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COE 341: Data & Computer Communications (T061) Dr. Radwan E. Abdel-Aal Chapter 4: Transmission Media Agenda Overview Guided Transmission Media Twisted Pair Coaxial Cable Optical Fiber Wireless Transmission Antennas Terrestrial Microwaves Satellite Microwaves Broadcast Radio Infrared

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agenda
Agenda
  • Overview
  • Guided Transmission Media
    • Twisted Pair
    • Coaxial Cable
    • Optical Fiber
  • Wireless Transmission
    • Antennas
    • Terrestrial Microwaves
    • Satellite Microwaves
    • Broadcast Radio
    • Infrared
overview
Overview
  • Media:
    • Guided – wire or fiber
    • Unguided - wireless
  • Transmission characteristics and quality determined by:
    • Signal
    • Medium
  • For guided, the medium is more important
  • For unguided, the bandwidth provided by the antenna is more important
design issues
Design Issues
  • Key communication objectives are:
    • High data rate
    • Low error rate
    • Long distance
    • Bandwidth economy: Tradeoff

- Want larger BW for higher data rates: C  B

- But limited by economy: Larger BW is costly e.g. Coaxial Vs TP

  • Transmission impairments
    • Attenuation: Twisted Pair > Cable > Fiber (best)
    • Interference:

Worse with unguided… (the medium is shared!)

  • Number of receivers
    • In multi-point links of guided media:

More connected receivers introduce more attenuation

the electromagnetic spectrum
The Electromagnetic Spectrum

Microwaves

Light

10 GHz

100 MHz

10 KHz

standard multiplier prefixes 1 18 to 10 18
Standard Multiplier Prefixes 1-18 to 10+18

exa- E 1018 = 1,000,000,000,000,000,000 peta- P 1015 = 1,000,000,000,000,000 tera- T 1012 = 1,000,000,000,000 giga- G 109 = 1,000,000,000 mega- M 106 = 1,000,000 kilo- K 103 = 1,000 milli- m 10-3 = 0.001 micro- 10-6 = 0.000,001 nano- n 10-9 = 0.000,000,001 pico- p 10-12 = 0.000,000,000,001 femto- f 10-15 = 0.000,000,000,000,001 atto- a 10-18= 0.000,000,000,000,000,001

electromagnetic spectrum

Ultra violet,

X-Rays,

Gamma-Rays

Electromagnetic Spectrum

Used for Communications

study of transmission media
Study of Transmission Media
  • Physical description
  • Main applications
  • Main transmission characteristics
guided transmission media
Guided Transmission Media
  • Twisted Pair
  • Coaxial cable
  • Optical fiber
transmission characteristics of guided media overview

Frequency Range

Typical Attenuation

Typical Delay

Repeater Spacing

Twisted pair (with loading coils)

0 to 3.5 kHz

0.2 dB/km @ 1 kHz

50 µs/km

2 km

Twisted pair(No loading coils)

e.g. for ADSL

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

Up to 9 km

Optical fiber

186 to 370 THz

0.2 to 0.5 dB/km

5 µs/km

40 km

Transmission Characteristics of Guided Media: Overview
  • Same delay

(except with loading)

Fewer

Repeaters

Required

Lower

Attenuation

Larger

Operating

Frequencies

twisted pair tp
Twisted Pair (TP)

Effect of loading with coils in series at intervals

But attenuation rises rapidly

Outside this narrow band.

No good for ASDL which tries to

get 1 MHz BW from the TP line!

Flat and low attenuation

Over the telephone voice band

(300-3400 Hz)

(Passive Equalizer)

u tp cables
UTP Cables

unshielded

twisted pair applications
Twisted Pair - Applications
  • Most commonly used guided medium
  • Telephone network (Analog Signaling)
    • Between houses and the local exchange (subscriber loop)
    • Originally designed for analog signaling.

