1 Chapter Three The Media – Conducted and Wireless

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Chapter Three

The Media –

Conducted and Wireless

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Introduction• The world of computer would not exist if there were

no medium( ناقل by which to transfer data (وسط• The two major categories of media include:

– Conducted media

– Wireless media

Chapter Three - The Media - Conducted and Wireless

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Twisted Pair Wire (Conducted media)• One or more pairs of single conductor(موصل) wires that have been twisted

around each other• Twisted pair wire is classified by category. Twisted pair is currently

Category 1 through Category 7, although Categories 2 and 4 are nearly obsolete(مهمل)

• two important laws from physics:– (1) A current passing through a wire creates a magnetic field around that wire– (2) a magnetic field passing over a wire induces a current in that wire.

• Therefore, a current or signal in one wire can produce an unwanted current or signal, called crosstalk, in a second wire.


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Twisted Pair Wire


• Twisting the wires helps to eliminate electromagnetic interference between the two wires

• Shielding can further help to eliminate interference

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• If the two wires run parallel to each other the chance for crosstalk increases.

• If the two wires cross each other at perpendicular angles the chance for crosstalk decreases.

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Twisted Pair Wire


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Coaxial Cable (Conducted media)• A single wire wrapped in a foam insulation( مادة عازلة )

surrounded by a braided metal shield to block electromagnetic signals from entering the cable and produce noise, then covered in a plastic jacket. Cable comes in various thicknesses

• Baseband coaxial technology uses digital signaling in which the cable carries only one channel of digital data

• Broadband coaxial technology transmits analog signals and is capable of supporting multiple channels of data simultaneously


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Coaxial Cable


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Coaxial Cable


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Fiber Optic Cable (Conducted media)• A thin glass cable approximately a little thicker than a

human hair surrounded by a plastic coating and packaged into an insulated(معزول ) cable

• A photo diode or laser generates pulses of light which travel down the fiber optic cable and are received by a photo receptor



• How does a thin glass cable transmit data? – A light source, called a photo diode, is placed at the transmitting end

and quickly switched on and off to produce light pulses. – These light pulses travel down the glass cable and are detected by an

optic sensor called a photo receptor on the receiving end. – The light source can be either a simple and inexpensive light-emitting

diode (LED), laser. – The laser is much more expensive than the LED, and it can produce

much higher data transmission rates. • Fiber optic cable advantages:

– providing high-speed, low-error data transmission rates.– Small noise as the light pulses bounce around inside the glass cable,

this noise is significantly less than noise generated in twisted pair wires or coaxial cables

– More secure than twisted pair wires or coaxial cables, it is impossible to wiretap. Without physically break into the line

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• fiber-optic cable has two small disadvantages:• Light pulses can travel in one direction only. Thus, to support a two-way transmission of data, two fiber-optic cables are necessary.

• Its higher cost than twisted pair wires or coaxial cables

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Fiber Optic Cable• Fiber optic cable is capable of supporting millions of bits per

second for 1000s of meters

• Thick cable (62.5/125 microns) causes more ray(شعاع ) collisions, so you have to transmit slower. This is step index multimode fiber. Typically use LED for light source, shorter distance transmissions

• Thin cable (8.3/125 microns) – very little reflection, fast transmission, typically uses a laser, longer transmission distances; known as single mode fiber


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Fiber Optic Cable


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Fiber Optic Cable• Fiber optic cable is susceptible(عرضة ) to reflection

(where the light source bounces around inside the cable) and refraction(االنكسار ) (where the light source passes out of the core and into the surrounding cladding(الكسوة ))

• Thus, fiber optic cable is not perfect either. Noise is still a potential problem


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Fiber Optic Cable


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Fiber Optic Cable• It is very common to mix fiber with twisted pair in LANs


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Wireless Media• Radio, satellite transmissions, and infrared light are

all different forms of electromagnetic waves that are used to transmit data

• Technically speaking – in wireless transmissions, space is the medium

• Note in the following figure how each source occupies a different set of frequencies


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Terrestrial(أرضي ) Microwave• Transmission systems transmit tightly focused beams of radio

signals from one ground-based microwave transmission antenna to another.

• Land-based, line-of-sight transmission• Approximately 20-30 miles between towers• Transmits data at hundred of millions of bits per second• Signals will not pass through solid objects• Popular with telephone companies and business to business

transmissions• might be less expensive in the long run than leasing a high-

speed telephone line with monthly payment.• once the system is purchased and installed, no telephone service

fees are necessary.


Terrestrial(أرضي ) Microwave

• Many microwave antennas are located on top of free-standing towers, The higher the tower, the farther the possible transmission distance.

• Another factor that limits transmission distance is the number of objects that might obstruct the path of transmission signals.

• Buildings, hills, forests, and even heavy rain and snowfall all interfere with the transmission of microwave signals.

• Disadvantages: – loss of signal strength and interference from other signals, – in addition to the costs of either leasing the service or installing and

maintaining the antennas.

