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UGANDA CHRISTIAN UNIVERSITYUGANDA CHRISTIAN UNIVERSITYFACULTY OF SCIENCE AND TECHNOLOGYFACULTY OF SCIENCE AND TECHNOLOGY
Department of Information Technology Department of Information Technology
Computer NetworksProgram: BSCS 2
January Semester – 2013
4. Transmission Media
Lecturer: Rebecca Asiimwe Phone Number:+256 712-997- 544
/0704 522 081 Email: [email protected]
Transmission Media
• What is Transmission Media?◦ Material over which a signal travels as it moves
from one network component/device to another.◦ actual path over which an electronic signal
travels as it moves from one component to another or one device to another.
• Classes/ Categories of Transmission Media• Guided• Unguided 2
Media Specifications• Different media types have different specifications
and expectations relating to performance:◦ Speed: Data transmission speeds are achieved using a
particular type of cable. The bit transmission speed is extremely important.
affected by the kind of conduit used. May affect the cost of the cable too
◦ Transmission being considered: Whether digital or analog.
digital or baseband transmission and analog or broadband transmission are the two choices
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Baseband Vs Broadband • Baseband:
Digital signals are usedFrequency division multiplexing is not possibleBaseband is bi-directional transmissionShort distance signal travellingEntire bandwidth of the cable is consumed by a single signal in a baseband transmission.
• Broadband:Analog signals are usedTransmission of data is unidirectionalSignal travelling distance is longFrequency division multiplexing is possibleThe signals are sent on multiple frequencies and allow all the multiple signals are sent simultaneously in broadband transmission. 4
Media Specifications
• Distance: Distance through which a signal travels through a particular type of cable before attenuation of the signal.
• The distance traveled by a signal through a cable directly affects attenuation of the signal.
• Degradation of the signal is directly related to the distance the signal travels and the type of cable used
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• The choice of medium depends on:
- Distance to be covered.-Desired Bit Rate (in bits per second, bps)-Cost Considerations
Numerous transmission media types are used for Data Communication
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Guided Transmission Media
a) Guided Transmission Media uses a "cabling" system that guides the data signals along a specific
path. The data signals are bound by the "cabling" system. Guided Media (also known as Bound)
• Guided Transmission Media include:• Fiber optics• Twisted pair cables• Coaxial cables
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Coaxial Cable
• It has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield. The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.
• Coaxial Categories:• RG-59 (Cable TV)• RG-58 (Thin Ethernet/ thinnet)• RG-11 (Thick Ethernet/ thicknet)
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Coaxial cable
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Coaxial Cable
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Braided copper shielding
Outer jacketCopper conductor
Plastic Insulation
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Coaxial Cable
• Advantages of coaxial cable on LANs:• can be run over longer distances than shielded twisted pair
(STP) and unshielded twisted pair (UTP) cable without the need for repeaters;• less expensive than fiber-optic cable;• the technology is well known;• has been used for many years for many types of data communication including cable television.
• It is important to consider its size; as the thickness of the cable increases, so does the difficulty of
working with it.
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Thicknet Coaxial Cable
• Different sizes of coaxial cable.
•Thicknet: The largest diameter specified for use as Ethernet backbone cable
•Greater transmission length •Noise rejection characteristics.
•generally, the more difficult the network media is to install, the more expensive it is to install.
•more expensive to install than twisted-pair cable-too thick to be bent.
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Thinnet Coaxial Cable
•Thinnet: Especially useful for cable installations that demanded the cable to make many twists and turns.
•Easier to install•Cheaper
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Coaxial Cable Connectors
• Coaxial Cable Connectors: The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector.
• Different types of adapters are available for BNC connectors, including a T-connector, barrel connector and
terminator. Connectors on the cable are the weakest point in any network. To help avoid problems with your network, it is advisable to use the BNC connectors that crimp,
rather than screw, onto the cable.
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Twisted-Pair Cable
• Twisted-pair cable is a type of cable that consists of insulated pair wires twisted around one another.
