Transmission Media( Guided and Wireless Transmission )

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Overview• Guided – provide a physical path along which

the signals are propagated—Wire: Coaxial Cable, Optical Fiber, Twisted Pair

• Unguided –employ an antenna for transmitting through air, vacuum or water.— wireless

• Characteristics and quality determined by medium and signal—For guided, the medium is more important—For unguided, the bandwidth produced by the antenna

is more important

• Key concerns are data rate and distance

Key Points

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Design Factors• Bandwidth

—Higher bandwidth gives higher data rate

• Transmission impairments—Attenuation

• Interference• Number of receivers

—In guided media—More receivers (multi-point) introduce more

attenuation

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Electromagnetic Spectrum

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2.2.1 Guided Transmission Media

To become part of network, a PC uses a network interface card (NIC).(For portable comp., NIC can be in form of a PC Card/ USB adapter)

•Backbone ,connection leading to and from it might use:

• Twisted Pair• Coaxial cable• Optical fiber

PC’s RAMNIC

LAN’s backbone

signal

signal

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Transmission Characteristics of Guided Media

 

  Frequency Range

Typical Attenuatio

n

Typical Delay

Repeater Spacing

Twisted pair (with loading)

0 to 3.5 kHz 0.2 dB/km @ 1 kHz

50 µs/km 2 km

Twisted pairs (multi-pair cables)

0 to 1 MHz 0.7 dB/km @ 1 kHz

5 µs/km 2 km

Coaxial cable

0 to 500 MHz

7 dB/km @ 10 MHz

4 µs/km 1 to 9 km

Optical fiber 186 to 370 THz

0.2 to 0.5 dB/km

5 µs/km 40 km

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Twisted Pair – Physical Description

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Twisted Pair - Applications

Most common transmission medium for analog/digital signal

• Telephone network—Between house and

local tel. exchange (subscriber loop)

• Com. within buildings—To private branch exchange (PBX)

• For local area networks (LAN)—10Mbps or 100Mbps

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Twisted Pair - Pros and Cons

Pros• Cheap• Easy to work with

Cons• Low data rate• Short range

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Twisted Pair - Transmission Characteristics

• Analog —Amplifiers every 5km to 6km

• Digital—Use either analog or digital signals—repeater every 2km or 3km

• Limited distance• Limited bandwidth (1MHz)• Limited data rate (100MHz)• Susceptible to interference and noise

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Coaxial Cable - Physical Description

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Coaxial Cable - ApplicationsMost versatile medium• Television distribution

—Arial to TV—Cable TV

• Long distance telephone transmission—Can carry 10,000 voice calls

simultaneously—Being replaced by fiber optic

• Short distance computer systems links —Connections between devices

• Local area networks

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Coaxial Cable - Benefits

• Less susceptible to interference and crosstalk**

• Longer distance** **(compared to twisted pair)

• Wide variety of application

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Coaxial Cable - Transmission Characteristics

• Analog—Amplifiers every few km—Closer if higher frequency—Up to 500MHz

• Digital—Repeater every 1km—Closer for higher data rates

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Optical Fiber - Physical Description

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Optical Fiber - Benefits

• Greater capacity—Data rates of hundreds of Gbps

• Smaller size & weight• Lower attenuation• Electromagnetic isolation

—Not affected by external electromagnetic fields

• Greater repeater spacing—10s of km at least

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Optical Fiber - ApplicationsTel. network• Long-haul trunks(laluan)

—aver. 1500 km, 20k-60k voice channels

• Metropolitan trunks• aver. 12 km, 100k voice channels• Rural exchange trunks

—Links town-village 40-160 km

• Subscriber(Pelanggan) loops—Central exchange - subsciber

• LANs—100 Mbps to 1Gbps

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Optical Fiber Transmission Modes

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Wireless Transmission Frequencies• 2GHz to 40GHz

—Microwave frequencies• Highly directional (one way)• Point to point transmission• Satellite

