Robert DengXuhua Ding

1
Network Layering Physical Layer
2
Outline

• Network Layering
• Signal and Frequency Domains
• Transmission Impairments
• Signal Transmission (Signal Encoding)
• Channel Capacity
• Error Control Process

3
Organization of Air Travel

• A series of steps

4
Layering of Airline Functionality
ticket
ticket (purchase) baggage (check) gates
(load) runway (takeoff) airplane routing
ticket (complain) baggage (claim gates
(unload) runway (land) airplane routing
baggage
gate
takeoff/landing
airplane routing
arrival airport
intermediate air-traffic control centers
departure airport

• Layers each layer implements a service
• via its own internal-layer actions
• relying on services provided by layer below

5
Why Layering

• Dealing with complex systems
• Explicit structure allows identification of
  relationship of complex systems pieces
• Modularization eases maintenance, updating of
  system
• Change of implementation of layers service
  transparent to rest of system
• e.g., change in gate procedure doesnt affect
  rest of system

6
Internet Protocol Layering Stack

• Application supporting network applications
• SMTP (Simple Mail Transport Protocol)
• Transport host-host data transfer
• TCP (Transport Control Protocol), UDP (User
  Datagram Protocol)
• Network routing of datagrams from source to
  destination
• Internet Protocol, routing protocols
• Link data transfer between neighboring network
  nodes
• PPP, Ethernet
• physical bits on the wire

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Encapsulation
Source
message
application transport network link physical
segment
datagram
frame
Destination
router
application transport network link physical
8
Outline

• Network Layering
• Signal and Frequency Domains
• Transmission Impairments
• Signal Transmission (Signal Encoding)
• Channel Capacity
• Error Control Process

9
What Is Signal?

• Signals electromagnetic representation of data

• Digital signal Series of voltage pulses (square
  wave)

• Analog signal Signal intensity varies in a
  smooth fashion no breaks

10
Periodic Signals Sine Wave

• Frequency is the rate (in cycles per second or
  hertz (Hz)) at which the signal repeats
• Frequency f 1/T, T is the period

11
Period Signals Square wave

• Can be represented by infinite number of Sine
  waves

• f 1/T is the fundamental frequency, all other
  frequency components of a signal are integer
  multiples of the fundamental frequency (applies
  only to periodic signals)

12
Frequency Domain Concepts

• Spectrum of a signal is the range of frequencies
  that it contains
• Bandwidth contained in a relatively narrow band
  of frequencies, where most of signals energy is
  found
• The greater the bandwidth, the higher the
  information-carrying capacity of the signal

13
Outline

• Network Layering
• Signal and Frequency Domains
• Transmission Impairments
• Signal Transmission (Signal Encoding)
• Channel Capacity
• Error Control Process

14
Transmission Impairments

• Channel physical path between and include
  transmitter and receiver
• 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 (Guided Media)

• Attenuation when the signal is transmitted along
  any medium, it will gradually becomes weaker at
  greater distance.
• Delay distortion the velocity of propagation of
  a signal through a cable is different for
  different frequency. A major limitation to the
  data rate for digital data
• Noise unwanted electromagnetic energy inserted
  somewhere between the transmitter and the
  receiver
• Thermal (aka white noise) Due to thermal
  agitation of electrons in a conductor, uniformly
  distributed, cannot be eliminated

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Transmission Impairments (Wireless)

• Free-Space Lose the signal disperses with
  distance
• Atmospheric Absorption water vapor and oxygen
  contributes most to attenuation. Rain and fog
  cause scattering of radio waves, esp. for higher
  frequency signals.
• Multipath due to reflection, multiple copies of
  signal with varying delays can be received

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Outline

• Network Layering
• Signal and Frequency Domains
• Transmission Impairments
• Signal Transmission (Signal Encoding)
• Channel Capacity
• Error Control Process

18
Transmission

• Analog Without regard for data content
  attenuation overcome with amplifiers not
  evaluated or regenerated
• Digital Concerned with data content uses
  repeaters rather than amplifiers evaluated and
  regenerated
• Advantages of Digital Transmission
• Cost cheaper due to VLSI
• Data Integrity no cumulative error
• Capacity Utilization data compression
• Security and Privacy encryption is readily
  applied
• Integration all signals have the same form and
  can be treated similarly.

