Telecommunication Network

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TelecommunicationNetwork

• Telecommunication Transmission
• Semester 1, 2006

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Electromagnetic Spectrum
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Transmission Media - Overview

• Transmission Medium
• Physical path between transmitter and receiver
• Guided Media
• Waves are guided along a solid medium
• e.g., copper twisted pair, copper coaxial cable,
  optical fiber
• Unguided Media
• Provides means of transmission but does not guide
  electromagnetic signals
• Employ an antenna for transmission
• e.g., atmosphere, outer space, satellites,
  terrestrial microwave, broadcast radio

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Transmission Media - Overview

• Characteristics and quality determined by medium
  and signal
• For guided
• Medium is more important
• For unguided
• Bandwidth produced by the antenna is more
  important
• Key concerns are
• Data rate and Distance
• Higher data rate and longer distance is better
• Design factor
• Bandwidth
• All other factors remaining constant, 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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Open-Wire Pairs
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Twisted Pair

• Most common medium
• Two separately insulated wires twisted together
  in a helical manner (like DNA) and often bundled
  together
• Advantages
• Cheap
• Easy to work with
• Disadvantages
• Low data rate
• Short range
• Applications
• Telephone network
• Between house and local exchange
• Within buildings
• To private branch exchange (PBX)
• For local area networks (LAN)
• 10 Mbps or 100 Mbps

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Twisted Pair

• Transmission Charateristics
• Analog
• Amplifiers every 5 km to 6 km
• Digital
• Use either analog or digital signals
• Repeater every 2 km or 3 km
• Limited in
• Distance
• Bandwidth (1 MHz)
• Data rate (100 Mbps)
• Susceptible to interference and noise

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UTP vs. STP

• Unshielded Twisted Pair (UTP)
• Ordinary telephone wire
• Cheapest
• Easiest to install
• Suffers from external Electromagnetic (EM)
  interference
• Shielded Twisted Pair (STP)
• Metal braid or sheathing that reduces
  interference
• More expensive
• Harder to handle (thick, heavy)
• UTP Categories
• Cat 3
• Up to 16 MHz
• Voice grade found in most offices
• Twist length of 7.5 cm to 10 cm
• Cat 4
• Up to 20 MHz
• Cat 5
• Up to 100 MHz
• Commonly pre-installed in new office buildings

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Coaxial Cable

• Most versatile medium

Braided shield is also referred to as the outer
conductor
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Coaxial Cable

• Applications
• Television distribution
• Cable TV
• Long distance telephone transmission
• Can carry 10,000 voice calls simultaneously
• Being replaced by fiber optic
• Short distance computer systems links
• LANs
• Transmission characteristic
• Analog
• Amplifiers every few km
• Closer if higher frequency
• Up to 500 MHz
• Digital
• Repeater every 1 km
• Closer for higher data rates

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Thin Ethernet 10 Base 2
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Thick Ethernet - 10 Base 5

http//www.ictp.trieste.it
http//www.ictp.trieste.it
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Optical Fiber

• Advantages
• Greater capacity - data rates of hundreds of Gbps
• Smaller size weight
• Lower attenuation
• Electromagnetic isolation
• Greater repeater spacing
• - 10s of km at least
• System components
• Transmission medium - fiber optic cable
• Light source
• LED (cheaper, wider operating temp range, last
  longer)
• Injection laser diode (ILD) (More efficient,
  greater data rate)
• Detector - photodiode

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Optical Fiber - Applications

• Telephone Network Applications
• Long-haul, metropolitan, rural, and subscriber
  loop circuits
• Local Area Networks
• Optical fiber networks
• Data rates from 100 Mbps to 1 Gbps
• Support hundreds (or even thousands) of stations

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Optical Fiber - Transmission Characteristics

• Light Sources
• 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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Cost of Wired Transmission Media

Source Cyganski, D., Orr, J.A., Information
Technology Inside and Outside, (USA Prentice
Hall, 2000)
Optical fiber cable 2.76-4.00/ft
Source http//www.dealtime.com, 2003
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Transmission Medium
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Transmission Medium
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Transmission Medium
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Voice Communication Services

• Leased Line High quality connections. High
  speed data transmission available known as
  conditioned leased line. Risky in case the line
  is down. No automatically re-route the call,
  Point-to-Point connection, Less reliability
• Dial-up Line Different route available
  depending on dialing number. High reliability but
  uncertain quality connections
• Private Branch Exchange PBX internal call
  never leave the customers premises. Only
  external calls are sent to the central office
  therefore few central office trunk are needed.
  And the customers central office charges are far
  less than those associated with Centrex service.
  But the customer must purchase or lease the PBX
  to achieve these economies.