Digital data transmitted using modems at low data rates

  • Within buildings (short distances): (Digital Signaling)
    • To private branch exchange (PBX) (64 Kbps)
    • For local area networks (LAN) (10-100Mbps)

Example:

10BaseT: Unshielded Twisted Pair, 10 Mbps,100m range

Digital signal travels in its base band i.e. without modulating a carrier

(short distances)

twisted pair pros and cons compared to other guided media
Twisted Pair - Pros and ConsCompared to other guided media

Pros:

  • Low cost
  • Easy to work with (pull, terminate, etc.)

Cons:

  • Limited bandwidth
    • Limited data rate
  • Large Attenuation
    • Limited distance range
  • Susceptible to interference and noise (exposed construction)
twisted pair transmission characteristics
Twisted Pair - Transmission Characteristics
  • Analog Transmission
    • For analog signals only
    • Amplifiers every 5km to 6km
    • Bandwidth up to 1 MHz (several voice channels): ADSL
  • Digital Transmission
    • For either analog or digital signals (carrying digital data)
    • Repeaters every 2km or 3km
    • Data rates up to few Mbps (1Gbps: very short distance)
  • Impairments:
    • Attenuation: A strong function in frequency ( Distortion, need for equalization)
    • EM Interference: Crosstalk, Impulse noise, Mains interference, etc.
ways to reduce em interference

WK 7

Ways to reduce EM interference
  • Shielding the TP with a metallic braid or sheathing
  • Twisting also reduces low frequency interference
  • Different twisting lengths for adjacent pairs help reduce crosstalk
unshielded utp and shielded stp
Unshielded (UTP) and Shielded (STP)
  • Unshielded Twisted Pair (UTP)
    • Ordinary telephone wire: Abundantly available in buildings
    • Cheapest
    • Easiest to install
    • Suffers from external EM interference
  • Shielded Twisted Pair (STP)

Shielded with foil, metal braid or sheathing:

      • Reduces interference
      • Reduces attenuation at higher frequencies (increases BW)

 Better Performance:

        • Increased data rates used
        • Increased distances covered

 But becomes:

        • More expensive
        • Harder to handle (thicker, heavier)
tp categories eia 568 a standard 1995 cabling of commercial buildings for data
TP Categories: EIA-568-A Standard (1995) (cabling of commercial buildings for data)
  • Cat 3: Unshielded (UTP)
    • Up to 16MHz
    • Voice grade
    • In most office buildings
    • Twist length of 7.5 cm to 10 cm
  • Cat 5: Unshielded (UTP)
    • Up to 100MHz
    • Data grade
    • Pre-installed now in many new office buildings
    • Twist length 0.6 cm to 0.85 cm

(Tighter twisting increases cost but improves performance)

  • Newer, shielded twisted pair: (150 W STP)
    • Up to 300MHz
near end crosstalk next
Near End Crosstalk (NEXT)
  • Coupling of signal from one wire pair to another
  • Coupling takes place when a transmitted signal entering a pair couples into an adjacent receiving pair at the same end
  • i.e. near transmitted signal is picked up by near receiving pair

Transmitted Power, P1

Disturbing pair

Coupled Received Power, P2

Disturbed pair

“NEXT” Attenuation = 10 log P1/P2 dBs

The larger … the smaller the crosstalk (The better the performance)

“NEXT” attenuation is a desirable attenuation- The larger the better!

transmission properties for shielded unshielded tp

Signal Attenuation (dB per 100 m)

Near-end Crosstalk Attenuation (dB)

Frequency (MHz)

Category 3 UTP

Category 5 UTP

150-ohm STP

Category 3 UTP

Category 5 UTP

150-ohm STP

1

2.6

2.0

1.1

41

62

68?

4

5.6

4.1

2.2

32

53

58

16

13.1

8.2

4.4

23

44

50.4

25

10.4

6.2

41

47.5

100

22.0

12.3

32

38.5

300

21.4

31.3

Transmission Properties for Shielded & Unshielded TP

Desirable Attenuation- Larger is better!