19
Digital Encoding of Analog Data Codec

• Nyquists sampling theorem
• If a signal is sampled at regular intervals of
  time and at a rate 2B samples/s, where B is the
  bandwidth, the samples contain all the
  information of the signal.
• Pulse-code modulation (PCM)
• Sampling based on Nyquist sampling theorem.
• Assign a value to a sample in a particular range
• Coder analog to digital
• Decoder digital to analog
• Codec Coder - decoder

20
Analog Encoding of Digital Data Modem

• Data encoding and decoding technique to represent
  data using the properties of analog waves
• Modulation the conversion of digital signals to
  analog form (digital signals control amplitude,
  frequency, or phase of a sine wave)
• Demodulation the conversion of analog data
  signals back to digital form
• Modem Modulator-demodulator

21
Analog Encoding of Digital Data ASK

• Amplitude shift keying (ASK)
• binary data are represented by different
  amplitudes of the carrier sine wave
• susceptible to sudden gain changes

• B-ASK

22
Outline

• Network layering
• Signal and Frequency Domains
• Transmission Impairments
• Signal Transmission (Signal Encoding)
• Channel Capacity
• Error Control Process

23
Bit, Baud and Symbol Rate

• Data rate is number of bits transmitted per
  second (bps)
• Baud rate or symbol rate is number of signal
  elements transmitted per second
• In general
• Bit Rate (Baud rate)?( of bits per signal
  element)
• In Quadruple-ASK, each signal elements carries 2
  bits, assuming baud rate 1200 elements per
  second, the data rate 2 ? 1200 2400 bps
• Nyquists sampling rate is 2B (B is bandwidth of
  a signal), then assuming noiseless channels
• Max data Rate 2B?( of bits per signal element)

24
Channel Capacity

• Shannons Theorem
• Max data rate CBlog2(1SNR decimal)
• C is called channel capacity, in bits per second
  in a noisy channel. It is the upper bound
  independent of the number of signaling levels and
  sampling rate
• B bandwidth in Hertz
• SNR signal-to-noise ratio (normally given in dB,
  i.e., SNR dB 10log10(SNR decimal)

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Outline

• Network Layering
• Signal and Frequency Domains
• Transmission Impairments
• Signal Transmission (Signal Encoding)
• Channel Capacity
• Error Control Process

26
Error Control Process

• All transmission media have potential for
  introduction of errors
• Error control can be performed at any layer of a
  communication network, depends on the
  transmission media and application
• Error control process has two components
• Error detection
• Error correction

27
Error Detection Parity Bits

• Bit added to each character to make all bits add
  up to an even number (even parity) or odd number
  (odd parity) of 1s
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit
  character12.5)

28
Error Correction Two-Dimensional Parity

• Arrange a string of bits as a two-dimensional
  array and compute parity over each row and each
  column of array
• Error detection
• Any number of errors in a single row
• Or any number of errors in a single column
• Error Correction
• Any single bit error

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Summary

• Network Layering
• Signal and Frequency Domains
• analog and digital signals, frequency
  representations, signal bandwidth
• Transmission Impairments
• various impairments and noises
• Signal Transmission (Signal Encoding)
• analog to digital (Nyquists sampling theorem,
  codec), digital to digital, digital to analog
  (modem)
• Channel Capacity
• Bit, baud and symbol rates, Shannons Theorem and
  how it differs from Nyquists sampling theorem
• Error Control Process
• Single bit parity, two-dimensional parity

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Reading Assignments

• Chapter 15
• Chapter 16
• Chapter 14 14.2