Lecture 3 - Data Transmission

Data Communication

Definition

• Data communications deals with the transmission of signals in a reliable and efficient manner
• Ultimately, it's about transmitting data (i.e., bits) across some physical transmission medium:
  • Electricity - copper wire, twisted pair, undersea cable
  • Light - infra-red through air, laser through fibre-optic cable
  • Electromagnetic radiation - radio, microwave, satellite

What is Communication?

• The term communication is defined as the act of disseminating information
• Prerequisites for communication:
  • There is information to disseminate (เผยแพร่)
  • The desire or requirement to disseminate exists
  • There is an agency to send/transmit information
  • There is a means of encoding information
  • There is a medium to carry the information
  • There is a recipient to receive the information
  • The recipient is capable of understanding the information received

Communication Properties

Required properties between two entities:

1. Modulation - Conversion of brain waves into speech
2. Signal compatibility - Agreement on vocabulary to use
3. Signal strength - Agreement on volume level for comfortable hearing
4. Data rate - Agreement on talking rate for understanding
5. Protocol - Agreement on rules for when to speak and listen
6. Demodulation - Conversion of audio signals into brain waves

Transmission Terminology

Medium Types

• Guided medium
  • Waves are guided along a physical path
  • Examples: twisted pair, coaxial cable, optical fiber
• Unguided/wireless medium
  • Provide means for transmitting electromagnetic waves but don't guide them
  • Examples: air, water, vacuum

Cable Types and Characteristics

Twisted Pair Cable

• Transmission: Electrical form over metallic conducting wires
• Advantages:
  • Cheapest
  • Easy to install
• Disadvantages:
  • Low noise immunity
  • Low bandwidth
  • Very high attenuation
  • Gets disrupted by external magnetic field

Coaxial Cable

• Transmission: Electrical form over inner conductor
• Characteristics:
  • Higher noise immunity than twisted pair
  • Moderate cost
  • Moderate high bandwidth
  • Low attenuation
  • Easy to install
  • Gets disrupted by external magnetic field

Optic Fiber Cable

• Transmission: Optical form over glass fiber
• Advantages:
  • Highest noise immunity
  • Very high bandwidth
  • Very low attenuation
  • Not affected by external magnetic field
  • Most efficient
• Disadvantages:
  • High cost
  • Difficult to install

Link Types

• Direct link: No intermediate devices
• Point-to-point: Direct link with only 2 devices sharing the link
• Multi-point: More than two devices share the link
  

Data Flow Types

• Simplex: One direction only (e.g., television)
• Half duplex: Either direction, but only one way at a time (e.g., police radio)
• Full duplex: Both directions at the same time (e.g., telephone)

Time and Frequency Domain

Time Domain Concepts

• Analog signal: Varies smoothly over time with no breaks or discontinuity
• Digital signal: Maintains constant level then changes to another constant level
• Periodic signal: Pattern repeated over time
• Aperiodic signal: Pattern not repeated over time

Sine Wave Parameters

• Peak amplitude (A): Maximum strength of signal (volts)
• Frequency (f): Rate of change of signal (Hertz/Hz or cycles per second)
• Period (T): Time for one repetition, so $T=\frac{1}{f}$
• Phase (φ): Measure of relative position in time within a single period

Sine Wave Formula

Wavelength ($\lambda$)

• Distance occupied by one cycle
• Between two points of corresponding phase in consecutive cycles
• Formula: $\lambda=vT=\frac{v}{f}$
• When $v=c$ (speed of light): $f=\frac{c}{\lambda}$
• $c=3\times10^8$ m/s (speed of light in free space)

Frequency Domain Concepts

• Signals are made up of many frequencies
• Components are sine waves
• Fundamental Frequency: Lowest frequency component
• DC Component: Component of zero frequency (direct current)

Spectrum and Bandwidth

Definitions

• Spectrum: Range of frequencies contained in signal
• Absolute bandwidth: Width of spectrum ($f_H-f_L$)
• Effective bandwidth: Narrow band of frequencies containing most energy
  • ถ้าพูดถึง Bandwidth เฉย ๆ ก็จะหมายถึง Effective bandwidth เนี่ยแหละนะ

Bandwidth Characteristics

• For digital devices: Expressed in bits per second (bps) or bytes per second (Bps)

• For analog devices: Expressed in cycles per second or Hertz (Hz)

• Range: From lowest frequency (fL) to highest frequency (fH)

• Half-power point: -3dB point where output power is half of maximum 0dB value

• Bandwidth is the range of frequencies within a given band, in particular that used for transmitting a signal.

• Graphical representations of frequency response curves are called Bode Plots

• The -3dB point is also know as the “half-power” points since the output power at this corner frequencies will be half that of its maximum 0dB value as shown.