• The number of twists per unit length of the cable
varies along the cable length.
• Twisted-pair cable comes in several varieties:• Unshielded Twisted Pair (UTP),• Shielded Twisted Pair (STP),• Screened Shielded Twisted Pair (S/STP),• Foiled Twisted Pair (FTP),• Screened Unshielded Twisted Pair (S/UTP), and• Screened Foiled Twisted Pair (S/FTP).
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Twisted-Pair Cable• The purpose of the twisting is to cancel out effects of electromagnetic interference (EMI) from external sour-
ces, and reduce crosstalk from the neighboring pairs.
• When electrical current flows through a wire, it creates a small, magnetic field around the wire. When two
wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields cancel each other out.
• They also cancel out any outside magnetic fields. Twi-sting the wires can enhance this cancellation effect.
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Twisted-Pair Cable
The greater the number of twists, the more crosstalk is reduced. The number of twists per meter
makes up part of the specification for a given type of cable.
Data transmission at low frequencies may not be a problem but at high frequencies, it poses an obvious problem – noise and crosstalk.
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STP
• STP combines the techniques of shielding, cancellation and twisting of wires.
• Each pair of wires is wrapped in metallic foil. The four pairs of wires are wrapped in an overall metallic braid or foil.• It reduces electrical noise within the cable such
as pair to pair coupling and crosstalk. • It reduces electronic noise from outside the cable
EMI and RFI (Radio Frequency Interference).• It affords greater protection from all types of
external interference, but is more expensive and difficult to install than UTP 19
UTP
• Widely used in a variety of networks. The cable relies entirely on the cancellation effect produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI.
• Crosstalk between the pairs is further reduced by varying the number of twists. Like STP cable, it follows exact specifications as to how many twists are permitted per foot of cable. 20
UTP
Advantages:• Easy to install• Less expensive than other types of networking
media.• Because of its small size (small external diameter),
UTP does not fill up wiring ducts as rapidly as other types of cable.
• When installed using an RJ-45 connector, potential sources of network noise are greatly reduced and a good solid connection is practically guaranteed.
Disadvantages: • More prone to electrical noise and interference
than other types of networking media.• The distance between signal boosts is shorter for
UTP than it is for coaxial and fiber optic cables.21
UTP Implementations
• Types of cable connections used between internetwork devices:
• Straight-through cable: used to connect a
LAN switch to a computer. (Different devices)
• Crossover cable: used to connect two switches. (Similar devices)
• Rollover cable: used to connect a RJ-45 adapter on the com port of the computer to the console port of the router or switch.
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• The cables are defined by the type of connections, or pinouts, from one end to the other end of the cable.• Straight through cables have both ends with
identical color patterns.• For a cross-over cable, the color of pins #1 and
#2 will appear on the other end at pins #3 and #6, and vice-versa because the transmit and receive pins are in different locations.
• On a rollover cable, the color combination from left to right on one end should be exactly opposite to the color combination on the other end.
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UTP Implementations
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TIP
1.Blue Sky2.Orange Sun3.Green Vegetation4.Brown Earth
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UTP Implementations
T568A T568B3 21 2 1 34 4
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Networking Media Tools
• Crimper - The most essential tool and critical to the cable making process.
• Cable Tester (Optional) - Having a good cable tester can prevent and solve cable wiring configuration and installation problems.
• RJ45 Connectors• Boots (optional but preferred)• Straight edge wire cutter• The Cable itself
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Making a Network Cable
1. Unroll the required length of network cable and adding a little extra. If a boot is to be fitted, do so before stripping away the sleeve and ensure the boot faces the correct way.
2. Carefully remove the outer jacket of the cable, exposing just over 1“ (one inch) of the twisted pairs. Be careful when stripping the jacket as to not nick or cut the internal
wiring. After removing the outer case, you will notice 8 wires twisted in 4 pairs.
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Making a Network Cable Each pair will have one wire of a certain color and another
wire that is white with a colored stripe matching its partner (this wire is called a tracer). Sometimes a rip cord (white thread) is also present.