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Data Transmission

• An ideal data transmission system gives an output
  which is identical to the input.
• Three problems in data transmission and long-haul
  communication
• noise
• attenuation (amplification, line loading)
• distortion (equalization)

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Noise

• A variation in output not caused by a variation
  in the measurand is noise
• It gives an error in the measurement unless it
  is removed
• It is quantified as the signal-to-noise ratio
  (SNR)

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Attenuation

• Energy losses in the transmission medium mean
  that the amplitude of the signal is reduced
• Reduces the SNR
• Reduces the signal level

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Distortion

• Arises when the frequency response of
  transmission system is inadequate to deal with
  the frequencies in the signal.

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Analogue data transmission

• Generally done by using conducting wires to feed
  the transducer output to the signal processing,
  recording and/or display unit.
• Wires may be simple single strand conductors, or
  may be co-axial cables.
• Co-axial cables consist of
• inner conductor
• insulating layer
• outer earthing and screening conductor
• final insulating layer outside.

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Analogue data transmission

• Thickness and purity of both conductors and
  insulators vary the cost varies accordingly.
• Reduction of attenuation and noise mean
  increased cost.

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Effect of noise on transmitted analogue data

• Low levels of noise enable the signal to be
  detected with very small errors
• High levels of noise may totally obscure the
  signal.
• Noise arises from
• external sources
• noise generated in the conductor itself.

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Effect of noise on transmitted analogue data

• Coaxial cables reduce these problems compared
  with single wires
• Internal noise is related to the size, length
  and quality of the conductors.
• Isolation from external pickup reduces with
  thicker, better quality insulation

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Effect of attenuation on transmitted analogue
data

• Attenuation is determined by the thickness and
  quality of the conductor in the cable.
• Also determined by distance (length of cable)

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Effect of distortion on transmitted analogue data

• Distortion effects are related to frequency
  response
• The transmission system deals adequately with the
  low frequency
• A higher frequency is, however, barely
  transmitted.
• The frequency response of co-axial cable varies
  with cost.

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Transmission distance

• The greater the distance, the more serious all
  these problems are
• Simple conducting wires may be adequate to carry
  a signal over a distance of a metre
• More expensive coaxial cable will be required to
  carry the same signal to the same display system
  over a longer distance.

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Near End Crosstalk (NEXT)

• Coupling of signal from one pair to another
• The tighter the twist in the cable, the more
  effective the cancellation

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Echo

• The effect resulting from a delayed reflection of
  a signal. Echo in a telephone circuit manifests
  to talker as slightly delayed repeat of his own
  voice, returned from the distant end, just like a
  sound echo.
• Echo can occur at the talker and listener.
• An echo canceller is used in voice and data
  circuit to suppress telephone echo by simulating
  a negative version of the outgoing signal in the
  receive path.
• An echo suppressor is a device to suppress
  retransmission of incoming receive path signals
  by inserting a very large attenuation into the
  transmit path whenever a signal is detected in
  the receive path.

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Echo

• Listener Echo
• SymptomListener and talker echo sound similar
  although the signal strength of listener echo may
  be lower. The essential difference between them
  is who hears the echo and where it is produced.
  Listener echo is the component of the talker echo
  that leaks through the near-end hybrid and
  returns again to the listener causing a delayed
  softer echo. The listener hears the talker twice.
• CauseCommon causes are
• Insufficient loss of the echo signal.
• Long echo tail.
• Echo cancellers in the gateway adjacent to the
  near-end hybrid not activating.
• Talker Echo
• SymptomTalker echo is the signal which leaks in
  the far-end hybrid and returns to the sender
  (talker). The talker hears an echo of his own
  voice.
• CauseCommon causes are
• Insufficient loss of the echo signal.
• Echo cancellers in the gateway adjacent to the
  far-end hybrid not activating.
• Acoustic echo caused by the listener's phone.

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Echo Suppressor/Cancellers

• Echo Suppressors Device uses to suppress noise.
  Allowing only a one-way communications path. This
  works well for normal conversation when we stop
  talking, and the other person begins, their voice
  take over. If they interrupt us at mid-sentence,
  they probably are speaking louder than we are so
  we can hear them. The suppressor hears the
  volume difference and gives them the line
• Echo Cancellers Device uses to help eliminate
  echoes. Allowing a continuous two-way
  communications, but are able to remove your own
  echo before it returns to your telephone. A
  sophisticated version of acoustical ceiling tiles
  used to absorb noise. And detects the difference
  between true conversation and an echo, and
  selectively absorb only the echo.