Data Rate (Data Transfer Rate)

• Definition: Amount of data being transferred through a connection (medium) per second
• Limitation: Cannot be higher than the bandwidth of the connection
• Measurement: Bits per second (bps), also called bit rate
• Relationship: Higher data rate requires greater effective bandwidth
  • ลองดูว่า relationship ขาดหายอะไรมั้ย
  • The higher the data rate of a signal, the greater is its required effective bandwidth.

Signal Types

Audio Signals

• Frequency range: 20 to 20,000 Hz (human hearing limits)
• Voice channel standard: 300 to 3400 Hz
• Conversion: Easily converted to electromagnetic signals
• Representation: Varying volume to varying voltage

Digital Data

• Generated by computers
• Has two DC components
• Bandwidth depends on data rate

• ใช้ 0.02 msec to transmit one bit, is called ”Bit Duration”
  • สามารถเอาไปคำนวณได้

Digital vs Analog Signals

Analog Signals

Digital Signals

Advantages of Digital Signals

• Generally cheaper than analog signaling
• Easier to implement
• Less susceptible to noise interference
• More preferred choice

Disadvantages of Digital Signals

• Greater attenuation (gradual loss in intensity through medium)

Transmission Impairments

Overview

• Signal received (ตรงปลายสาย - at the end) may differ from signal transmitted
• Effects:
  • Analog: degradation of signal quality
  • Digital: bit errors

Three Main Impairments

1. Attenuation

• Definition: Reduction in signal strength over distance
• Requirements for received signal:
  • Strong enough to be detected
  • Sufficiently higher than noise for error-free reception
• Solutions: Use amplifiers (for analog)/repeaters (for digital)
• Frequency dependency: Increases with frequency
• Equalization: Use loading coils or amplifiers

2. Delay Distortion

• Occurrence: Only in guided media
• Cause: Signal velocity and frequency variations
• Effect: Different frequency components arrive at different times
• Critical for: Digital data (causes inter-symbol interference)

3. Noise

Types of noise:

Thermal Noise

• Due to thermal agitation of electrons
• Uniformly distributed
• Also called white noise

Intermodulation Noise

• Signals that are sum and difference of original frequencies sharing a medium

Crosstalk

• Signal from one line picked up by another line

• Near End Crosstalk (NEXT): Interference measured at same end as transmitter
• Power Sum NEXT (PSNEXT): NEXT including all adjacent pairs
• Far End Crosstalk (FEXT): Interference measured at opposite end
• Equal Level FEXT (ELEFEXT): FEXT with attenuation compensation
• Power Sum FEXT (PSFEXT): Sum of all interference signals

Impulse Noise

• Irregular pulses or spikes
• External electromagnetic interference
• Short duration, high amplitude
• Minor annoyance for analog signals
• Major source of error in digital data

Channel Capacity

Definition

• Maximum possible data rate on communication channel

Factors Affecting Capacity

• Data rate: In bits per second
• Bandwidth: In cycles per second or Hertz
• Noise: On communication link (if we’re taking it in consideration)
• Error rate: Of corrupted bits
• Physical limitations

Types of Channels

• Noiseless Channel: Nyquist Bit Rate
• Noisy Channel: Shannon Capacity

Nyquist Theorem

A method to find maximum data rate (with fixed bandwidth) - เราจึงต้อง increase signal level

Definition

• Principle for digitization of analog signals
• Specifies sampling rate for faithful reproduction
• Key principle: Highest frequency component determines bandwidth

Nyquist Bandwidth

• Formula: $C=2B\log_2 M$
• Where:
  • $B$ = bandwidth
  • $M$ = signal levels
• For binary signals: $2B$ bps needs bandwidth $B$ Hz
• Trade-off: Increase rate by increasing signal levels at cost of receiver complexity

Signal Levels Example

• M = 2 (binary): 1 bit per baud
• M = 4: 2 bits per baud
• Formula application: $\log_2 M$ determines bits per symbol

Example Calculation

• Voice channel bandwidth: 3100 Hz
• Nyquist capacity: $C = 2B = 3100 \times 2 = 6200$ bps
• With M = 8: $C=2B\log_2 8 = 6200\times 3 = 18,600$ bps

Shannon Capacity Formula

Relationship

• Considers data rate, noise, and error rate
• Higher data rates mean shorter bit duration
• Noise bursts affect more bits at higher rates
• Higher rates with given noise level = higher errors

Formula

Where:

• SNR = Signal Power / Noise Power
• C: Theoretical maximum capacity
• Practical rates: Lower than theoretical maximum

Example

• หา Bandwidth ก่อน ก็คือ high frequency - lower frequency
  

Summary

Key Topics Covered

• Data transmission issues
• Frequency, spectrum & bandwidth concepts
• Analog vs digital signals comparison
• Types and effects of transmission impairments
• Channel capacity calculations
• Nyquist and Shannon theorems