PHYSICAL LAYER IN DATA AND COMMUNICATION

The PHYSICAL LAYER IN DATA AND COMMUNICATION is the foundational layer of the OSI (Open Systems Interconnection) model. It is responsible for the physical transmission of data over a communication channel. Essentially, it deals with the raw bit stream and how it’s physically conveyed between devices. This layer is concerned with the characteristics of the physical media (like cables, radio waves, or optical fibers) and the signals used to represent data on those media.

1. Data and Signals {PHYSICAL LAYER IN DATA AND COMMUNICATION}

1.1 Analog and Digital Data

Analog Data	Digital Data
Continuous in nature.	Discrete (binary 0s and 1s).
Example: Human voice, temperature.	Example: Text files, computer data.

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1.2 Analog and Digital Signals

Analog Signal	Digital Signal
Continuous waveform.	Square pulses (discrete values).
Varies smoothly over time.	Only two levels: High (1) and Low (0).
Example: Sine wave, radio signals.	Example: Ethernet signal, USB data.

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2. Periodic and Nonperiodic Signals

2.1 Periodic Signals

• Signals that repeat over time.
• Example: Sine wave, clock signal. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

2.2 Nonperiodic Signals (Aperiodic)

• Signals that do not repeat. {PHYSICAL LAYER IN DATA AND COMMUNICATION}
• Example: Data from typing on keyboard, speech signals.

Periodic Signal	Nonperiodic Signal
Repeats after a fixed time.	Random, no repetition pattern.
Used in analog and clock signals.	Used in data communication.

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3. Composite Signals

• A combination of multiple sine waves(with different frequencies, amplitudes, phases).
• Any complex signal (non-sine wave) can be broken into simpler sine waves (Fourier Series).
• Example: Voice, music, video signals.

Composite Signal
Combination of multiple frequencies.
Can be analyzed as sum of simple sine waves. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Mathematical Formula:

Where:

n = Harmonic number (n = 1, 2, 3, …).

x(t) = Composite signal as a function of time.

a0= Average (DC component) of the signal.

an,bn​ = Fourier coefficients (amplitude of cosine & sine components).

f0​ = Fundamental frequency of the signal.

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4. Bandwidth

• Bandwidth is the range of frequencies that a channel can transmit.
• Calculated as: Bandwidth=Highest Frequency − Lowest Frequency
• Measured in Hertz (Hz). {PHYSICAL LAYER IN DATA AND COMMUNICATION}
• Higher bandwidth = Higher data transmission capacity.

Type	Explanation
Analog Bandwidth	Range of frequencies for analog signal transmission.
Digital Bandwidth	Data rate (bits per second – bps) of a digital signal.

5. Digital Signals

Digital signals are made up of discrete binary values (0s and 1s). These signals are typically used in computer networks and digital communication systems. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

5.1 Bit Rate (Data Rate)

• Bit rate is the number of bits transmitted per unit of time.
• Unit: Bits per second (bps). {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Formula:

Bit Rate (R)=Total Number of Bits / Time Taken (seconds)

5.2 Bit Length

• Bit length refers to the physical length of a bit on the transmission medium, or how long a bit takes to travel through the medium. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Formula:

Bit Length (L)=Propagation Speed (v) / Bit Rate (R)

Bit Rate =(R)

Propagation Speed =(v)​

Where:

• v = Propagation speed of the signal in the transmission medium (speed of light in fiber, or speed of electrical signals in copper wire).
• R= Bit rate (number of bits per second {PHYSICAL LAYER IN DATA AND COMMUNICATION}

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5.3 Digital Signal as a Composite Analog Signal

• In digital communication, a sequence of discrete binary bits (0s and 1s) is transmitted as a sequence of voltage pulses over a transmission medium. {PHYSICAL LAYER IN DATA AND COMMUNICATION}
• A digital signal can be seen as a series of high and low voltage levels, with each “high” representing a binary “1” and each “low” representing a binary “0”.
• Digital signals are typically transmitted using Pulse Code Modulation (PCM), which converts digital data into a sequence of analog pulses. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

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2. Transmission of Digital Signals

Digital signals can be transmitted over various media, including:

1. Copper wires (Twisted pairs, Coaxial cables).
2. Fiber optic cables.
3. Wireless transmission (Radio waves, microwaves).

Transmission Techniques

Passband Transmission: The digital signal is modulated to a higher frequency, and it is then transmitted over the medium (typically used in internet connections). {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Baseband Transmission: The digital signal is transmitted directly over the medium without any frequency modulation. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Diagram: Digital Signal Representation as a Composite Analog Signal

Digital Signal:
        ____        ____        ____    ____
_______|    |______|    |______|    |__|____|_____
(High=1)         (Low=0)       (High=1)      (Low=0)

Composite Analog Signal:
        ~~~~~~~~   ~~~~~~~~   ~~~~    ~~~~
    ~~~~         ~~~~         ~~~      ~~~~
------------------------> Time ------>

TRANSMISSION IMPAIRMENT

Transmission impairment refers to the various factors that degrade the quality of a signal as it travels through a medium. These impairments result in errors or loss of data. The three main types of transmission impairments are attenuation, distortion, and noise. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Attenuation

• Definition: Attenuation is the gradual loss of signal strength as the signal travels through the transmission medium (e.g., copper wires, fiber optics, air for wireless).
• Cause: Loss occurs due to the resistance, impedance, and the physical properties of the medium (like copper, air, or fiber). {PHYSICAL LAYER IN DATA AND COMMUNICATION}
• Effect: The signal weakens over distance and eventually becomes too weak to be detected correctly by the receiver.

