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William Stallings Data and Computer Communications. Chapter 4 Transmission Media. Overview. Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important

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William stallings data and computer communications l.jpg

William StallingsData and Computer Communications

Chapter 4

Transmission Media

Overview l.jpg

  • Guided - wire

  • Unguided - 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

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Design Factors

  • Bandwidth

    • Higher bandwidth gives higher data rate

  • Transmission impairments

    • Attenuation

  • Interference

  • Number of receivers

    • Major factor in guided media

    • More receivers (multi-point) introduce more attenuation

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Guided Transmission Media

  • the transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint

  • e.g.,

  • Twisted Pair

  • Coaxial cable

  • Optical fiber

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Twisted Pair

  • consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs

  • often used at customer facilities and also over distances to carry voice as well as data communications

  • low frequency transmission medium

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Twisted Pair - Applications

  • Most common medium

  • Telephone network

    • Between house and local exchange (subscriber loop)

  • Within buildings

    • To private branch exchange (PBX)

  • For local area networks (LAN)

    • 10Mbps or 100Mbps

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Twisted Pair - Pros and Cons

  • Cheap

  • Easy to work with

  • 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) using different modulation & signaling techniques

  • Susceptible to interference and noise

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Unshielded and Shielded TP

  • Unshielded Twisted Pair (UTP)

    • Ordinary telephone wire

    • Cheapest

    • Easiest to install

    • Suffers from external electromagnetic interference (EM)

  • Shielded Twisted Pair (STP)

    • the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference

    • More expensive

    • Harder to handle (thick, heavy)

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UTP Categories

  • Cat 3

    • up to 16MHz

    • Voice grade found in most offices

    • Twist length of 7.5 cm to 10 cm

  • Cat 4 (least common)

    • up to 20 MHz

  • Cat 5

    • up to 100MHz

    • Commonly pre-installed in new office buildings

    • Twist length 0.6 cm to 0.85 cm

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Twisted Pair Advantages

  • inexpensive and readily available

  • flexible and light weight

  • easy to work with and install

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Twisted Pair Disadvantages

  • susceptibility to interference and noise

  • attenuation problem

    • For analog, repeaters needed every 5-6km

    • For digital, repeaters needed every 2-3km

  • relatively low bandwidth

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Coaxial Cable Applications

  • Most versatile medium

  • Television distribution

    • Aerial 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

  • Local area networks

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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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Coax Advantages

  • higher bandwidth

    • 400 to 600Mhz

    • up to 10,800 voice conversations

  • can be tapped easily (pros and cons)

  • much less susceptible to interference than twisted pair

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Coax Disadvantages

  • high attenuation rate makes it expensive over long distance

  • bulky

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Optical Fiber - Benefits

  • Greater capacity

    • Data rates of hundreds of Gbps

  • Smaller size & weight

  • Lower attenuation

  • Electromagnetic isolation

  • Greater repeater spacing

    • 10s of km at least

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Optical Fiber - Applications

  • Long-haul trunks

  • Metropolitan trunks

  • Rural exchange trunks

  • Subscriber loops

  • LANs

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Optical Fiber - Transmission Characteristics

  • Act as wave guide for 1014 to 1015 Hz

    • Portions of infrared and visible spectrum

  • Light Emitting Diode (LED)

    • Cheaper

    • Wider operating temp range

    • Last longer

  • Injection Laser Diode (ILD)

    • More efficient

    • Greater data rate

  • Wavelength Division Multiplexing

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Fiber Optic Types

  • multimode step-index fiber

    • the reflective walls of the fiber move the light pulses to the receiver

  • multimode graded-index fiber

    • acts to refract the light toward the center of the fiber by variations in the density

  • single mode fiber

    • the light is guided down the center of an extremely narrow core

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Fiber Optic Signals

fiber optic multimode


fiber optic multimode


fiber optic single mode

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Fiber Optic Advantages

  • greater capacity (bandwidth of up to 2 Gbps)

  • smaller size and lighter weight

  • lower attenuation

  • immunity to environmental interference

  • highly secure due to tap difficulty and lack of signal radiation

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Fiber Optic Disadvantages

  • expensive over short distance

  • requires highly skilled installers

  • adding additional nodes is difficult

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Wireless Transmission

  • Unguided media

  • Transmission and reception via antenna

  • Two techniques are used:

  • Directional

    • Focused beam

    • Careful alignment required

  • Omnidirectional

    • Signal spreads in all directions

    • Can be received by many antennas

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  • 2GHz to 40GHz

    • Microwave

    • Highly directional

    • Point to point

    • Satellite

  • 30MHz to 1GHz

    • Omnidirectional

    • Broadcast radio

  • 3 x 1011 to 2 x 1014

    • Infrared

    • Local

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Wireless Examples

  • terrestrial microwave transmission

  • satellite transmission

  • broadcast radio

  • infrared

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Terrestrial Microwave

  • uses the radio frequency spectrum, commonly from 2 to 40 Ghz

  • transmitter is a parabolic dish, mounted as high as possible

  • used by common carriers as well as by private networks

  • requires unobstructed line of sight between source and receiver

  • curvature of the earth requires stations (called repeaters) to be ~30 miles apart

