Data Transmission

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Data Transmission
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Terminology

• Data transmission occurs between a transmitter
  and a receiver.
• The media may be guided or unguided
• guided twisted pair, coaxial cable, and fiber.
• unguided trough air, water, or vacuum.
• Either type of transmission is based on
  electromagnetic waves.
• A direct link is the signal transmission path
  between two devices with no intermediate device
  other than repeaters and amplifiers.

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• A guided medium is point-to-point if
• it provides a direct link between two devices
• the medium is shared by only those two devices
• In a multi-point configuration, more than two
  devices share the transmission medium.
• We distinguish 3 forms of transmision
• Simplex
• Half Duplex
• Full Duplex

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• Simplex Transmission in only one direction one
  station is the transmitter, the other the
  receiver. Examples
• One-Way Street
• Keyboard-Computer connection
• Computer-Monitor connection
• TV Broadcast
• Can you think of other simplex examples?

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• Half Duplex Transmission in both directions
  possible, but NOT at the same time. Here, the
  attached stations are both, sender and receiver.
  Examples
• One-Lane Road with access control lights. While
  cars go in one directions, cars going the
  opposite way must wait.
• Walkie-Talkies
• CB-Radios
• Traditional Ethernet (Coax or 10baseT)

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• Full Duplex Transmission in both directions
  simultaneously. Both stations can send and
  receive at the same time. Examples
• Regular 2-way street
• Full-Duplex repeated Ethernet (Gbit Ethernet)
• Full Duplex transmission can be accomplished in
  two ways
• Separated physical transmission media
• Divided channel capacity and separation of
  signals in different directions.

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Signals etc.

• Signals can be expressed in two ways
• in the Time-Domain, the signal intensity varies
  over time i.e., as a function of time, f(t)
• in the Frequency-Domain, the signal is expressed
  as a function of the constituent frequencies,
  the set of sinusoid signals which make up the
  signal.
• We need to distinguish between 2 types of
  signals
• Continuous
• Discrete

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• A continuous signal is one in which the signal
  intensity varies in a smooth fashion over time.
  There are no breaks (poles) or discontinuities.
• A discrete signal is one in which the signal
  intensity maintains a constant level for some
  period of time and then changes to another
  constant level.
• Note A discrete signal may consist of more than
  just 2 constant levels i.e., discrete does not
  mean binary!

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• The simplest sort of signal is a periodic signal.
• Here, T is said to be the period. T is the
  smallest value that satisfies the equation.

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• The sine wave is the fundamental continuous
  signal. We can represent the sine wave by 3
  parameters
• Amplitude (A)
• Frequency (f)
• Phase (?)

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• Amplitude (A) is the peak value or strength of
  the signal over time. (in Volts, Watts, etc.)
• Frequency (f) is the rate (in cycles per second,
  or Hertz (Hz)) at which the signal repeats.
• The period T can be computed as T1/f. T is the
  amount of time taken for one repetition.
• Phase (?) is the measure of the relative
  position in time within a single period of the
  signal.

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• The Wavelength (?) of a signal is the distance
  occupied by a single cycle (or period). In other
  words, it is the distance between to points of
  corresponding phase of two consecutive cycles.
• Here, v represents the velocity of the signal.

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• The Frequency-Domain Concept allows us to
  represent a signal as the sum of constituent
  frequencies. For example
• The components of s(t) are sine waves of
  frequencies f1 and 3f1.

s(t) sin(2?f1t) 1/3 sin(2?(3f1)t)
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• When all of the frequency components are integer
  multiples of one frequency f1, f1 is called the
  fundamental frequency.
• The period of the total signal is equal to the
  period of the fundamental frequency.
• The spectrum of a signal is the range of
  frequencies that it contains. In our example, the
  spectrum extends from f1 to 3 f1.

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Bandwidth

• The absolute bandwidth is the width of the
  spectrum. In our example the bandwidth is 3f1-
  f1 2f1.
• Note that most of the energy in the signal is
  contained in a relative narrow band of
  frequencies. This is referred to as the effective
  bandwidth.

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Bandwidth and Data Rate

• Consider the following signal
• The corresponding Bandwidth is 4MHz.
• The period of the fundamental frequency f1 is
  computed as T1/ f1 1?sec.
• Assuming that we transmit a bit stream of 1s and
  0s, the width of a bit is 0.5 ?sec.

s(t)sin((2??106)t)1/3 sin((2??3?106)t) 1/5
sin((2??5?106)t)
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• What is the corresponding Data Rate?
• 1 bit every 0.5 ?sec gt 2Mbps
• What happens if we use the same type of signal
  but double the bandwidth?
• (5?2 ?106) - (2 ?106) 8MHz
• As a result, one bit occurs every 0.25?sec for a
  data rate of 4Mbps.

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• All other things being equal, by doubling the
  bandwidth, we double the potential data rate!
• But..
• Consider the case where it suffices to
  approximate the square wave with the following
  signal composition
• Here, f12Mhz. What is the potential data rate?

s(t)sin((2??2?106)t)1/3 sin((2??3?2?106)t)
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• Clearly, the bandwidth is
• (3?2 ?106) - (2 ?106) 4MHz
• The period of the signal is 1/f1 0.5?sec
• Hence the width of each bit is 0.25?sec which
  results in a potential data rate of 4Mbps.
• Note A given bandwidth can support a variety of
  data rates.