Undesirable Attenuation- Smaller is better

newer twisted pair categories and classes

Category 3 Class C

Category 5 Class D

Category 5E

Category 6 Class E

Category 7 Class F

Bandwidth

16 MHz

100 MHz

100 MHz

200 MHz

600 MHz

Cable Type

UTP

UTP/FTP

UTP/FTP

UTP/FTP

SSTP

Link Cost (Cat 5 =1)

0.7

1

1.2

1.5

2.2

Newer Twisted Pair Categories and Classes

UTP: Unshielded Twisted Pair

FTP: Foil Twisted Pair

SSTP: Shielded-Screen Twisted Pair

coaxial cable
Coaxial Cable

Physical Description:

1 - 2.5 cm

Designed for operation

over a wider frequency range

coaxial cable applications
Coaxial Cable Applications

Most versatile medium:

  • Television distribution (FDM Broadband)
    • Cable TV (CATV): 100’s of TV channels over 10’s Kms
  • Long distance telephone transmission
    • Can carry 10s of thousands of voice channels simultaneously (though FDM multiplexing) (Broadband)
    • Now facing competition from optical fibers and terrestrial microwave links
  • Local area networks, e.g. Thickwire Ethernet cable

(10Base5): 10 Mbps, Baseband signal, 500m segment

(5 time TP distance)

coaxial cable transmission characteristics improvements over tp
Coaxial Cable - Transmission Characteristics:Improvements over TP
  • Extended frequency range
    • Up to 500 MHz
  • Reduced EM interference and crosstalk
    • Due to enclosed concentric construction
    • EM fields terminate within cable and do not stray outside causing interference
  • Remaining limitations:
    • Attenuation
    • Thermal and inter modulation noise (FDM)
coaxial cable transmission characteristics
Coaxial Cable - Transmission Characteristics
  • Analog Transmission
    • Amplifiers every few kms
    • Closer amplifier spacing for higher operating frequencies
  • Digital Transmission
    • Repeater every 1km
    • Closer repeater spacing for higher data rates
optical fiber
Optical Fiber
  • A thin (2-125 mm) flexible strand of glass or plastic
    • Light entering at one end travels confined within the fiber until it leaves it at the other end
    • As fiber bends around corners, the light remains within the fiber through multiple internal reflections
    • Lowest losses (attenuation) with ultra pure fused silica glass… but expensive and more difficult to manufacture
    • Reasonable losses with multi-component glass and with plastic

Quality, Cost, Difficulty of Handling

Attenuation (Loss)

Pure

Glass

Multi-component Glass

Plastic

optical fiber construction

Individual Fibers:

(Each having its core & Cladding)

Multiple Fiber Cable

Single Fiber Cable

Optical Fiber: Construction
  • An optical fiber consists of three main parts
    • Core
      • A narrow cylindrical strand of glass/plastic, with refractive index n1
    • Cladding
      • A tube surrounding each core, with refractive index n2
      • The core must have a higher refractive index than the cladding to keep the light beam trapped inside: n1 > n2
    • Protective outer jacket
      • Protects against moisture, abrasion, and crushing

Important: Each core surrounded by a cladding

reflection and refraction
Reflection and Refraction
  • At a boundary between a denser (n1) and a rarer (n2) medium, n1 > n2 (e.g. water-air, optical fiber core-cladding) a ray of light will be refracted or reflected depending on the incidence angle

Increasing Incidence angle,

1

Angles

With the

Normal

2

rarer

v2 = c/n2

n2

denser

2

1

n1

critical

1

n1 > n2

v1 = c/n1

Total internal reflection

Critical angle

Refraction

optical fiber33
Optical Fiber

Refraction at boundary

for . Escaping light is absorbed in jacket

i <

critical

n2

Rarer

Denser

Denser

n1

n1

Rarer

i

Total Internal Reflection at

core-cladding boundary for

i >

critical

n1 > n2

optical fiber benefits
Optical Fiber - Benefits
  • Greater capacity
    • Fiber: 100’s of Gbps over 10’s of Kms
    • Cable: 100’s of Mbps over 1’s of Kms
    • Twisted pair: 100’s of Mbps over 10’s of meters
  • Lower/more uniform* attenuation (Fig. 4.3c)
    • An order of magnitude lower
    • Relatively constant over a larger range of frequencies*
  • Electromagnetic isolation
    • Not affected by external EM fields:
      • No interference, impulse noise, crosstalk
    • Does not radiate:
      • Not a source of interference
      • Difficult to tap (data security)

