Transmission Problems

1
Transmission Problems Encoding
2
Transmission Problems Encoding

• This causes the received signal to differ from
  original, transmitted signal.
• Analog data quality degradation
• Digital data bit errors.
• Types of impairments include
• Attenuation.
• Delay distortion.
• Noise.

3
Attenuation

• Weakening of the signals power as it propagates
  through medium.
• Function of medium type
• Guided medium logarithmic with distance.
• Unguided medium more complex (function of
  distance and atmospheric conditions).
• Problems and solutions
• Insufficient signal strength for receiver to
  interpret it use amplifiers/repeaters to
  boost/regenerate signal.
• Error due to noise interference (level is not
  high enough to be distinguished from noise) use
  amplifiers/repeaters.
• Attenuation increases with frequency special
  amplifiers to amplify high-frequencies.

4
Delay Distortion

• Speed of propagation in guided media varies with
  frequency.
• Different frequency components arrive at receiver
  at different times.
• Solution equalization techniques to equalize
  distortion for different frequencies.
• Noise
• Noise undesired signals inserted anywhere in the
  source/destination path.
• Different categories thermal (white), crosstalk,
  impulse, etc.

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Decibel Signal-to-Noise Ratio

• Decibel (dB) measures relative strength of 2
  signals.
• Example S1 and S2 with powers P1 and P2.
• NdB 10 log10 (P1/P2)
• Signal-to-noise ratio (S/N)
• Measures signal quality.
• S/NdB 10 log10 (signal power/noise power)

6
Channel Capacity

• Rate at which data can be transmitted over
  communication channel.
• Noise-free channel Nyquist Theorem
• Limitation of data rate is signals bandwidth.
• Given channel bandwidth W, highest signal rate is
  2W.
• From receivers point of view sampling at rate
  2W can reconstruct signal.
• Using data rate,
• C 2W log2V, where V is number voltage levels.
• Same bandwidth, increasing number of signal
  levels, increases data rate, but more complex
  signal recognition at receiver and more
  noise-prone.
• This is a theoretical upper bound, since channels
  are noisy.

7
Transmission Media

• Physically connect transmitter and receiver
  carrying signals in the form electromagnetic
  waves.
• Types of media
• Guided waves guided along solid medium such as
  copper twisted pair, coaxial cable, optical
  fiber.
• Unguided wireless transmission (atmosphere,
  outer space).

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Guided Media Examples

• Twisted Pair
• 2 insulated copper wires arranged in regular
  spiral. Typically, several of these pairs are
  bundled into a cable.
• Cheapest and most widely used limited in
  distance, bandwidth, and data rate.
• Applications telephone system (home-local
  exchange connection).
• Unshielded and shielded twisted pair.
• Coaxial Cable
• Hollow outer cylinder conductor surrounding inner
  wire conductor dielectric (non-conducting)
  material in the middle.
• Applications cable TV, long-distance telephone
  system, LANs.
• s Higher data rates and frequencies, better
  interference and crosstalk immunity.
• -s Attenuation and thermal noise.

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Examples continued.

• Optical Fiber
• Thin, flexible cable that conducts optical waves.
• Applications long-distance telecommunications,
  LANs.
• s greater capacity, smaller and lighter, lower
  attenuation, better isolation,

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Unguided, Wireless Media

• Microwave directional, LOS transmission.
• Satellite directional, LOS, large delay, high
  bandwidth.
• Radio omnidirectional (broadcast), single hop
  (cellular), multi-hop (ad hoc nets).
• Infrared directional, LOS transmission, cannot
  penetrate obstacles and used outdoors.

11
Data Encoding

• Transforming original signal just before
  transmission.
• Both analog and digital data can be encoded into
  either analog or digital signals.

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Digital/Analog Encoding
Encoding
g(t)
g(t)
Digital Medium
(D/A)
Encoder
Decoder
Source
Destination
Source System
Destination System
Modulation
g(t)
g(t)
Analog Medium
(D/A)
Modulator
Demodulator
Source
Destination
Source System
Destination System
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Encoding Considerations

• Digital signaling can use modern digital
  transmission infrastructure.
• Some media like fiber and unguided media only
  carry analog signals.
• Analog-to-analog conversion used to shift signal
  to use another portion of spectrum for better
  channel utilization (frequency division muxing).

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Digital Transmission Terminology

• Data element bit.
• Signaling element encoding of data element for
  transmission.
• Data rate rate in bps at which data is
  transmitted for data rate of R, bit duration
  (time to emit 1 bit) is 1/R sec.
• Modulation rate baud rate (rate at which signal
  levels change).

15
Digital Transmission Receiver-Side Issues

• Clocking determining the beginning and end of
  each bit.
• Transmitting long sequences of 0s or 1s can
  cause synchronization problems.
• Signal level determining whether the signal
  represents the high (logic 1) or low (logic 0)
  levels.
• S/N ratio is a factor.

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Digital-to-Analog Encoding

• Transmission of digital data using analog
  signaling.
• Example data transmission of a PTN.
• PTN voice signals ranging from 300Hz to 3400 Hz.
• Modems convert digital data to analog signals
  and back.
• Techniques ASK, FSK, and PSK.

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Amplitude Frequency Shift Keying

• 2 binary values represented by 2 amplitudes.
• Typically, 0 represented by absence of carrier
  and 1 by presence of carrier.
• Prone to errors caused by amplitude changes.
• 2 binary values represented by 2 frequencies.
• Frequency Shift Keying
• Frequencies f1 and f2 are offset from carrier
  frequency by same amount in opposite directions.
• Less error prone than ASK.

