ICSA 733: Week 3 Transmission Basics

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ICSA 733 Week 3Transmission Basics

• Elizabeth Lane Lawley, Instructor

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Data Communication Basics

• Analog or Digital
• Three Components
• Data
• Signal
• Transmission

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Electromagnetic Signals

• Function of time
• Analog (varies smoothly over time)
• Digital (constant level over time, followed by a
  change to another level)
• Function of frequency
• Spectrum (range of frequencies)
• Bandwidth (width of the spectrum)

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Periodic Signal Characteristics

• Amplitude (A) signal value, measured in volts
• Frequency (f) repetition rate, cycles per second
  or Hertz
• Period (T) amount of time it takes for one
  repetition, T1/f
• Phase (F) relative position in time, measured in
  degrees

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Analog Signaling

• represented by sine waves

phase difference
1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
cycles per second
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Digital Signaling

• represented by square waves or pulses

1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
cycles per second
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Digital Text Signaling

• Transmission of electronic pulses representing
  the binary digits 1 and 0
• How do we represent letters, numbers, characters
  in binary form?
• Earliest example Morse code (dots and dashes)
• Most common current form ASCII

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ASCII Character Codes

• Use 7 bits of data (1 byte) to transmit one
  character
• 7 binary bits has 128 possible outcomes (0 to
  127)
• Represents alphanumeric characters, as well as
  special characters
• Eighth bit in a byte can be used for formatting
  also known as high-order bit

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Digital Image Signaling

• Pixelization and binary representation

Code 00000000 00111100 01110110 01111110 011
11000 01111110 00111100 00000000
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Why Study Analog?

• Telephone system is primarily analog rather than
  digital (designed to carry voice signals)
• Low-cost, ubiquitous transmission medium
• If we can convert digital information (1s and 0s)
  to analog form (audible tone), it can be
  transmitted inexpensively

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Voice Signals

• Easily converted from sound frequencies (measured
  in loudness/db) to electromagnetic frequencies,
  measured in voltage
• Human voice has frequency components ranging from
  20Hz to 20kHz
• For practical purposes, the telephone system has
  a narrower bandwidth than human voice, from 300
  to 3400Hz

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Bandwidth

• Width of the spectrum of frequencies that can be
  transmitted
• if spectrum300 to 3400Hz, bandwidth3100Hz
• Greater bandwidth leads to greater costs
• Limited bandwidth leads to distortion
• Analog measured in Hertz, digital measured in baud

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BPS vs. Baud

• BPSbits per second
• Baud of signal changes per second
• Each signal change can represent more than one
  bit, through variations on amplitude, frequency,
  and/or phase

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Analog Data Choices
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Digital Data Choices
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Transmission Choices

• Analog transmission
• only transmits analog signals, without regard for
  data content
• attenuation overcome with amplifiers
• Digital transmission
• transmits analog or digital signals
• uses repeaters rather than amplifiers

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Data, Signals, and Transmission
A
Data
D
D
Transmission System
A
D
A
Signal
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Advantages of Digital Transmission

• The signal is exact
• Signals can be checked for errors
• Noise/interference are easily filtered out
• A variety of services can be offered over one
  line
• Higher bandwidth is possible with data compression

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Analog Encoding of Digital Data

• data encoding and decoding technique to represent
  data using the properties of analog waves
• modulation the conversion of digital signals to
  analog form
• demodulation the conversion of analog data
  signals back to digital form

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Modem

• an acronym for modulator-demodulator
• uses a constant-frequency signal known as a
  carrier signal
• converts a series of binary voltage pulses into
  an analog signal by modulating an audible carrier
  signal
• the receiving modem translates the analog signal
  back into digital data

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Methods of Modulation

• amplitude modulation (AM) or amplitude shift
  keying (ASK)
• frequency modulation (FM) or frequency shift
  keying (FSK)
• phase modulation or phase shift keying (PSK)

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Amplitude Shift Keying (ASK)

• In radio transmission, known as amplitude
  modulation (AM)
• the amplitude (or height) of the sine wave varies
  to transmit the ones and zeros
• major disadvantage
• telephone lines are very susceptible to
  variations in transmission quality that affect
  amplitude

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ASK Illustration
1
0
0
1
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Frequency Shift Keying (FSK)

• in radio transmission, known as frequency
  modulation (FM)
• the frequency of the carrier wave varies in
  accordance with the signal to be sent
• signal is transmitted at constant amplitude
• more immune to noise than ASK
• requires more analog bandwidth than ASK

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FSK Illustration
1
1
0
1
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Phase Shift Keying (PSK)

• also known as phase modulation (PM)
• frequency and amplitude of the carrier signal are
  kept constant
• the carrier is shifted in phase according to the
  input data stream
• each phase can have a constant value, or value
  can be based on whether or not phase changes
  (differential keying)

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PSK Illustration
0
0
1
1
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Differential Phase Shift Keying (DPSK)
0
0
1
1
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Complex Modulations