* With careful selection of operating band

optical fiber benefits contd
Optical Fiber – Benefits, Contd.
  • Greater repeater spacing: Lower cost, Fewer Units
    • Fiber: 10-100’s of Kms
    • Cable, Twisted pair: 1’s Kms
  • Smaller size and weight:
    • An order of magnitude thinner for same capacity
      • Useful in cramped places
      • Reduced cost of digging in populated areas
      • Reduced cost of support structures
optical fiber applications
Optical Fiber - Applications
  • Long-haul trunks
    • Telephone traffic over long routes between cities, and undersea:
      • Fiber & Microwave now replacing coaxial cable
      •  1500 km, Up to 60,000 voice channels
  • Metropolitan trunks
    • Joining exchanges inside large cities:
      •  12 km, Up to 100,000 voice channels
  • Rural exchange trunks
    • Joining exchanges of towns and villages:
      •  40-160 km, Up to 5,000 voice channels
  • Subscriber loops
    • Joining subscribers to exchange:
      • Fiber replacing TP, allowing all types of data
  • LANs, Example:

10BaseF 10 Mbps, 2000 meter segment

Exchange

City

City

Main

Exchange

optical fiber transmission characteristics
Optical Fiber - Transmission Characteristics
  • Acts as a wave guide for light (1014 to 1015 Hz)
    • Covers portions of infrared and visible spectrum
  • Transmission Modes:

Multimode

Single Mode

Graded Index

Step Index

optical fiber transmission modes

i <

critical

n2

n1

Dispersion:

Spread in ray

arrival time

Optical Fiber Transmission Modes

Refraction

Shallow reflection

Deep reflection

n2

n1

Large Spread

Core

Cladding

2 ways:

Curved path: n is not uniform- decreasing

Smaller

  • v = c/n
  • n1 is made lower away from center…this speeds up deeper rays
  • and compensates for their larger distances, arrive together with shallower rays

Smallest

  • Smaller spread  Narrower pulses
  • Higher data rates supported
optical fiber transmission modes40
Optical Fiber – Transmission modes
  • Spread of received light pulse in time (dispersion) is bad:
    • Causes inter-symbol interference  bit errors (similar to delay distortion)
    • Limits usable data rate and usable transmission distance
  • Caused by propagation through multiple reflections at different angles of incidence
  • Dispersion increases with:
    • Larger distance traveled
    • Thicker fibers with step index
    • Less focused sources
  • Can be reduced by:
    • Limiting the distance
    • Thinner fibers and a highly focused light source

 Single mode (in the limit): High data rates, very long distances

    • Or Graded-index multimode thicker fibers: The half-way (lower cost) solution
optical fiber transmission system light source fiber light detector
Optical Fiber Transmission System– Light Source + Fiber + Light Detector

Light Sources

  • Light Emitting Diode (LED)
    • Incoherent light- More dispersion  Lower data rates
    • Low cost
    • Wider operating temp range
    • Longer life
  • Injection Laser Diode (ILD)
    • Coherent light- Less dispersion  Higher data rate
    • More efficient
    • Faster switching  Higher data rate
optical fiber wavelength division multiplexing wdm
Optical Fiber – Wavelength Division Multiplexing (WDM)
  • A form of FDM (Channels sharing the medium by occupying different frequency bands)
  • Multiple light beams at different light frequencies (wavelengths) transmitted on the same fiber
  • Each beam forms a separate communication channel
  • Separated at destination by filters
  • Example:

256 channels

@ 40 Gbps each

 10 Tbps total data rate

WDM

optical fiber four transmission bands windows in the infrared ir region
Optical Fiber – Four Transmission bands (windows) in the Infrared (IR) region
  • Band selection is a system decision based on:
    • Attenuation of the fiber
    • Properties of the light sources
    • Properties of the light receivers

S

L

C

Bandwidth, THz

33

12

4

7

Note: l in fiber = v/f = (c/n)/f = (c/f)/n = l in vacuum/n

i.e. l in fiber < l in vacuum

wireless transmission
Wireless Transmission
  • Free-space is the transmission medium
  • Need efficient radiators, called antennas
    • Signal fed from transmission line (wireline) and radiated it into free-space (wireless)
  • Popular applications
    • Radio & TV broadcast
    • Cellular Communications
    • Microwave Links
    • Wireless Networks
wireless transmission frequency ranges
Wireless Transmission Frequency Ranges
  • Radio: 30 MHz to 1 GHz
    • Omni directional
      • Broadcast radio
  • Microwaves: 1 GHz to 40 GHz
    • Highly directional beams
      • Point to point (Terrestrial)
      • Satellite
  • Infrared Light: 0.3 THz to 20 THz (below light)
    • Localized communications (confined spaces)
antennas
Antennas
  • Electrical conductor (or system of conductors) used to radiate / collect electromagnetic energy into/from surrounding space
  • Transmission
    • Radio frequency electrical energy from transmitter
    • Converted into electromagnetic energy
    • Radiated into surrounding space
  • Reception
    • Electromagnetic energy impinging on antenna
    • Converted to radio frequency electrical energy
    • Fed to receiver
  • Same antenna often used for both TX and RX in 2-way communication systems
radiation pattern
Radiation Pattern
  • Power radiated in all directions, but usually not with the same efficiency
  • Isotropic antenna
    • A hypothetical point source in space

(Small dimensions relative to l)

    • Radiates equally in all directions – A spherical radiation pattern
    • Used as a reference for other antennae
  • Directional Antenna
    • Concentrates radiation in a given desired direction – hence point-to-point, line of sight

communications

    • Gives antenna ‘gain’ in that direction

relative to isotropic for both TX and RX

    • Larger dimensions relative to l  Greater directivity

Radiation Patterns

Isotropic

Directional

parabolic reflective antenna
Parabolic Reflective Antenna

WK 8

  • Used for terrestrial and satellite microwave
  • Source placed at the focal point will produce waves that get reflected from parabola parallel to the parabola axis
    • Creates a (theoretically) parallel beam of light/sound/radio that does not spread (disperse) in space
    • In practice, some divergence (dispersion) occurs, because source at focus has a finite size (not exactly a point!)
  • On reception, only signal from the axis direction is concentrated at focus, where detector is placed. Signals from other directions miss the focus  negligible O/P
  • The larger the antenna

(in wavelengths) the better

the directionality  so, using

Higher frequency is advantageous

Parabola

Focus

antenna gain g
Antenna Gain, G
  • A measure of antenna directionality
  • Power output of the antenna in a particular direction compared to that produced by a perfect isotropic antenna
  • Can be expressed in decibels (dB, dBi) (i = relative to isotropic)
  • Increased power radiated in one direction causes less power radiated in another direction (Total power is fixed)
  • Effective area Ae:
    • Related to size and shape of antenna
    • Determines the antenna gain,

Ae is the effective area

antenna gain g effective areas
Antenna Gain, G: Effective Areas
  • An isotropic antenna has a gain G = 1 (0 dBi)
  • i.e.
  • A parabolic antenna has:
  • Substituting we get:
  • Gain in dBi = 10 log G
  • Important: Gains apply to both TX and RX antennas

A = Actual Area = p r2

propagation attenuation
Propagation Attenuation
  • As signal propagates in space, its power drops with distance according to the inverse square law