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Singing

• If both paths of 4-wire circuit are connected
  directly to the 2-wire circuit at each end, a
  signal can circulate round the complete loop thus
  created. This will results in continuous
  oscillation, known as singing, unless the sum of
  the gains in the two direction were less than
  zero. To avoid this a transhybrid transformer
  (4-wire/2-wire terminating set) is used.
• Stability (related to singing)
• -singing path loss Ls2(BL2)
• -Condition of stability Lsgt0

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Sidetone
the name given to an effect on 2-wire system
(e.g. basic analogue telephones) where the
speaker hears his own voice in his own earphone
while speaking. Too little sidetone can make
speakers think their telephone is dead, but too
much leads them to lower their voices.
Anti-sidetone circuit is normally incorporated
into the telephone circuitry to control the level
of sidetone. ITU-T recommends sidetone reference
equivalents of at least 17 dB.
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The Maximum Data Rate of a Channel

• The maximum data rate of a noiseless channel with
  a bandwidth of H and V number of levels is
• 2H log2(V) bits per second (bps)
• due to H. Nyquist in 1924
• hence if V 2 (binary encoding) and channel
  bandwidth 3000 Hz then maximum data rate is
  23000 6000 bps

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Transmitting Signals

• Major problems
• Attenuation
• weakening of signal as it propagates forward
• depends on frequency of signal
• Noise
• unwanted energy or signals from sources other
  than the transmitter

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Data transfer in the presence of noise

• Shannons Law
• C B log2 (1 S/N) where
• C achievable channel capacity
• B Bandwidth of line (in Hz)
• S Average signal power
• N Average Noise power
• S/N Signal to Noise Ratio
• this is usually measured in decibels (dB)
• where dB 10 log10 (S/N)

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Decibels (dB)
YdB 10 log (x) where x is in power units
(Watts) YdB 10 log (10) 10 dB YdB 10 log
(100) 20 dB YdB 10 log (20) 13 dB
YdB 10 log (v2) for voltage across say 1W
resistor YdB 20 log (v)
Decibels represent 1. Level (absolute or
relative) 2. Ratio (gain or loss)
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Decibels (Level)
The RF output power level of a transmitter is
specified in dBm (microwave radio) or dBw (mobile
radio).
Example Microwave radio transmitter 30 dBm (1
W) Mobile radio base station transmitter 20 dBw
(100 W)
dBw 10 log (P/1W) dBm 10 log (P/1mW)
1 Watt 0 dBw 1 m Watt 0 dBm
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Examples of dBw and dBm
1 W 2 W 4 W 8 W 16 W
0 dBw 3 dBw 6 dBw 9 dBw 12 dBw
30 dBm 33 dBm 36 dBm 39 dBm 42 dBm
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Decibels (Ratio)
Pi Vi Ii
Po Vo Io
System
dB 10 log (Po/Pi) power ratio dB 20 log
(Vo/Vi) voltage ratio dB 20 log
(Io/Ii) current ratio
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Examples
dBi
antenna gain with reference to isotropic antenna
(unity gain)
dBd
antenna gain with reference to dipole antenna
(2.2 dBi)
As an example, a 0.6 m parabolic antenna (dish)
operating at 18 GHz will have a nominal gain of
38 dBi Doubling the diameter (1.2) adds 6 dB i.e.
44 dBi
A 3.0 m parabolic antenna (dish) operating at 1.8
GHz will have a nominal gain of 32 dBi
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Noise Factor / Figure
Linear System
Si/Ni
So/No
Noise Factor (f) (S/N) i / (S/N) o
Noise Figure (NF) 10 log ( f )
Note Signal and Noise are in Power Unit
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Return Loss
Relates to the degree of matching the source to
the load
Reflection Coefficient
Return Loss in dB
RLdB 20 log (1/r)
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dBr
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2-Wireand 4-Wire
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4-Wire Circuit
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Echo and Singing
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Echo Path
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Singing

• Echoes part of the signal return from the other
  direction
• talker echo and listener echo.
• Attenuation (2W-2W) L2 6 G4 dB
• G4net gain of one side of 4W circuit Total
  Ampl. Gain-Total loss
• Transhybrid loss (loss in hybrid transformer)
  6B dB B (balance-return loss due to impedance
  mismatch) 20 log10 (NZ)/(N-Z) Z impedance
  of two wire N impedance of balance network
• Attenuation of talkers echo LT 2L2B dB
• Echo delay time DT 2T4
• Attenuation of listeners echo Ll 2L22B dB
• Singing path Ls 2(B 6 G4) 2(B L2) dB
  (singing point Ls 0)
• For stability Ls gt 0, (B L2) gt 0, G2 lt B where
  G2 - L2
• Stability margin M B L2
• Echo cancellers to increase the echo attenuation.