3. Untwist the pairs to lie flat between your fingers. The white piece of thread can be cut off even with the jac- ket and disposed. For easier handling, cut the wires so that they are 3/4“ long from the base of the jacket.
4. Arrange the wires based on the wiring specifications you are following, either the TIA 568A or the 568B.
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Cable Categories
• Cat 1: Currently unrecognized by TIA/EIA. Previously used for POTS telephone communications, ISDN and doorbell wiring.
• Cat 2: Currently unrecognized by TIA/EIA. Previo-usly was frequently used on 4 Mbit/s token ring
networks.
• Cat 3: Currently defined in TIA/EIA-568-B, used for data networks using frequencies up to 16 MHz. Historically popular for 10 Mbit/s Ethernet networks. 32
• Cat 4: Currently unrecognized by TIA/EIA. Provided performance of up to 20 MHz, and was frequently used on 16 Mbit/s token ring networks.
• Cat 5: Currently unrecognized by TIA/EIA. Provided performance of up to 100 MHz, and was frequently used on 100 Mbit/s Ethernet networks. May be unsuitable for 1000BASE-T gigabit Ethernet.
• Cat 5e: Currently defined in TIA/EIA-568-B. Provides performance of up to 100 MHz, and is frequently used for both 100 Mbit/s and Gigabit Ethernet networks.
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• Cat 6: Currently defined in TIA/EIA-568-B. Provides performance of up to 250 MHz, more than double category 5 and 5e.
• Cat 6a: Currently defined in ANSI/TIA/EIA-568-B.2-10. Provides performance of up to 500 MHz, double that of category 6. Suitable for 10GBase-T.
• Cat 7: An informal name applied to ISO/IEC 11801 Class F cabling. This standard specifies four individually-shielded pairs (STP) inside an overall shield. Designed for transmission at frequencies up to 600 MHz.
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Fiber Optic Cable• An optical fiber cable has a cylindrical shape and consists
of three concentric sections: 1. the core,2. the cladding, and 3. the jacket
The core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic where light travels. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core. The interface between the core and cladding acts as a reflector to confine light that would otherwise escape the core.
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Fiber Optic Cable• The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket / buffer jacket.
• The jacket is composed of plastic and other material layered to protect against moisture, scratching and
crushing, and other environmental dangers. This makes
it ideal for certain environments that contain a large amo-unt of electrical interference. It has also made it the stan-dard for connecting networks between buildings, due to
its immunity to the effects of moisture and lighting. 36
Parts of fiber optics
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Fiber Optic Cable
• Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and
twisted pair. It also has the capability to carry infor-mation at vastly greater speeds. This capacity
broadens communication possibilities to include services such as video conferencing and interact-
ive services.
• The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify.
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How fiber optics operate• The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the
cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does
not absorb any light from the core, the light wave can travel great distances
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Transmission is through electroluminescence(an optical phenomenon and electrical phenomenon in
which a material emits light in response to the passage). The emitted light is incoherent with a relatively
wide spectral width of 30-60 nm.
Receivers- the main component of an optical recei-ver is a photo-detector that converts light into elec-tric signals through the photoelectric effect . 41
Transmitter-the most commonly used optical transmitters are semiconductor devices such as light-emitting diodes (LEDs) and laser diodes.
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Fiber-Optic Cable
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Two types of fiber-optic cables
a) Single-modeAllows only one mode of light to propagate through the fiber. Capable of higher bandwidth, and it is often used as a backbone - (10kms).
b) Multi-mode Multimode fiber cable allows multiple modes of
light to propagate through the fiber. It uses light-emitting diodes (LEDs) as a light-generating device (2km)
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Signals and Noise in Optical Fibers
• Not affected by external noises that cause problems on copper media since external light
cannot enter the fiber except at transmitter ends.
• Light on one fiber in a cable does not generate interference disturbances on any other fiber. No crosstalk like copper media.
• Fiber-optic transmission allows the Ethernet protocol to be used on MANs and WANs.