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• Often the microwave antennas are on towers or buildings

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Satellite Microwave• Similar to terrestrial microwave except the signal

travels from a ground station on earth to a satellite and back to another ground station

• Can also transmit signals from one satellite to another• Satellites can be classified by how far out into orbit

each one is (LEO, MEO, GEO, and HEO)


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Satellite Microwave


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Satellite Microwave• LEO – Low Earth Orbit – 100 to 1000 miles out. Used for wireless e-

mail, special mobile telephones, pagers, spying, videoconferencing• MEO – Middle Earth Orbit – 1000 to 22,300 miles. Used for GPS

(global positioning systems) and government• GEO – Geosynchronous( األرض مع Earth Orbit – 22,300 (المتزامن

miles. Always over the same position on earth (and always over the equator( االستواء Used for weather, television, government .((خطoperations

• HEO - highly elliptical orbit, which is used by governments for spying and by scientific agencies for observing celestial bodies( السماوية .(األجرام– It follows an elliptical pattern. When the satellite is at its perigee (closest point to

the Earth), it takes photographs of the Earth. When the satellite reaches its apogee (farthest point from the Earth), it transmits the data to the ground station. At its apogee, the satellite can also photograph objects in space.


Satellite Microwave

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Satellite Microwave

• Satellite microwave can also be classified by its configuration:– Bulk carrier configuration

• the satellite system and all its assigned frequencies are devoted to one user.

• transmitting large amounts of data in a very short time, used for large application.

• entire satellite system by one user. For example, a telephone company use a bulk carrier satellite system to transmit thousands of long-distance telephone calls.

– Multiplexed configuration (Multiple users )• The ground station accepts input from multiple sources.

– Single-user earth station configuration.• each user employs his or her own ground station to transmit

data to the satellite


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Infrared Transmissions• Transmissions that use a focused ray of light in the

infrared frequency range• Very common with remote control devices, but can

also be used for device-to-device transfers, such as PDA to computer


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Bluetooth• is a wireless technology that uses low power, short-range radio frequencies to

communicate between two or more devices.• Bluetooth is a specification for short-range, point-to-point or point-to-

multipoint voice and data transfer• Bluetooth can transmit through solid, non-metal objects Thus, a device that is

transmitting Bluetooth signals can be carried in a pocket or briefcase.• Its typical link range is from 10 cm to 10 m, but can be extended to 100 m by

increasing the power


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Bluetooth• Bluetooth will enable users to connect to a wide

range of computing and telecommunication devices without the need of connecting cables

• Typical uses include phones, pagers, modems, LAN access devices, headsets, notebooks, desktop computers, and PDAs


Bluetooth

• To appreciate the potential power of Bluetooth technology, consider examples:

• You can automatically synchronize all e-mail messages between your PDA and your desktop/laptop computer;

• as you approach your car, your PDA will tell the car to unlock its doors and change the radio to your favorite station;

• as you walk up to the front door of your house, your PDA will instruct your house to unlock the front door, turn on the lights, and turn on an entertainment system,

• and as you sit in a business meeting, your PDA/laptop will wirelessly transmit your slide presentation to a projector and your notes to each participant’s PDA/laptop.

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Wireless LAN (IEEE 802.11)• This technology transmits data between workstations and local area networks using

high-speed radio frequencies

• access point is the connection into the wired portion of the local area network.

• Current technologies allow up to 54 Mbps (theoretical) data transfer at distances up to hundreds of feet)

• IEEE 802.11n. supporting a 100-Mbps signal between wireless devices and uses multiple antennas to support multiple independent data streams.

• All protocols—802.11a, 802.11b, 802.11g, and 802.11n—are now called Wi-Fi.


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Free Space Optics

• Uses lasers, or, in some cases, infrared transmitting devices, to transmit data between two buildings over short distances, such as across the street.

• Data transfer speeds can be as high as 1.25 Gbps, higher in the future.

• Lasers lose their strength when transmitting through fog. Thus, if the fog is thick, transmission distances can be cut down to less than 50 meters

• Line of sight between buildings

• Typically short distances, such as across the street

• Newer auto-tracking systems keep lasers aligned when buildings shake from wind and traffic


ZigBee

• relatively new wireless technology supported by the IEEE 802.15.4 standard. It has been designed for data transmission between smaller, often embedded, devices that require low data transfer rates (20–250 Kbps) and low power consumption.

• For example, home and building automation (heating, cooling, security, lighting, and smoke detectors), industrial control, automatic meter reading, and medical sensing and monitoring.

• power consumption is so low that some suppliers claim that their ZigBee-equipped devices will last multiple years on the original battery.

• Bluetooth is best at replacing cables for short distances, while ZigBee will be good at sending low-speed signals over short to medium distances.

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Media Selection Criteria• Cost• Speed• Distance and expandability• Environment• Security


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Media Selection Criteria - Cost• Different types of cost

– Initial cost – what does a particular type of medium cost to purchase? To install?

– Maintenance / support cost

• ROI (return on investment) price/performance ratio – if one medium is cheaper to purchase and install but is not cost effective, where is the savings?


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Media Selection Criteria - Speed• Two different forms of speed:

– Propagation speed – the time to send the first bit across the medium. This speed depends upon the medium. Airwaves and fiber are speed of light. Copper wire is two thirds the speed of light

– Data transfer speed – the time to transmit the rest of the bits in the message. This speed is measured in bits per second


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1 Chapter Three The Media – Conducted and Wireless