• Combining modulation techniques allows us to
  transmit multiple bit values per signal change
  (baud)
• Increases information-carrying capacity of a
  channel without increasing bandwidth
• Increased combinations also leads to increased
  likelihood of errors
• Typically, amplitude and phase modulation are
  combined

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Quadrature Amplitude Modulation (QAM)

• the most common method for quadbit transfer
• combination of 8 different angles in phase
  modulation and two amplitudes of signal
• provides 16 different signals, each of which can
  represent 4 bits

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Quadrature Amplitude Modulation Illustration
90
135
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amplitude 1
0
180
amplitude 2
225
315
270
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Quadrature Amplitude Modulation Uses

• CCITT V.22 bis modem
• the "bis" qualifier is a French term for "duo" or
  "twice"
• supports transmission of full-duplex 2400 bps
  synchronous or asynchronous data over a switched,
  2-Wire, voice circuit
• the modulation rate is 600 baud, with each baud
  representing four data bits

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Trellis Coded Modulation (TCM)

• sophisticated mathematics are used to predict the
  best fit between the incoming signal and a large
  set of possible combinations of amplitude and
  phase changes
• a Forward Error Correcting (FEC)
• used in the V.32 modem (9600 bps) and all the
  higher speed modems

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Digital Encodingof Digital Data

• Most common, easiest method is different voltage
  levels for the two binary digits
• Typically, negative1 and positive0
• Known as NRZ-L, or nonreturn-to-zero level,
  because signal never returns to zero, and the
  voltage during a bit transmission is level

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Differential NRZ

• Differential version is NRZI (NRZ, invert on
  ones)
• Change1, no change0
• Advantage of differential encoding is that it is
  more reliable to detect a change in polarity than
  it is to accurately detect a specific level

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Problems With NRZ

• Difficult to determine where one bit ends and the
  next begins
• In NRZ-L, long strings of ones and zeroes would
  appear as constant voltage pulses
• Timing is critical, because any drift results in
  lack of synchronization and incorrect bit values
  being transmitted

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Biphase Alternatives to NRZ

• Require at least one transition per bit time, and
  may even have two
• Modulation rate is greater, so bandwidth
  requirements are higher
• Advantages
• Synchronization due to predictable transitions
• Error detection based on absence of a transition

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Manchester Code

• Transition in the middle of each bit period
• Transition provides clocking and data
• Low-to-high1 , high-to-low0
• Used in Ethernet

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Differential Manchester

• Midbit transition is only for clocking
• Transition at beginning of bit period0
• Transition absent at beginning1
• Has added advantage of differential encoding
• Used in token-ring

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Digital Encoding Schemes
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Transmitting Digital Data

• Codes determine what needs to be transmitted, not
  how to transmit
• Two primary transmission methods
• Serial
• Parallel

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Parallel Transmission

• sending a character at a time
• the components of each character are transmitted
  in parallel
• common transmission method between a personal
  computer and a printer
• multiple wires are required for transmission

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Parallel Illustration
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Serial Transmission

• sending bits one after another rather than
  several at the same time
• requires only one wire to transmit data
• slower than parallel transmission
• used when transmitting data over a telephone line
  as there is only one set of wires

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Serial Illustration
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Asynchronous Synchronous Transmission

• Concerned with timing issues in serial
  communication
• How does the receiver know when the bit period
  begins and ends?
• Small timing difference become more significant
  over time if no synchronization takes place
  between sender and receiver

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Timing of Serial Data

• asynchronous transmission
• synchronous transmission

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Asynchronous Transmission

• Data transmitted 1 character at a time
• Character format is 1 start 1 stop bit, plus
  data of 5-8 bits
• Character may include parity bit

• Resynchronization each start bit
• Uses simple, cheap technology
• Wastes 20-30 of bandwidth
• Example VT100 terminal

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Synchronous Transmission

• Large blocks of bits transmitted without
  start/stop codes
• Synchronized by clock signal or clocking data,
  usually sent over a separate wire or channel
• Data framed by preamble and postamble bit patterns

• More efficient than asynchronous
• Overhead typically below 5
• Used at higher speeds than asynchronous
• Requires error checking
• Example IBM3270 terminal

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Communication Paths

• Simplex
• Half-Duplex
• Full-Duplex

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Simplex Transmission

• only transmit in one direction
• rarely used in data communications
• e.g., receiving signals from the radio station or
  CATV
• the sending station has only one transmitter the
  receiving station has only one receiver

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Simplex Illustration
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Half Duplex Communication

• data may travel in both directions, but only in
  one direction at a time
• provides nonsimultaneous two-way communication
• computers use special control signals to
  negotiate which system will send data and which
  will receive data
• the amount of time it takes computers to switch
  between sending and receiving is called
  turnaround time

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Half Duplex Illustration
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Full Duplex Communication

• complete two-way simultaneous transmission
• faster than half-duplex communication because no
  turnaround time is needed
• requires higher bandwidth

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Full Duplex Illustration
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Digital Interfaces

• The point at which one device connects to another
• Standards define what signals are sent, and how
• Some standards also define physical connector to
  be used

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RS-232C (EIA 232C)