18
Phase-Shift Keying

• Phase of carrier is shifted to represent data.
• Phase shift of 90o can represent more bits aka,
  quadrature PSK.

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Analog-to-Digital Encoding

• Analog data transmitted as digital signal, or
  digitization.
• Codec device used to encode and decode analog
  data into digital signal, and back.
• 2 main techniques
• Pulse code modulation (PCM).
• Delta modulation (DM).

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Pulse Code Modulation

• Based on Nyquist (or sampling) theorem if f(t)
  sampled at rate gt 2signals highest frequency,
  then samples contain all the original signals
  information.
• Example if voice data is limited to 4000Hz, 8000
  samples/sec are sufficient to reconstruct
  original signal.
• Analog signal -gt PAM -gt PCM.
• PAM pulse amplitude modulation samples of
  original analog signal.
• PCM quantization of PAM pulses amplitude of PAM
  pulses approximated by n-bit integer each pulse
  carries n bits.

21
Delta Modulation (DM)

• Analog signal approximated by staircase function
  moving up or down by 1 quantization level every
  sampling interval.
• Bit stream produced based on derivative of analog
  signal (and not its amplitude) 1 if staircase
  goes up, 0 otherwise.
• Parameters sampling rate and step size.

22
Spread Spectrum

• Used to transmit analog or digital data using
  analog signaling.
• Spread information signal over wider spectrum to
  make jamming and eavesdropping more difficult.
• Popular in wireless communications
• 2 schemes
• Frequency hopping signal broadcast over random
  sequence of frequencies, hoping from one
  frequency to the next rapidly receiver must do
  the same.
• Direct Sequence each bit in original signal
  represented by series of bits in the transmitted
  signal.

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Asynchronous Transmission

• Avoid synchronization problem by including sync
  information explicitly.
• Character consists of a fixed number of bits,
  depending on the code used.
• Synchronization happens for every character
  start (0) and stop (1) bits.
• Line is idle transmits 1.
• Example sending ABC in ASCII
• 0 10000010 1 0 01000010 1 0 110000 1 1111
• Timing requirements are not strict.
• But problems may occur.
• Significant clock drifts high data rate
  reception errors.
• Also, 2 or more bits for synchronization
  overhead!

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Synchronous Xmission

• No start or stop bits.
• Synchronization via
• Separate clock signal provided by transmitter or
  receiver doesnt work well over long distances.
• Embed clocking information in data signal using
  appropriate encoding technique such as Manchester
  or Differential Manchester.
• Need to identify start/end of data block.
• Block starts with preamble (8-bit flag) and may
  end with postamble.
• Other control information may be added for data
  link layer.

8 -bit flag
8 -bit flag
Control
Data
Control
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Data Link Layer

• So far, sending signals over transmission medium.
• Data link layer responsible for error-free
  (reliable) communication between adjacent nodes.
• Functions framing, error control, flow control,
  addressing (in multipoint medium).

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Flow Control

• What is it?
• Ensures that transmitter does not overrun
  receiver limited receiver buffer space.
• Receiver buffers data to process before passing
  it up.
• If no flow control, receiver buffers may fill up
  and data may get dropped.

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Stop-and-Wait

• Simplest form of flow control.
• Transmitter sends frame and waits.
• Receiver receives frame and sends ACK.
• Transmitter gets ACK, sends other frame, and
  waits, until no more frames to send.
• Good when few frames.
• Problem inefficient link utilization.
• In the case of high data rates or long
  propagation delays.

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Sliding Window

• Allows multiple frames to be in transit at the
  same time.
• Receiver allocates buffer space for n frames.
• Transmitter is allowed to send n (window size)
  frames without receiving ACK.
• Frame sequence number labels frames.
• Receiver acks frame by including sequence number
  of next expected frame.
• Cumulative ACK acks multiple frames.
• Example if receiver receives frames 2,3, and 4,
  it sends an ACK with sequence number 5, which
  acks receipt of 2, 3, and 4.

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..Sliding Window

• Sender maintains sequence numbers its allowed to
  send receiver maintains sequence number it can
  receive. These lists are sender and receiver
  windows.
• Sequence numbers are bounded if frame reserves
  k-bit field for sequence numbers, then they can
  range from 0 2k -1 and are modulo 2k.
• Transmission window shrinks each time frame is
  sent, and grows each time an ACK is received.

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Example 3-bit sequence number and window size 7

• A B
• 0 1 2 3 4 5 6 7 0 1 2 3 4... 0 1 2 3 4 5
  6 7 0 1 2 3 4

0
1
2
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
RR3
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
3
0 1 2 3 4 5 6 7 0 1 2 3 4
4
5
0 1 2 3 4 5 6 7 0 1 2 3 4
RR4
6
0 1 2 3 4 5 6 7 0 1 2 3 4
0 1 2 3 4 5 6 7 0 1 2 3 4
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Practical Work
Q. A signals power can weaken as it propagates
through a medium. Describe the solution for (a)
Insufficient signal strength for receiver to
interpret it and (b) Errors due to noise
interference (level is not high enough to be
distinguished from noise).

• Insufficient signal strength for receiver to
  interpret it use amplifiers/repeaters to
  boost/regenerate signal.
• Error due to noise interference (level is not
  high enough to be distinguished from noise) use
  amplifiers/repeaters.