Telecommunications Technology Signal Formats

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Telecommunications TechnologySignal Formats

• Class 5

ICS 620
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Overview

• Signal formats (review)
• Multiplexed Signal Formats
• Noise types
• Signal to Noise Ratio

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Signal Encoding Techniques
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Reasons for Choosing Encoding Techniques

• Digital data, digital signal
• Equipment less complex and expensive than
  digital-to-analog modulation equipment
• Analog data, digital signal
• Permits use of modern digital transmission and
  switching equipment

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Reasons for Choosing Encoding Techniques

• Digital data, analog signal
• Some transmission media will only propagate
  analog signals
• E.g., optical fiber and unguided media
• Analog data, analog signal
• Analog data in electrical form can be transmitted
  easily and cheaply
• Done with voice transmission over voice-grade
  lines

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Signal Encoding Criteria

• What determines how successful a receiver will be
  in interpreting an incoming signal?
• Signal-to-noise ratio
• Data rate
• Bandwidth
• An increase in data rate increases bit error rate
• An increase in SNR decreases bit error rate
• An increase in bandwidth allows an increase in
  data rate

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Factors Used to CompareEncoding Schemes

• Signal spectrum
• With lack of high-frequency components, less
  bandwidth required
• With no dc component, ac coupling via transformer
  possible
• Transfer function of a channel is worse near band
  edges
• Clocking
• Ease of determining beginning and end of each bit
  position

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Factors Used to CompareEncoding Schemes

• Signal interference and noise immunity
• Performance in the presence of noise
• Cost and complexity
• The higher the signal rate to achieve a given
  data rate, the greater the cost

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Basic Encoding Techniques

• Digital data to analog signal
• Amplitude-shift keying (ASK)
• Amplitude difference of carrier frequency
• Frequency-shift keying (FSK)
• Frequency difference near carrier frequency
• Phase-shift keying (PSK)
• Phase of carrier signal shifted

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Basic Encoding Techniques
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Amplitude-Shift Keying

• One binary digit represented by presence of
  carrier, at constant amplitude
• Other binary digit represented by absence of
  carrier
• where the carrier signal is Acos(2pfct)

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Amplitude-Shift Keying

• Susceptible to sudden gain changes
• Inefficient modulation technique for data
• On voice-grade lines, used up to 1200 bps
• Used to transmit digital data over optical fiber

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Binary Frequency-Shift Keying (BFSK)

• Two binary digits represented by two different
  frequencies near the carrier frequency
• where f1 and f2 are offset from carrier frequency
  fc by equal but opposite amounts

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Binary Frequency-Shift Keying (BFSK)-contd

• Less susceptible to error than ASK
• On voice-grade lines, used up to 1200bps
• Used for high-frequency (3 to 30 MHz) radio
  transmission
• Can be used at higher frequencies on LANs that
  use coaxial cable

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Multiple Frequency-Shift Keying (MFSK)

• More than two frequencies are used
• More bandwidth efficient but more susceptible to
  error
• f i f c (2i 1 M)f d
• f c the carrier frequency
• f d the difference frequency
• M number of different signal elements 2 L
• L number of bits per signal element

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Multiple Frequency-Shift Keying (MFSK)-contd

• To match data rate of input bit stream, each
  output signal element is held for
• TsLT seconds
• where T is the bit period (data rate 1/T)
• So, one signal element encodes L bits

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Multiple Frequency-Shift Keying (MFSK)-contd
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Phase-Shift Keying (PSK)

• Two-level PSK (BPSK)
• Uses two phases to represent binary digits

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Phase-Shift Keying (PSK)-contd

• Differential PSK (DPSK)
• Phase shift with reference to previous bit
• Binary 0 signal burst of same phase as previous
  signal burst
• Binary 1 signal burst of opposite phase to
  previous signal burst

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Phase-Shift Keying (PSK)-contd

• Four-level PSK (QPSK)
• Each element represents more than one bit

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Phase-Shift Keying (PSK)-contd