• Uses NRZ-L encoding
• Defines two types of interface
• DTE Data Terminal Equipment
• DCE Data Circuit-Terminating Equipment
• We often define entire devices based on their
  interface (e.g terminalDTE, or modemDCE)

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DTE and DCE
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RS-232C DB-25 Connectors

• For pin assignments, see page 58 of the textbook

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RS-232C Examples
Odd Parity
Even Parity
No Parity
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Initial Handshaking

• DTE raises DTR (data terminal ready) signal to
  DCE
• DCE raises DSR (data set ready) signal
• DTE raises RTS (request to send) signal
• DCE raises CTS (clear to send) signal
• DCE sends a carrier signal
• Remote DCE detects carrier and raises DCD (data
  carrier detect) signal to DCE
• DTE sends data on TD (transmit data)

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Completion Handshaking

• DCE modulates data onto the carrier wave
• Remote DCE demodulates data onto RD (receive
  data)
• DTE lowers RTS signal
• DCE drops CTS and carrier wave
• Remote DCE drops DCD
• Transmission is complete

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Null Modem Cable

• special wiring of an RS-232-C cable to enable
  computers to talk to one another without a modem

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Null Modem Cable
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EIA-232-D

• new version of RS-232-C adopted in 1987
• improvements in grounding shield, test and
  loop-back signals
• the prevalence of RS-232-C in use made it
  difficult for EIA-232-D to enter into the
  marketplace

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RS-449

• an EIA standard that improves on the capabilities
  of RS-232-C
• provides for a 37-pin connection, cable lengths
  up to 200 feet, and data transmission rates up to
  2 million bps
• equates with the functional and procedural
  portions of R-232-C
• the electrical and mechanical specifications are
  covered by RS-422 and RS-423

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Baseband vs. Broadband

• Baseband transmission
• A single data signal transmitted directly on a
  wire (as in RS-232)
• Commonly used for LANs
• Broadband transmission
• Data is sent using a carrier signal
• Different frequencies allow multiple simultaneous
  signals
• Cable TV is a good example

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Transmission Media

• the physical path between transmitter and
  receiver
• design factors
• bandwidth
• attenuation weakening of signal over distances
• interference
• number of receivers

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Impairments and Capacity

• Impairments exist in all forms of data
  transmission
• Analog signal impairments result in random
  modifications that impair signal quality
• Digital signal impairments result in bit errors
  (1s and 0s transposed)

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Transmission Impairments

• Attenuation
• loss of signal strength over distance
• Attenuation Distortion
• different losses at different frequencies
• Delay Distortion
• different speeds for different frequencies
• Noise

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Types of Noise

• Thermal (aka white noise)
• Uniformly distributed, cannot be eliminated
• Intermodulation
• when different frequencies
• Crosstalk
• Impulse noise
• Less predictable

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Types of Transmission Media

• conducted or guided media
• use a conductor such as a wire or a fiber optic
  cable to move the signal from sender to receiver
• wireless or unguided media
• use radio waves of different frequencies and do
  not need a wire or cable conductor to transmit
  signals

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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
• Examples
• twisted pair wires
• coaxial cables
• optical fiber

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Twisted Pair Wires

• 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 Wires

• two varieties
• STP (shielded twisted pair)
• the pair is wrapped with metallic foil or braid
  to insulate the pair from electromagnetic
  interference
• UTP (unshielded twisted pair)
• each wire is insulated with plastic wrap, but the
  pair is encased in an outer covering

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Twisted Pair Wires

• Category 3 UTP
• data rates of up to 16mbps are achievable
• Category 5 UTP
• data rates of up to 100mbps are achievable
• more tightly twisted than Category 3 cables
• more expensive, but better performance
• STP
• More expensive, harder to work with

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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 (3000Hz)

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Coaxial Cable (or Coax)

• bandwidth of up to 400 MHz
• has an inner conductor surrounded by a braided
  mesh
• both conductors share a common center axial,
  hence the term co-axial

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Coax Layers
outer jacket (polyethylene)
shield(braided wire)
insulating material
copper or aluminum conductor
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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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Fiber Optic Cable

• relatively new transmission medium used by
  telephone companies in place of long-distance
  trunk lines
• also used by private companies in implementing
  local data communications networks
• require a light source with injection laser diode
  (ILD) or light-emitting diodes (LED)

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Fiber Optic Layers

• consists of three concentric sections

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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 step-index
fiber optic multimode graded-index
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 (Unguided Media) Transmission

• transmission and reception are achieved by means
  of an antenna
• directional
• transmitting antenna puts out focused beam
• transmitter and receiver must be aligned
• omnidirectional
• signal spreads out in all directions
• can be received by many antennas

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Wireless Examples

• terrestrial microwave transmission
• satellite transmission
• broadcast radio
• infrared

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Terrestrial Microwave Transmission

• 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
• bypass application

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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 Transmission

• a microwave relay station in space
• can relay signals over long distances
• 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
satellite transponder
dish
dish
22,300 miles
uplink station
downlink station
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Satellite Transmission Applications

• television distribution
• a network provides programming from a central
  location
• direct broadcast satellite (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