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Signals and Noise in Optical Fibers
• Attenuation of the signal due to factors of the nature of fiber itself:
◦Scattering caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy.
◦Absorption is another cause of light energy loss. Parts of energy are absorbed by impurities when light rays strike impurities in a fiber. This light
energy is lost as heat energy, making the light signal dimmer. 46
Signals and Noise in Optical Fibers
• Manufacturing irregularities/roughness in the core-to-cladding boundary. Power is lost from
the light signal because of the less than perfect total internal reflection in that rough area of the fiber. Any microscopic imperfections in the thickness or symmetry of the fiber cuts down on total internal reflection and the cladding absorbs some light energy.
• Dispersion of a light flash - the spreading of pulses of light as they travel down the fiber also limits transmission distances on a fiber. 47
Advantages
• Fibers do not leak light and are quite difficult to tap (Secure).• Handles much higher bandwidth than copper wires.• The loss of signal in optical fiber is less than in copper wire. Repeaters are needed at 30 km Vs 5 for
copper. • Not affected by electromagnetic interference. Unlike
electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers
in the same cable. 48
Disadvantages
• They are quite difficult to install • Its an expensive technology. Its
expensive to install.• Very delicate / fragile.
49
Wireless communication• Is the transfer of information over a dista-
nce with the use of electromagnetic waves.• Electromagnetic waves are formed when an
electric field couples with a magnetic field.
• Does not use cables Wireless50
Examples of unguided media
1. Radio wave transmission2. Micro wave transmission3. Light wave transmission4. Infrared
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1. Radio transmission
• Omnidirectional – no need for dish-shaped antennas and no need for them to be rigidly mounted to a precise alignment unlike microwave which is directional.
• Radio-term used for frequencies in the range of 3Hz to 300GHz
• Range covers FM radio and • UHF and VHF television
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Types of Radio Frequency Propagation
• There are 3 types of RF (Radio Frequency) Propagation:
1. Ground Wave2. Ionospheric and 3. Line of Sight (LOS) Propagation.
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i) Ground Wave Propagation
• Follows the curvature of the Earth. Ground Waves have carrier frequencies up to 2M-Hz. AM radio is an example of Ground Wave Propagation.
• Radio, television and micro-waves are types of electromagnetic waves. They diff-er from each other in wavelength.
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Ground Wave Propagation
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ii) Ionospheric Propagation
• Bounces off of the Earths Ionospheric Layer in the upper atmosphere. It is sometimes called Double Hop Propagation. It operates in the frequency range of 30 - 85 MHz
• Signal from earth based antenna is reflected from ionised layer (ionsphere) of upper atmosphere back
down to earth, effect caused by refraction and not as though the waves were reflected from some hard surface.
• Also known as Sky Wave Propagation 57
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Sky Wave Propagation
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iii) Line of Sight Propagation
• Line of Sight Propagation transmits exactly in the line of sight. The receive station must be in view of the transmit station. It is sometimes called Space Waves or Tropospheric Propagation.
• • Not reflected by ionosphere so signal can be tran-
smitted between an earth station and a satellite overhead that is not beyond the horizon.
• Examples of Line of Sight Propagation are: FM Radio, Microwave and Satellite.
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Line of Sight Propagation
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Characteristics of radio transmission
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•Easily generated.•Omni-directionally travel a long distance.•Can penetrate buildings.•Frequency dependant.•Relatively low bandwidth for data comm-unication.•Tightly licensed by governments.
2. Microwaves Transmission
Frequency of above 100MHz, Travel in straight lines Can be narrowly focused.Use a parabolic antenna. Both the transmitting and the receiving
antennas should be accurately aligned with each other.
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• Its used for long distance communication, • Can not easily pass through strong
buildings and can easily be absorbed by the rain.
• Affected by rain, vapor, dust, snow, clouds, mist and fog, heavy moisture, depending on chosen frequency.
• It’s used for long distance communication.