While with a guided medium, signal drops exponentially with distance… giving larger attenuation and lower repeater spacing

d’ = distance in l’s

i.e. loss in signal power over distance traveled, d

  • Show that L increases by 6 dBs for every doubling of distance d.
  • For guided medium, corresponding attenuation = a d dBs, a in dBs/km

A disadvantage for operating at higher frequency?

microwave link calculations
Microwave Link Calculations

TX-RX Net attenuation, A = L – G1 – G2, dBs

S dBm = P dBm – A dBs

terrestrial microwave
Terrestrial Microwave
  • Parabolic dish
  • Focused beam (with antenna gain)
  • Line of sight requirement:
    • Beam should not be obstructed
    • Curvature of earth limits maximum range  Use relays to increase range (multi-hop link)
    • Link performance sensitive to antenna alignment
  • Applications:
    • Long haul telecommunications Many voice/data channels over long distances between large cities, possibly through intermediate relays: Competes with coaxial cable and fiber
    • Short wireless links between buildings:
      • CCTV links
      • Wireless links between LANs in close-by buildings
    • Cellular Telephony
terrestrial microwave transmission properties
Terrestrial Microwave: Transmission Properties
  • 1 - 40 GHz
  • Higher f Advantages:
    • Larger bandwidth, B  higher data rate (Table 4.6)
    • Smallerl  smaller (lighter, cheaper) antenna for a required antenna gain (see gain eqn.)
  • But Higher f  larger attenuation due propagation and absorption by rain
  • So,
    • Long-haul links (long distances) operate at lower frequencies (4-6 GHz,11 GHz) to avoid large attenuation
    • Short links between close-by buildings operate at higher frequencies (e.g. 22 GHz) (Attenuation is not a big problem for the short distances, smaller antenna size)
satellite microwave
Satellite Microwave
  • Satellite acts as a relay station

for the link

  • Satellite receives on one frequency (uplink), amplifies or repeats signal and re-transmits it on another frequency (downlink)
  • Spatial angular separation (e.g. 3)to avoid interference from neighboring TXs
  • Require a geo-stationary orbit (satellite rotates at the same speed of earth rotation, so appears stationary):
    • Height: 35,784km (long link, large transmission delays)
  • Applications:
    • Television direct broadcasting
    • Long distance telephony
    • Private business networks linking multiple company sites worldwide
a satellite point to point link
a. Satellite Point to Point Link

Relay

Downlink

Uplink

Earth curvature

Obstructs line of sight

for large distances

b satellite broadcast link
b. Satellite Broadcast Link

Direct Broadcasting Satellite

transmission characteristics
Transmission Characteristics
  • 1-10 GHz
  • Frequency Trade offs:
    • Lower frequencies: More noise and interference
    • Higher frequencies: Larger rain attenuation, but smaller antennas
  • Downlink/Uplink frequencies recently going higher: 4/6 GHz  12/14  20/30 (better receivers becoming available)
  • Delay = 0.25 s  noticeable for telephony
  • Inherently a broadcasting facility
broadcast radio 30 mhz 1 ghz
Broadcast Radio: 30 MHz – 1 GHz
  • Omni directional (no need for antenna directionality horizontally)
    • No dishes
    • No line of sight requirement
    • No antenna alignment requirement/problems
  • Applications:
    • FM radio
    • UHF and VHF television
  • Choice of frequency range:

Reflections from ionosphere < 30 MHz -1 GHz < Rain

  • Propagation attenuation:

Lower than for Microwaves (as l is larger)

  • Problems caused by omni directionality: Interference due to

multi-path reflections

    • e.g. TV ghost images
multi path effects of omni directionality
Multi-Path effects of omni-directionality

Omni-Directional TV Broadcasting

Antenna

TV ghost images

infrared
Infrared
  • Data Modulates a non coherent infrared light
  • Relies on line of sight (or reflections through walls or ceiling)
  • Blocked by walls (unlike microwaves)
  • No licensing required for frequency allocation
  • Applications:
    • TV remote control
    • Wireless LAN within a room