Diagram

Signal Strength over Distance:
   ______
  /      \
 /        \______
 |                           |
 |__________________________|
|__________________________|
            Distance

Distortion

• Definition: Distortion occurs when the shape or form of the signal is altered during transmission. It usually happens because different frequencies within a signal travel at different speeds through the medium, causing the signal to change its shape. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Cause of Distortion:

• Different frequencies of the signal may experience different delays (or propagation speeds), leading to a change in the waveform
• Example: In voice communication, distortion can cause the audio to sound garbled or unclear.

Types of Distortion:

• Amplitude Distortion: Changes the amplitude of different frequencies.
• Phase Distortion: Changes the phase relationship between the signal’s components.
• Delay Distortion: Causes certain frequency components of the signal to arrive at the receiver at different times. {PHYSICAL LAYER IN DATA AND COMMUNICATION}

Effect:

• The signal at the receiver will be distorted, resulting in errors in data interpretation

Diagram:

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Original Signal: 
    ____        ____        ____
___|    |______|    |______|    |____

Distorted Signal:
    ___       ____     ___       ____
___|   |_____|    |___|   |_____|    |____

3. Noise

• Definition: Noise is any unwanted electrical or electromagnetic signal that interferes with the original signal during transmission. Noise can be generated from various sources such as electrical equipment, the environment, or the medium itself.

Types of Noise:

1. Thermal Noise (Johnson-Nyquist Noise): Caused by the random motion of electrons in a conductor.
2. Interference: Noise from other signals in the environment, like radio interference or crosstalk between wires.
3. Impulse Noise: Short bursts of high energy noise (e.g., lightning strikes, power surges).
4. Crosstalk: When one signal interferes with another in the same transmission medium.

Effect:

• Noise introduces errors, making it difficult for the receiver to accurately detect the signal.
• Signal-to-noise ratio (SNR) is an important metric to assess the quality of a transmission. Higher SNR indicates a cleaner, clearer signal.

Diagram

Original Signal:   ____    ____    ____
                  |    |  |    |  |    |
Noisy Signal:     ___|  |__|    |__|  |__
                  |____|  |____|   |____|

Rate Limit (in Data Communication & Networking)

Definition:
Rate limiting is the process of controlling the amount of data that can be transmitted or processed in a given time frame, usually to avoid network congestion or to prevent overuse of resources.

1.1 Nyquist Bit Rate

The Nyquist Bit Rate defines the maximum theoretical bit rate for a noiseless channel. It calculates how many bits can be transmitted over a channel, given the bandwidth.

Formula:

Nyquist Bit Rate=2×B×log⁡2(M)

Where:

• B = Bandwidth of the channel (in Hz).
• M = Number of discrete signal levels (for example, 2 for binary signaling).
• Explanation: The formula assumes that the channel is noiseless and the signal is sent at the highest possible efficiency.
• For binary signals (M=2) ,Nyquist Bit Rate=2×B bps

1.2 Shannon Capacity

The Shannon Capacity defines the maximum bit rate of a noisy channel, where noise and interference are present.

Formula:

2. Transmission Modes

2.1 Parallel Transmission

• Definition: In parallel transmission, multiple bits are transmitted simultaneously over multiple channels (or wires).
• Advantages:
  • Faster than serial transmission because multiple bits are sent at once.
  • Commonly used for short-distance communication (e.g., within a computer, between CPU and memory).
• Disadvantages:
  • Can suffer from crosstalk and signal degradation over longer distances.
  • Requires more wires, which increases complexity.
• Example: USB and parallel ports in older computers.

2.2 Serial Transmission

• Definition: In serial transmission, bits are transmitted one at a time over a single channel (or wire).
• Advantages:
  • Simple and requires fewer wires.
  • More suitable for long-distance communication.
• Disadvantages:
  • Slower than parallel transmission because only one bit is transmitted at a time.
• Example: Ethernet, RS-232 serial ports, and USB (modern).

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3. Digital-to-Analog Conversion

When transmitting digital data, we often need to convert the data into analog signals suitable for transmission over analog channels. The following methods are used:

3.1 Amplitude Shift Keying (ASK)

• Definition: In ASK, the amplitude of the carrier signal is varied in proportion to the data being transmitted. {PHYSICAL LAYER IN DATA AND COMMUNICATION}
• How it works:
  • A binary 1 is transmitted as a high amplitude, and a binary 0 is transmitted as a low amplitude.
• Advantages:
  • Simple to implement.
• Disadvantages:
  • Susceptible to noise and interference.

Example:

• A carrier signal with two different amplitudes: high (for 1) and low (for 0).

3.2 Phase Shift Keying (PSK)

• Definition: In PSK, the phase of the carrier signal is changed to represent the data.
• How it works:
  • A binary 1 could be represented by one phase (e.g., 0°), and a binary 0 could be represented by another phase (e.g., 180°).
• Types:
  • BPSK (Binary PSK): 2 phases (0° and 180°).
  • QPSK (Quadrature PSK): 4 phases, allowing 2 bits per symbol.
• Advantages:
  • More resilient to noise compared to ASK.
• Disadvantages:
  • Requires more complex modulation and demodulation schemes.

3.3 Frequency Shift Keying (FSK)

• Definition: In FSK, the frequency of the carrier signal is shifted between two distinct frequencies to represent the data.
• How it works:
  • A binary 1 is represented by one frequency, and a binary 0 is represented by another frequency.
• Advantages:
  • Less susceptible to amplitude noise than ASK.
  • Good for low signal-to-noise ratio (SNR) environments.
• Disadvantages:
  • Requires more bandwidth than ASK or PSK.

Example:

• A signal with two different frequencies, one for 0 (e.g., 1000 Hz) and another for 1 (e.g., 2000 Hz).