connectors.splitterA device that takes the light from one fiber and injects it into the cores of several other fibers.

step indexA fiber in which the core has a single uniform refractive index. Contrast with "graded index" in this list of terms.tight bufferA protective coating (typically 900 ¦m) that is extruded directly over the primary coating of fibers. Provides high tensile strength, durability, ease of handling and termination.transmitter (TX)An optoelectronic device that converts an electrical signal to an optical signal. It is usually an LED or laser diode.transceiverA transmitter and receiver combined in one device.

wavelengthThe length of a wave measured from any point on one wave to the corresponding point on the next, such as from crest to crest. Wavelength determines the nature of the various forms of radiant entry that comprise the electromagnetic spectrum. The wavelengths of light used in optical fiber communications are measured in nanometers. Common wavelengths are 850, 1300 and 1350 nm.

COMPARISON OF LAN TRANSMISSION MEDIA

Twisted Pair Coaxial Fiber Optic

Transmission- 10Mbps over 10Mbps over 100Mbps over

rate 100 meter 500 meter 2km.

Flexibility flexible stiff flexible

Ease of Insta- very easy relatively easy difficult

llation

Cost Inexpensive Moderate Expensive

Reliabilty Good Good Excellent

Factors to consider in selecting transmission mediareliability

ease of installation and maintenance

flexibility (ability to be reconfigured)

cost

conditions of use

maximum transmission rate

security of data

resistance to interference

WIRELESS TRANSMISSION

Electromagnetic Waves (EM)• the energy that exists in all things produced by

oscillating electrons• composed of sinusoidal electric and magnetic fields

that are perpendicular to each other

• the number of oscillations per second of the wave, or its frequency is expressed in Hertz (Hz) while the distance between two consecutive wave peaks, or the wavelength , is in meters.

• travels at a constant speed in vacuum, no matter what its frequency is. Its speed c (a.k.a. speed of light) is 3 x 108 m/s. This speed is related to the frequency and wavelength by c = f

• no other wave or object can be faster than the EM wave - it is the limit.

Electromagnetic Spectrum

• A broad range of EM waves with varying frequencies.

• Parts of the radio spectrum are still unasigned, but will eventually be used for some commercial communications purpose.

• Radio, microwave, visible and infrared portions are used to transmit signals.

• Ultraviolet, x-ray and gamma rays are not used because they are difficult to modulate, are dangerous to human beings, and are not satisfactorily propagated through walls.

Gamma rays

X- rays

Ultraviolet

VISIBLE LIGHT

Infrared

Radio waves(amateur radio,aeronautical,cellular phone,taxis, aircraft,TV, FM, AM )

largerwavelengths

lowerfrequencies

higherfrequencies

smallerwavelengths

Microwaves

4 - 7.5 x 10

10 - 10

10 - 10

10 - 10

10 - 10

< 3 x 10

3 x 10 - 10

<10 m

1 nm - 1 pm

400 nm - 1 nm

25 µm - 2.5 µm

> 1 mm

1 mm - 25 µm

750 nm - 400 nm

Type Frequencyrange (Hz)

Wavelengthrange

20 24 -12

17 20

1715

14

1413

11 13

11

Near-infrared 1 x 10 - 4 x 10 2.5 µm - 750 nm14 14

Electromagnetic Spectrum

Frequency range Name Popular applications

30 – 300 kHz Low Frequency navigation

300 kHz – 3 MHz Medium Frequency commercial AM radio

3 – 30 MHz High Frequency shortwave radio (SW)

30 300 MHzVery HighFrequency

VHF TV and FM radio

300 MHz – 3GHzUltra HighFrequency

UHF TV, terr.microwave

3 – 30 GHzSuper HighFrequency

terr. And sat.microwave

30 – 300 GHzExtremely HighFrequency

Radio and Microwave Bands

RadioThe transmission of electromagnetic energy (radiation) over the air or through a hollow tube called a waveguide. Although radio is often thought of as only AM or FM, all airborne transmission is radio, including satellite and line-of-sight microwave.Radio transmitters are omnidirectional, meaning the signal is transmitted in all directions from the source. They easily penetrate walls and can reach long distances.Common Uses• AM• FM• VHF • UHF• SW radio• Packet radio

Transmission Properties

• has longer wavelengths which keeps it free from so much attenuation.

• subject to interference from motors and electrical equipment.

• at low frequency it passes through obstacles but the power decreases with distance from the source.

• at high frequency radio waves have the tendency to travel in straight lines and bounce at obstacles.

• quickly absorbed by rain.

• in data communications, the biggest disadvantage of using radio is the low data rate it can offer because radio waves have low frequency.

Microwave (Terrestrial)An electromagnetic wave that vibrates at 1GHz and above. Microwaves are the transmission frequencies used in communications satellites as well as in line-of-sight systems on earth.Electromagnetic waves above 100 MHz can travel in straight lines. Parabolic dish antennas can be used to focus narrow microwave beams and aimed to the other antenna along a (straight) line-of-sight. Microwave covers a part of the UHF and the whole SHF band.Common Uses• voice and television transmission• used for long-haul telephone transmission before the introduction of fiber optics.• cordless phones, wireless hifi speakers, microwave ovens.• applied to short links between buildings.

Transmission Properties• antenna height determines the maximum distance

between two antennas without any obstacle in between. The higher the antenna, the longer the maximum distance between two antennas.

• amplifiers are inserted between points to be linked when distance is beyond maximum.

• Signal attenuation depends on the distance between the antenna, the wavelength, and rainfall.

Early Microwave TowerThis microwave radio relay station was installed in 1968 at Boulder Junction, Colorado. (Photo courtesy of AT&T.)

Satellite Transmission (Microwave) A radio relay station in orbit above the earth that

receives, amplifies and redirects analog and digital signals contained within a carrier frequency.

Three kinds1. Geostationary (GEO) satellites are in ordbit 22,282 miles above the earth and rotate with the earth, thus appearing stationary. The downlink from GEOs back to earth can be localized into small areas or cover as much as a third of the earth's surface.2. Low-earth orbit (LEO) satellites reside no more than 1,000 miles above the earth and revolve around the globe every couple of hours. They are only in view for a few minutes, and multiple LEOs are required to maintain continuous coverage.3. Medium-earth orbit (MEO) satellites are in the middle, taking about six hours to orbit the earth and in view for a couple of hours. See Teledesic, Iridium, DSS, DirecPC and bent pipe architecture.

Communications SatelliteThere are hundreds of commercial communications satellites in orbit providing services for both industry and consumers. By the 21st Century, it is expected that Internet access via satellite will be popular.

Broadcast Satellite

Point-to-point Satellite

Common Uses• television broadcast distribution• long distance telephone transmission, extremely useful

for those airborne or at sea.• private business network

Transmission Properties• most common frequency used for satellite

communications is the 6/4 GHz 14/12 GHz bands (uplink/downlink).