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Microwave Transmission Applications

  • long-haul telecommunications service for both voice and television transmission

  • short point-to-point links between buildings for closed-circuit TV or a data link between LANs

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Microwave Transmission Advantages

  • no cabling needed between sites

  • wide bandwidth

  • multichannel transmissions

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Microwave Transmission Disadvantages

  • line of sight requirement

  • expensive towers and repeaters

  • subject to interference such as passing airplanes and rain

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Satellite Microwave

  • a microwave relay station in space

  • Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency

  • geostationary satellites

    • remain above the equator at a height of 22,300 miles (geosynchronous orbit)

    • travel around the earth in exactly the time the earth takes to rotate

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Satellite Transmission Links

  • earth stations communicate by sending signals to the satellite on an uplink

  • the satellite then repeats those signals on a downlink

  • the broadcast nature of the downlink makes it attractive for services such as the distribution of television programming

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Satellite Transmission Process





22,300 miles

uplink station

downlink station

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Satellite Transmission Applications

  • television distribution

    • a network provides programming from a central location using direct broadcast satellites (DBS)

  • long-distance telephone transmission

    • high-usage international trunks

  • private business networks

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Principal Satellite Transmission Bands

  • C band: 4(downlink) - 6(uplink) GHz

    • the first to be designated

  • Ku band: 12(downlink) -14(uplink) GHz

    • rain interference is the major problem

  • Ka band: 19(downlink) - 29(uplink) GHz

    • equipment needed to use the band is still very expensive

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Satellite Advantages

  • can reach a large geographical area

  • high bandwidth

  • cheaper over long distances

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Satellite Disadvantages

  • high initial cost

  • susceptible to noise and interference

  • propagation delay

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Broadcast Radio

  • Omnidirectional

  • FM radio

  • UHF and VHF television

  • Requires line of sight

  • Suffers from multipath interference

    • Reflections

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  • Achieved using tranceivers that modulate noncoherent infrared light

  • Requires line of sight (or reflection)

  • Blocked by walls

  • e.g. TV remote control, Infrared port

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Common Carriers

  • a government-regulated private company

  • involved in the sale of infrastructure services in transportation and communications

  • required to serve all clients indiscriminately

  • services and prices from common carriers are described in tariffs

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Leased (or Dedicated) Lines

  • permanently or semi-permanently connect between two points

  • economical in high volume calls between two points

  • no delay associated with switching times

  • can assure consistently high-quality connections

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Leased (or Dedicated) Lines

  • voice grade channels

    • normal telephone lines

    • in the range of 300 Hertz to 3300 Hertz

  • conditioning or equalizing

    • reduces the amount of noise on the line, providing lower error rates and increased speed for data communications

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T-1 Carrier

  • also referred to as DS-1 signaling

  • provides digital full-duplex transmission rates of 1.544Mbps

  • usually created by multiplexing 24 64-Kbps voice or 56-Kbps data lines

  • higher speeds are available with T-3 (45Mbps) [sometimes referred to a DS-3 lines; can be multiplexed into 28 T-1 signals; T-3 consists of 672 individual channels, each of which supports 64-Kbps] and T-4 services (274Mbps)

  • in Europe, E-1 (2.048Mbps) is used instead of T-1

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Integrated Services Digital Network (ISDN)

  • all-digital transmission facility that is designed to replace the analog PSTN

  • basic ISDN (basic rate access)

    • two 64Kbps bearer channels + 16Kbps data channel (2B+D) = 144 Kbps

  • broadband ISDN (primary rate access)

    • twenty-three 64Kbps bearer channels + 64 data channels (23B+D) = 1.536 Mbps

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Past Criticism of ISDN

  • “Innovations Subscribers Don’t Need”

  • “It Still Doesn’t Network”

  • “It Still Does Nothing”

  • Why so much criticism?

    • overhyping of services before delivery

    • high price of equipment

    • delay in implementing infrastructure

    • incompatibility between providers' equipment.

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ISDN Channel Definitions

  • B (bearer) channels

    • 64 kbps channels that may be used to carry voice, data, facsimile, or image

  • D (demand) channels

    • mainly intended for carrying signaling, billing and management information to control ISDN services (out-of-band control messages)

    • may be either 16 or 64 kbps

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Two Levels of ISDN Service

  • basic rate interface (BRI)

    • 2B (64 kbps) + D (16 kbps) = 144 kbps

  • primary rate interface (PRI)

    • 23B (64 kbps) + D (64 kbps) = 1.536 Mbps

      • North American standard

    • 30B (64 kbps) + D (64 kbps) = 1.984 Mbps

      • European standard