• Multilevel PSK
• Using multiple phase angles with each angle
  having more than one amplitude, multiple signals
  elements can be achieved
• D modulation rate, baud
• R data rate, bps
• M number of different signal elements 2L
• L number of bits per signal element

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Quadrature Amplitude Modulation

• QAM is a combination of ASK and PSK
• Two different signals sent simultaneously on the
  same carrier frequency

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Quadrature Amplitude Modulation
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Decibels

• It is essential to realize that decibels are a
  measure of the RATIO between two quantities.
  Decibels are a measure of how one quantity
  compares to some reference quantity. Decibels are
  NOT an absolute measure.

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Decibels - contd
dB 20 log 100/1 20 log 10 20
(2) 40 dB
IN
OUT
2
1v
100v
IN
OUT
dB 20 log 1/100 20 log 10 20
(-2) -40 dB
-2
100v
1v
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Noise Types

• White Noise
• Static -- Lightning
• Thermal -- Pervasive
• Galactic -- Not a problem except in satellite
• Single Frequency
• Hum -- 60 Hz

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Noise Types (Cont'd)

• Shot noise
• Phenomenon in integrated circuits
• Coherent interference
• Cross-talk -- NeXT, FeXT,Intermodulation
  distortion

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Signal to Noise Ratio

• Ratio of Signal Power wrt Noise
• Measures difference in strengths
• Used to determine minimum allowable signal
• SNR 10 Log (S/N)

Signal Strength
- 25 dBm
SNR 35 dB
- 60 dBm
Noise Strength
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Channel Capacity

• In the presence of noise there is a limit to the
  bit rate that a channel can carry
• Shannon proved
• CSh BW Log2 (PSignal/Pnoise )
• CSh is in bits/sec

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Example of Capacity

• Voice circuit has SNR 26 dB
• What is capacity of this circuit for voice
  bandwidth of 3300 Hz?
• Start with
• CSh BW Log2 (PSignal/Pnoise)

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Example of Capacity
SNR 26 dB
BW 3300 Hz
CSh BW Log2 (PSignal/PNoise CSh 3.3 K Log2 (
26 dB)
)
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Example of Capacity
SNR 26 dB
BW 3300 Hz
CSh BW Log2 (PSignal/PNoise) CSh 3.3 K Log2 (
26 dB)
SNR 10 Log (PSignal/PNoise)
(PSignal/PNoise) 10 SNR/10
CSh 3.3K Log2 (10 2.6)
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Example of Capacity
SNR 26 dB
BW 3300 Hz
CSh BW Log2 (PSignal/PNoise) CSh 3.3 K Log2 (
26 dB)
SNR 10 Log (PSignal/PNoise)
(PSignal/PNoise) 10 SNR/10
CSh 3.3K Log2 (10 2.6)
A handy substitution Log2 (A) ln A ? ln 2
2.6
CSh 3.3 K x ln 10 ln 2 CSh 3.3 K x
5.986 x .6931 28.5 Kbps
CSh 28.5 Kbps
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SONET

• Synchronous Optical NETwork
• SONET is the multiplexing protocol used to in
  fiber optical connections. Its basic
  configuration is based on 51.84 Mb/s (Synchronous
  Transport Signal level one - STS - 1)

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SONET - contd
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STS-1 Frame Structure

• An STS-1 frame is divided into two main
  categories The Transport Overhead and the
  Synchronous Payload Envelope (SPE). The frame is
  made up of 90 columns in 9 rows (810 bytes or
  6480 bits).

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STS-1 Frame Elements
9 rows x 90 bytes (columns) x 8 bits/byte x 8000
frames/s equals 51,840, 000 b/s or 51.840 Mbps
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Virtual Tributaries (VT)
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SONET Line Switched Ring Topology
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Network Design Considerations for SONET
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DWDM
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Summary

• dB
• Signal formats
• Noise types
• SNR (Shannon Model)
• SONET basics