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• Notes
• Any digital wave form will have infinite
  bandwidth.
• The nature of the medium will limit the bandwidth
  that can be transmitted.
• For any given medium - the greater the bandwidth
  transmitted, the greater the cost.

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Data, Signals, and Transmission

• We distinguish between digital and analog data.
• Analog ? Continuous
• Digital ? Discrete
• Note Analog ? indiscrete! -)
• Examples of digital data
• Text, Integers, Floating Point Numbers, etc.
• Examples of analog data
• voice, video signal, light intensity, etc.

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• Data are propagated in a communication system my
  means of electromagnetic signals.
• An analog signal is a continuously varying
  electromagnetic wave.
• A digital signal is a sequence of voltage pulses
  transmitted over a medium.
• a constant positive voltage level indicating a
  binary 1.
• Absence of a signal or a negative voltage level
  for 0.

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• We need to consider the following cases
• Analog data transmitted via analog signals
• Digital data transmitted via analog signals
• Analog data transmitted via digital signals
• Digital data transmitted via digital signals
• As long as the signal uses the same domain as the
  data, little conversion is necessary.
• Otherwise, we need to use modems and codec
  devices.

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• Last but not least, we need to consider the
  transmission medium over which the signals are
  transmitted.
• Analog transmission is a means of transmitting
  analog signals without regard to their contents!
• The signals ,may represent analog data (e.g.,
  voice) or digital data (e.g., binary data
  modulated via a modem).
• Analog signals suffer attenuation and may have to
  be amplified. Noise needs to be considered.

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• Digital transmission is concerned with the
  content of the signal. I.e., we need to be able
  to maintain the signals that represent the 1s and
  0s.
• Digital signals can only be transmitted over
  limited distance before attenuation jeopardizes
  the interpretation of the signal content.
• Repeaters are used to achieve greater distances.
• A repeater repeats rather than amplifies the
  signal. It receives and recovers the original
  signal and generates a new, clean signal. Here,
  noise is NOT repeated (not cumulative).

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Transmission Impairments

• In general, we must expect the received signal to
  be different from the transmitted signal. WHY?
• Answer The signal may be altered through a
  variety of impairments such as
• Attenuation and attenuation distortion
• Delay distortion
• Noise

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Attenuation

• The strength of a signal decreases with distance.
• The reduction in strength is generally
  logarithmic and expressed in decibel (dB).
• Decibel is used to express the difference of two
  quantities (usually signal strength) in
  logarithmic form. For 2 signals, P1 and P2 we can
  compute
• NdB 10log10(P1 / P2)

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Problems to solve

• A signal must have sufficient strength to be
  detectable by the receiver.
• The signal must maintain a level that is
  sufficiently higher than noise to be received
  without error.
• Attenuation is an increasing function with
  frequency.

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• The problems related to signal strength can be
  solved through the use of amplifiers or
  repeaters.
• The problem caused by non-uniform attenuation
  across the frequency spectrum can cause signal
  distortion.
• Techniques for equalizing the attenuation across
  the spectrum can be used.

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Delay Distortion

• Delay distortion is due to the fact that the
  velocity of propagation through a guided medium
  is frequency dependent.
• Remember that a signal may have several
  constituent frequencies.
• The various frequency components will arrive at
  the receiver at different times.
• Equalizer and bandwidth limitations are possible
  solutions.

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Noise

• In addition to the transmitted signal, other,
  unwanted signals may find their way into the
  transmission medium.
• These noise signals are then super-imposed on the
  data signal to distort the original signal.
• We need to provide error detection and error
  correction mechanisms to deal with bit-errors
  introduced by noise.

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Channel Capacity and Data Rate

• Nyquists Law
• If the signals to be considered are binary (two
  voltage levels), then the data rate that can be
  supported by W Hz is 2W bps.

Given a bandwidth of W, the highest signal rate
that can be carried is 2W.
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• If multilevel signaling is used, Nyquists Law is
  expressed as
• Where M is the number of discrete signal or
  voltage levels.

C 2W log2M
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Shannons Law

• Shannons Theorem provides an upper bound on the
  capacity of a link, in terms of bits per second,
  as a function of the signal-to-noise ration of
  the link, measured in decibels (dB)
• Question What data can we expect to achieve over
  a regular telephone line?
• Assume a phone-line which can support voice
  frequencies in the range of 300-3300 Hz.
• What is the Bandwidth B of this line?

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• B 3300 Hz - 300 Hz 3000 Hz
• Shannons Formula
• Here, C is the achievable channel capacity
• S/N is the signal-to-noise ratio

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• (S/N) is generally expressed in decibels (dB)
• If we assume a typical signal-to-noise ratio of
  30dB, what would be the value of (S/N)?

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• 30dB gt (S/N)1000
• Thus, we can compute the achievable channel
  capacity as
• This is roughly the limit of a 28.8-Kbps modem
• For higher data rates we need compression or
  telephone lines of higher quality.

