Review: The application layer.

1

• Review The application layer.
• Network Applications see the network as the
  abstract provided by the transport layer
• Logical full mesh among network end-points
• Reliable and unreliable communication among
  end-points
• Application layer protocol issues closely related
  to the particular application.
• Some example applications and their protocols
• Email system SMTP, POP3
• Web HTTP
• DNS

2

• The physical layer.
• Physical layer issues how to transfer bits
  correctly.
• How to physically connect computers (what kind of
  connectors should we use?)
• How to represent 0s and 1s? Timing?
• Components
• transmitter
• transmission medium
• receiver
• Example Telephone network.
• transmitter converts sound waves into vibrating
  currents gt electromagnetic waves down to the
  wire.
• receiver convert vibrating currents to voice.
• Telephone network analog transmission of analog
  signal

3

• Transmission medium
• Important parameters for the transmission medium
  are the capacity and the distance.
• Capacity depends on distance.
• 
  Capacity Distance
• --------------------------------------------------
  ---------------------------
• unshield twisted pair
  4000Hz lt 10 km
• 
  (10Mbps) (20m??)
• coaxial cable
• baseband(50-ohm ThinNet) 10-100Mbps
  200m (10Base2) broadband (75-ohm ThickNet)
  10-100Mbps 500m
• optical fiber
• multi-mode
  100 Mbps 30km
• single-mode
  10Gbps 30km

4

• Bandwidth and Capacity
• Bandwidth width of the frequency range of signal
  or transmission (Hz) e.g. human voice 100 3300
  Hz, bandwidth 3200, twisted pair 4kHz.
• Capacity rate in bits per second
• Baud rate how many symbols per second
• Bit rate number of bits / symbol Baud rate
• How to determine the number of bits per symbol?
• Number of bits/symbol log_2(number of symbols)
• E.g eight voltage outputs, how many bits per
  symbol?

5

• Bandwidth and Capacity
• Nyquist's theorem maximum baud rate for
  noiseless channel.
• max baud rate 2 Bandwidth
• Implication
• (1) max bit rate 2 Bandwidth of bits
  /symbol
• (2) also applies to the noisy channels.
• Example A 10kHz bandwidth channel is used to
  send binary signals, what is the maximum bit rate?

6

• Shannon's theorem maximum bit rate for noisy
  channel.
• C Bandwidth log_2 (1 S/N)
• (S strength of signal, N strength of noise)
• S/N is given in the units of decibel(dB),
  10log_10(S/N)
• signal_to_noise ratio 20 dB, S/N ?
• The typical local loop telephone line
  S/N1000, Bandwidth 4000 Hz, C ?
• Based on Nyquist and Shannon theorems, what is
  the key for the transmission media to achieve
  high data rate?

7

• The electromagnetic spectrum

104
105
106
108
107
109
1011
1010
1012
1013
1015
1014
F(Hz)
satellite
Twisted pair
Fiber optics
coax
FM
AM
TV
Conclusion?
8

• Analog vs. Digital
• analog continuous, digital discrete
• different contexts
• 
  Analog Digital
  ----------------------------------------
  ------------------------------------Data
  something that has a meaning voice
  text -------------------------
  --------------------------------------------------
  - Signal encoded data
  continuously sequence
• 
  varying of pulses
• 
  electromagnetic (1's, 0's)
• 
  wave
  ------------------------------------------
  ------------------------------------
  Transmission how data transmitted propagate
  propagate
• 
  waves pulses
• ---------------------------------------------
  ---------------------------------
• Computer networks transmit digital data
• Telephone networks transmit voice

9

• Data Encoding map data into signals
• Digital data to digital signal
• NRZ high 1, low 0
• Manchester 1 low-high transition, 0 high-low
  transition
• Digital data to analog signals (example modem)
• Square wave (digital signal) suffers from strong
  attenuation and delay distortion.
• modulation -- make analog signals.
• Amplitude modulation use two different voltage
  levels to represent 0 and 1.
• Frequency modulation use two different tones to
  represent 0 and 1.
• Phase modulation carrier wave is shifted at
  different intervals to represent 0 and 1.

10

• high speed modem
• Bandwidth in the local loop 3000HZ,
• maximum baud rate ???,
• how to achieve higher speed (56Kbps modem)?
• many bits per baud,
• a combination of modulation techniques.
• more amplitude levels and more phase intervals --
  QAM (quadrature Amplitue Modulation)
• Using 2400 baud rate, how many symbols are needed
  to achieve 56kbps?

11

• Analog data to digital signals (example digital
  voice)
• 300 - 3400 HZ human voice
• PCM 3000 HZ with protection 4000 HZ Samples
  Nyquist Theorem 8000 samples per second
• Digitization 8 or 7 bits per sample
  (logarithmically spaced) 64 kbps or 56 kbps
• methods to reduce the number of bits per sample
• differential pulse code
• delta modulation
• predictive encoding
• Analog data to analog signals
• radio, TV, telephone

12

• Simplex and duplex communication
• simplex communication data travel in one
  direction
• half-duplex communication data travel in either
  direction, but not simultaneously
• full-duplex communication data travel in both
  direction simultaneously.
• Multiplexing
• combines slow channels into faster channels.
• two schemes
• Time Division Multiplexing time domain is
  divided into slots, put channels in different
  time domain.
• Frequency division Multiplexing frequency
  spectrum is divided into logical channels.
• Code division multiplexing.

13

• In the telephone system
• basic voice channel 64kbps
• T1 line 24 basic channels 1 bits per 248 bits
  
• 193 bits per 125 us 1938000 1.544Mbps
• T2 line 4 T1 line (96 basic channels) extra
  bits for framing 6.176 Mbps, actually 6.312Mbps
• T3 line 7 T2 line (796 basic channels) extra
  bits for framing 44.184Mbps, actually 44.736Mbps
• T4 line 6 T3 line .......
• OC-1 51.84Mbps
• OC-3 3 OC-1 155.52
• OC-9 .....