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2. Microwaves Transmission
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3. Bluetooth and infrared• Bluetooth uses radio waves to transmit
between microchip devices like mobile phone and a Hands free device.
• Infrared (IR) uses electromagnetic waves for transmission as a smaller wavelength than radio. A TV remote control is an example of an Infrared appli-cation.
• Wireless Personal Area Networks (WPANs) are able to communicate using technologies like Bluetooth (IEEE 8.2.15.1) and infrared. 68
Infrared• Infrared signals can be used for short range comm-
unication. • Modulate noncoherent infrared light• Transceivers must be within Line of sight (or
reflection) of each other directly or via reflection from light colored surfaces such as the ceiling of a room.
• e.g. TV remote control• Infrared signals, having high frequencies, cannot
penetrate walls unlike microwave which does . This helps to prevent interference between one system and another.
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Infrared• Security and interference problems encountered in
microwave systems are not present in infrared. • Furthermore there are no frequency allocation issues
with infrared because no license is required.
• There are a number of computer devices which are used to send data through infrared medium e.g. a key board, mouse, PCs, phones and printers. There are some manufacturers who provide a special part called the IrDA (Infrared Data Association) port that allows a wireless keyboard to communicate with the PC.
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4.Light wave transmission
• Unguided optical signal such as laser.• Connect 2 LANS in two buildings via a laser
mounted on their roof.• Unidirectional, easy to install, don’t require
license.• Laser beams can be used for communication
but can not penetrate rain or thick fog.• Laser beams can easily be diverted by
turbulent air. 71
Frequencies• Gamma Rays (above 30GHz) • X-Rays• Ultra-Violet Light • Visible Light • Infrared Light • EHF - Extremely High Frequencies 30 GHz
Radar • SHF - Super High Frequencies 3 GHz Satellite
& Microwaves • UHF - Ultra High Frequencies 300 MHz, UHF TV
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• VHF - Very High Frequencies 30 MHz FM & TV• HF - High Frequencies 3 MHz Short Wave
Radio• MF – Medium Frequencies 300 kHz (kilo = 10^3) AM Radio • LF - Low Frequencies 30 kHz Navigation • VLF - Very Low Frequencies 3 kHz Submarine Commu-
nications • VF - Voice Frequencies 300 Hz Audio • ELF - Extremely Low Frequencies 30 Hz Power Transmi-
ssion73
Frequencies
Frequencies• 2GHz to 40GHz
• Microwave frequencies• Highly directional• Suitable for Point to point transmission• Also used for Satellite communication
• 30MHz to 1GHz• Suitable for Omnidirectional applications• This range is referred to as radio range
• 3 x 1011 to 2 x 1014
• Infrared• For Local applications• Point to point and multipoint within confined areas –
single room74
Antennas• For unguided media, transmission and reception are
achieved by means of an antenna.• Electrical conductor (or system of conductors) used to
radiate electromagnetic energy or collect electroma-gnetic energy
• For Transmission of a Signal• Electrical energy from transmitter is converted into
electromagnetic energy by the antenna and Radiated into surrounding environment (space, water...)
• For Reception• Electromagnetic energy impinging on antenna• Converted to electrical energy• Fed to receiver
• Same antenna often used for both
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Radiation Pattern
• An antenna will radiate power in all directions but typically does not perform equally well in all directions. A common way to characterize the performance of an antenna is the radiation pattern.
• Simplest pattern produced by an idealized antenna known as the isotropic antenna• Radiates power in all directions.• Actual radiation pattern is a sphere with the
antenna at the centre. 76
Parabolic Reflective Antenna• An important type of antenna is the parabolic
reflective antenna, which is used in terrestrial microwave and satellite applications.
• A parabolic dish antenna is based on the geometry of a parabola: Every line parallel to the line of symmetry (line of sight) reflects off the curve at angles such that all the lines intersect in a common point called the focus.
• The parabolic dish works as a funnel, catching a wide range of waves and directing them to a common point. In this way, more of the signal is recovered than would be possible with a single-point receiver.