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Data Encoding

• As seen earlier, we need to consider the
  following cases
• Digital Data / Digital Signal
• Analog Data / Digital Signal
• Digital Data / Analog Signal
• Analog Data / Analog Signal
• As long as we stay in the same domain,
  transmission is less complex and less expensive.

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Digital Data / Digital Signal

• The main issue here is how binary data is
  encoded. I.e., how can we represent the 1s an
  0s that make up the data as signals?
• Many encoding schemes have been developed we
  various advantages and disadvantages.
• We will be looking at only a small subset.

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Encoding

• Problem Encode the binary data that the source
  node wants to send to the destination node into
  the signal that propagates over to the
  destination node.
• How would you encode the 1s and 0s?

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Non-Return to Zero (NRZ)
0 0 1 0 1 1 1 1 0 1 0
0 0 0 1 0
Bits
NRZ

• 1s are encoded as high signal
• 0s are encoded as low signal

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• Problems to consider if a large number of
  consecutive 1s or 0s are transmitted
• Low signal (0) may be interpreted as no signal
• High signal (1) leads to baseline wander
• Unable to recover clock
• So, what can we do about it ? Discuss!

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NRZI and Manchester

• Non-return to Zero Inverted (NRZI) Make a
  transition from the current signal to encode a
  one, and stay at the current signal to encode a
  zero solves the problem of consecutive ones.
• Manchester Transmits the XOR of the NRZ encoded
  data and the clock only 50 efficient.

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0 0 1 0 1 1 1 1 0 1 0
0 0 0 1 0
Bits
NRZ
Clock
Manchaster
NRZI
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4B/5B

• Idea Every 4 bits of data is encoded in a 5-bit
  code,
• with the 5-bit codes selected to have no more
  than
• one leading 0 and no more than two trailing 0
  (i.e.,
• never get more than three consecutive 0s).
  Resulting
• 5-bit codes are then transmitted using the NRZI
• encoding. Achieves 80 efficiency.

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Digital Data - Analog Signals

• Some transmission systems can only transmit
  analog signals
• The telephone network (traditional)
• Fiber optic networks
• In order to transmit digital data we need to
  modulate the digital values onto the analog
  signal.
• In this course we consider 3 basic modulation
  techniques.

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• Amplitude-shift keying (ASK)
• Frequency-shift keying (FSK)
• Phase-shift keying (PSK)
• In all three cases, the resulting signal occupies
  a bandwidth centered on the carrier frequency.

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ASK

• Binary values 1 and 0 are represented by
  different signal amplitudes
• s(t)
• Here, A?cos(2?fct) is the carrier signal.
• ASK is used to transmit digital data over optical
  fiber.

A?cos(2?fct) binary 1 0 binary 0
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FSK

• In FSK, the two binary values are represented by
  two different frequencies near the carrier
  frequency
• s(t)
• f1 and f2 are typically offset from the carrier
  frequency fc by equal but opposite amounts.

A?cos(2?f1t) binary 1 A?cos(2?f2t) binary 0
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PSK

• In PSK, the phase of the carrier signal is
  shifted to represent data.
• s(t)
• with a phase shift of ? ? 180o, the
  modulation above is a two phase modulation.

A?cos(2?fct ?) binary 1 A?cos(2?fct) binary 0
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• A 4-phase modulation, known as quatrature
  phase-shift keying uses phase ? shifts of
  multiples of 90o.
• Thus, each signal element represents two bits
  rather than just one.
• s(t)

A?cos(2?fct 45o) 11 A?cos(2?fct 135o)
10 A?cos(2?fct 225o) 00 A?cos(2?fct 315o) 10
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• This scheme can be extended.
• With eight phase angles we can encode 3 values
• Further, each angle can have more than one
  amplitude!
• A standard 9600 bps modem uses 12 phases, four of
  which have two amplitude values.
• Other schemes are certainly possible.
• We stop here, before its getting too messy.

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Analog Data - Digital Signals

• In order to transmit analog data through digital
  signals, we first translate the analog data into
  digital form (digital data).
• The digital data can then be transmitted using
  any one of the mechanisms described before.
• The main issue to be considered in this section
  is HOW can we translate analog to digital data?

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PCM

• Pulse Code Modulation (PCM) is based on the
  sampling theorem, which states

If a signal f(t) is sampled at regular intervals
of time at a rate higher than twice the highest
significant signal frequency, then the samples
contain all the information of the original
signal. f(t) can be reconstructed from these
samples!
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Analog Data - Analog Signals

• If analog data is transmitted in their original
  spectrum (i.e., the signal of the data itself),
  we refer to the transmission as baseband
  transmission.
• One example of baseband transmission is the
  transmission of voice data over a telephone line.
• Other forms of transmission require the data
  signal to be modulated onto a transmission
  signal.
• As before, we can use the three basic
  characteristics of a signal for modulation!
• What are they?

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• .Amplitude, Frequency, and Phase
• The principal techniques are
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)
• See Figures 4.18, 4.20
• Read section 4.4 in textbook.