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Telephone Channel Capacity
Voice Channel 0-4,000 Hz
Voice Bandwidth 300-3,300 Hz
Voltage
Guard Band
Guard Band
0
300
3,300
4,000
Frequency (Hertz)
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CO Connectivity
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POTS Connectivity

• Small Cities have a COBig Cities have COs
• Hierarchical system, add
• High Usage Direct Lines between COs
• Tandem (Trunk-to-Trunk) Switches
• Minimum of two physically separate routes out of
  all switches desired
• Best compromise of cost reliability

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POTS

• Items in a typical phonemicrophone
  speakerhybriddialing circuitry (DTMF)on/off
  hook switchring circuitry
• Items in a typical COcrosspoint
  switchhybridsA/D D/A convertersecho
  cancelersTDM

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Home Phone
Speaker
Ring Circuitry
On Hook
Hybrid
Off Hook
Dialing Circuitry
Wall Socket
Microphone
4 Wire
2 Wire
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Home Phone
Speaker
Inbound Audio
Ring Circuitry
On Hook
Hybrid
Off Hook
Dialing Circuitry
Wall Socket
Microphone
4 Wire
2 Wire
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Home Phone
Speaker
Sidetone
Ring Circuitry
On Hook
Hybrid
Outbound Audio
Off Hook
Dialing Circuitry
Wall Socket
Microphone
4 Wire
2 Wire
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One Wire
Speaker
Microphone
Earth Ground

• To get audio out of speaker, need a voltage drop
  across the speaker inputs
• Need two 'wires' to get a voltage drop across a
  speaker
• one wire can be an actual wire
• second 'wire' can be the earth
• Very Susceptible to static

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Two Wires
Speaker
Microphone

• Resistant to static
• Susceptible to interference over long distances
• Twisting the wires slashes interference

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Two Wires
Speaker
Hybrid
Hybrid

• Hybrids allow Telco Two Wire lines to carry both
  outbound and inbound traffic
• short distances

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Four Wires
Speaker
Microphone
Amp
Amp
Speaker
Microphone

• Easier to amplify traffic moving one direction
• Telco Four Wire lines
• 2, one-way, 2 wire connections
• Long distance

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POTS Connectivity (1920)
Copper Local Loop
Copper Local Loop
Copper Long Haul
CO
CO
Phone
Phone
4 Wire
4 Wire
2 Wire
2 Wire
4 Wire
Analog
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POTS Connectivity (1970)
Copper Local Loop
Copper Local Loop
Copper Long Haul
CO
CO
Phone
Phone
4 Wire
4 Wire
2 Wire
2 Wire
4 Wire
Digital TDM 64 Kbps
Analog
Analog
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POTS Connectivity (1990)
Copper Local Loop
Copper Local Loop
Fiber Optic Trunk
CO
CO
Phone
Phone
4 Wire
4 Wire
2 Wire
2 Wire
4 Wire
Digital TDM 64 Kbps
Analog
Analog
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Simplified Central Office Switch
Space Switch
TDM deMux
D/A
Local Loops
Echo Canceler
Hybrid
TDM Mux
A/D

T1 Line
2 Wire
4 Wire
Analog
Digital
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Simplified CO-to-CO connectivity
Space Switch
TDM deMux
D/A
Local Loops
Echo Canceler
Hybrid
TDM Mux
A/D

Space Switch
TDM deMux
D/A
Local Loops
Echo Canceler
Hybrid
TDM Mux
A/D

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The phone system...

• Parts are 4 wire (headset and long haul)
• 4 wire two unidirectional signals
• unidirectional signals make amplification a lot
  easier
• Parts are 2 wire (local loop)
• 2 wire one bi-directional signal
• Turn-of-the-century decision to save and go 2
  wire on local loops
• Parts are analog (phone local loop)
• About 80 of U.S. Local Loops are copper
  all-the-way
• Parts are digital (long haul, many CO switches,
  some local loops)
• About 20 of U.S. Local Loops use ISDN or Digital
  Loop Carriers

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The phone system...

• 4 Wire to 2 Wire Conversion at Central Office
  Hybrids can cause some problems
• Singing (Cure Attenuation)
• Echoes (Cure Echo Canceler)
• Analog to Digital Conversion points also cause
  some problems
• CO Switch filters on analog voice lines,
  necessary to limit noise and interference on
  voice circuits, limit modem data speeds to about
  33 Kbps
• Trend is to an all-digital system
• U.S. long haul POTS voice circuits use digital
  Time Division Multiplexing

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PC Modems POTS

• Band Pass Filter suppresses energy outside voice
  bandwidth (500 - 3,500 Hz)

A/D Converter
Twisted Pair Cable
Band Pass Filter (.5 - 3.5 KHz)
Sampler Fs 8 KHz
Quantize 256 levels
Code 8 bits/sample
64 Kbps
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PC Modems POTS

• PC Bit Stream has a significant amount of energy
  below 0.5 KHz
• Modems shift the energy into the pass band of the
  filter

A/D Converter
Twisted Pair Cable
PC
Band Pass Filter (.5 - 3.5 KHz)
Sampler Fs 8 KHz
Quantize 256 levels
Code 8 bits/sample
64 Kbps