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• Such surfaces are used in headlights, optical and radio telescopes, and microwave antennas because of the following property:
• • If a source of electromagnetic energy (or
sound) is placed at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of the paraboloid
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• Figure (b) in the next slide shows this effect in cross section. In theory, this effect creates a parallel beam without dispersion. In practice, there will be some dispersion, because the source of energy must occupy more than one point.
• The larger the diameter of the antenna, the more tightly directional is the beam. On reception, if incoming waves are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus. 79
Parabolic Reflective Antenna
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Terrestrial Microwave
• Most common type of microwave antenna is a Para- bolic dish
• Antenna fixed rigidly and focuses a narrow beam to achieve Line of sight transmission
• Located above ground level to extend range between antennas and be able to transmit over intervening obstacles
• For long distance transmission, a series of microwave relay towers are used and point to point microwave links are strung together over the desired distance
• Higher frequencies give higher data rates81
Satellite Microwave
• Satellite is a microwave relay station• Links two or more ground based microwave
trans-mitter/receivers known as earth stations.• Satellite receives on one frequency band
(uplink), amplifies or repeats signal and transmits on another frequency (downlink)
• A single orbiting satellite will operate on a number of frequency bands called transponder channels or simply transponders.
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Satellite Point to Point Link
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Satellite Broadcast Link
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• For a communication satellite to function effectively, it is generally required that it remain stationary with respect to its position over the earth; otherwise it would not be within the line of sight of its earth stations at all times.
• To remain stationary, the satellite must have a period of rotation equal to the earths period of rotation.
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Applications• The communications satellite is a technological
revolutions as important as fiber optics. Among the most important applications for satellites are the following:
• Television distribution, Long distance telephone transmission and private business networks.
• Programs are transmitted to the satellite and then broadcast down to a number of stations which then distribute the programs to individual viewers.
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• Another application for satellite is based on busin-ess data. The satellite provider can divide the total capacity into a number of channels and lease these to individual business users.
• A user equipped with the antenna at a number of sites can use the satellite channel for private network.
• Such applications are very expensive and limited to large organizations with high-volume requirements.
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• A recent development is the very small aperture terminal (VSAT) systems which provide low cost alternatives.
• A number of subscriber stations are equipped with low cost VSAT antennas, these stations share a satellite transmission capacity for transmission to a hub station that can then exchange messages with each of the subscribers and can relay messages between suscribers. 88
Advantages associated to wireless communication
• It’s ideal for non-reachable places.• It’s ideal for temporary network
setups.• It supports more users unlike the
wired ones.• It’s neat and easy to install.• It’s flexible/can be moved easily.• It’s cheaper to maintain.
89
Disadvantages• Relatively lower speed - example: although currently 802.11/n
could reach 128 Mbps, some UTP cable categories can reach 1 Gbps. And more user mean each bandwidth get smaller. That is why currently wired backbone network is still preferred
• The network can be less stable. Wireless reception may be impaired by a number of factors including large distances or objects between wireless devices, and other wireless networks.
• The speed and the viability of the wireless signals drop as more and more users use the same frequency
• Health implications related with radio waves/EMF
90
• Because signals are spread in the air, it is convenient for hackers to catch wireless signals. Wireless networks require very tight security so that the unauthorized users cannot exploit the information. As more and users are making use of the wireless technologies, the risk of data being lost is increasing.
• Read from the following link for more disadvantages• http://freewimaxinfo.com/disadvantages-of-wireless-
networks.html&
• Read about wireless security protocols and wireless standards from books given. (http://www.rfidc.com/docs/introductiontowireless_standards.htm)
91
Required Reading
• Chapter 7 of • Behrouz A. Forouzan & Sophia Chung
Fegan’s book on Data Communications and Networking.
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NEXT WEEK'S TOPIC
Introduction to Local Area Networks: • Topologies • LAN protocol Architecture• LAN Devices• High speed LANS
• Ethernet• Token Ring• Fibre Channel
• READ IN ADVANCE 93