• 30MHz to 1GHz (radio range)—Omnidirectional—Broadcast radio

• 3 x 1011 to 2 x 1014 Hz—Infrared

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Antennas• Electrical conductor (or system of conductor)

used to radiate electromagnetic energy or collect electromagnetic energy

• Transmission of signal—Radio-frequency electrical energy from transmitter—Converted to electromagnetic energy—By antenna—Radiated into surrounding environment; atmosphere,

space, water• Reception of signal

—Electromagnetic energy impinging(melanggar) on antenna

—Converted to radio-frequency electrical energy—Fed to receiver

• Same antenna often used for both

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Terrestrial Microwave• Parabolic dish• Long haul telecommunications

(application)— alternative to coaxial cable or optical fiber.— requires far fewer amplifiers/repeaters than

coaxial cable over the same distance but requires LOS (line-of -sight) transmission.

• Higher frequencies give higher data rates— common frequencies used for transmission

are in the range 1 to 40GHz.

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Satellite Microwave• Satellite is relay station• Satellite receives on one frequency

(uplink), amplifies or repeats signal and transmits on another frequency (downlink)

• Applications— Television— Long distance telephone

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Satellite Point to Point Link

Uplink Downlink

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Satellite Broadcast Link

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Broadcast Radio• Omnidirectional• FM radio• UHF and VHF television

• Suffers from multipath interference—Reflections

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Infrared• Achieved using transmitter/receivers

(transceiver) that modulate non-coherent infrared light

• Transceivers must be within the line of sight of each other either directly or via reflection form a light-colored surface such as the ceiling of a room.

• Blocked by walls• e.g. TV remote control

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Wireless Propagation• Signal travels along three routes

—Ground wave• Follows contour of earth• Up to 2MHz• Best-known example of ground wave communication is AM

radio

—Sky wave• Used for amateur radio, BBC world service, Voice of

America• A signal from an earth-based antenna is reflected from

ionosphere layer of upper atmosphere back down to earth.

—Line of sight (LOS)• Above 30Mhz, neither ground wave or sky wave

propagation modes operate, and communication must be by LOS.

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Line of Sight Transmission’s Impairment

• Free space loss—Signal disperses with distance (signal spread over a larger and

larger area)

• Atmospheric Absorption—Water vapour and oxygen absorb radio signals—Water greatest at 22GHz, less below 15GHz—Oxygen greater at 60GHz, less below 30GHz—Rain and fog scatter radio waves that results in attenuation.

• Multipath—Better to get line of sight if possible—Signal can be reflected causing multiple copies to be received—May be no direct signal at all—May reinforce or cancel direct signal

• Refraction—May result in partial or total loss of signal at receiver

END!!

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Ground Wave Propagation

Sky Wave Propagation

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Line of Sight Propagation

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FreeSpaceLoss

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Multipath Interference

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Key Points• Transmission media – guided and unguided• For guided transmission:

—Twisted pair has been the workhorse for com. of all sorts.

—Higher data rates over longer distances can be achieved with coaxial cable, so coaxial cable has been often been used for high-speed LAN and for high-capacity long-distance trunk applications.

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Key Points(2)—However, tremendous capacity of optical fiber

has made that medium more attractive than coaxial cable, and thus optical fiber has taken over much of the market for high-speed LANs and for long-distance applications.

• For unguided transmission:—Commonly used for info. com. including

broadcast radio, terrestrial (daratan) microwave, and satellite. Infrared transmission is used in some LAN applications.

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Local Telephone Exchange

• A telephone exchange is the place that users voice and broadband is delivered to and from to their address.

• The exchange splits the signals that it receives so it can then send them on to the correct part of the exchange. Voice calls are sent to the PSTN (Public Switched Telephone Network) network and broadband/ADSL signals is sent to the DSLAM (Digital Subscriber Line Access Multiplexer).

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private branch exchange

• A PBX (private branch exchange) is a telephone system within an enterprise that switches calls between enterprise users on local lines while allowing all users to share a certain number of external phone lines. The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company's central office.