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Practice MCQ

Unit 2 - Notes

CSE306

Unit 2: PHYSICAL LAYER: Signal, Media, Modulation & Multiplexing

The Physical Layer is the first layer of the OSI model (Layer 1). It coordinates the functions required to carry a bitstream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium.

1. Basics of Data Communications and Signals

Data Communication Components

  1. Message: The information (data) to be communicated.
  2. Sender: The device that generates the data.
  3. Receiver: The device that receives the data.
  4. Transmission Medium: The physical path by which a message travels from sender to receiver.
  5. Protocol: A set of rules that govern data communications.

Data Flow Directions

  • Simplex: Unidirectional (e.g., Keyboard to CPU, TV broadcasting).
  • Half-Duplex: Both directions, but only one at a time (e.g., Walkie-talkie).
  • Full-Duplex: Both directions simultaneously (e.g., Telephone network).

Analog and Digital Signals

To be transmitted, data must be transformed into electromagnetic signals.

Analog Signals

  • Definition: A continuous waveform that changes smoothly over time.
  • Nature: Infinite range of values.
  • Example: Human voice, sine waves.
  • Sine Wave Parameters:
    • Peak Amplitude (): Absolute value of the highest intensity (Volts).
    • Frequency (): Number of cycles per second (Hertz). .
    • Phase (): The position of the waveform relative to time zero (Degrees or Radians).

Digital Signals

  • Definition: A discrete waveform that changes abruptly.
  • Nature: Limited set of values (usually 1 and 0).
  • Bit Interval: Time required to send one single bit.
  • Bit Rate: The number of bits sent in 1 second (bps).

Periodic vs. Non-Periodic

  • Periodic: Repeats a pattern within a measurable time frame (common in analog).
  • Non-Periodic: Does not exhibit a repeating pattern (common in data communication/digital signals).

2. Transmission Impairments and Performance

Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment.

Types of Impairments

1. Attenuation

  • Definition: Loss of signal energy (strength) as it travels through a medium due to resistance.
  • Measurement: Decibels (dB).
  • Formula:
    TEXT
        dB = 10 log10 (P2 / P1)
    Where P1 is input power and P2 is output power.
    • Negative dB = Attenuation (loss).
    • Positive dB = Amplification (gain).

2. Distortion

  • Definition: Alteration of the signal's original shape.
  • Cause: Occurs in composite signals where different frequencies travel at different propagation speeds, arriving at the receiver at slightly different times.

3. Noise

  • Definition: Random or unwanted signals that mix with the original signal.
  • Types:
    • Thermal Noise: Random motion of electrons (white noise).
    • Induced Noise: From external sources (motors, appliances).
    • Crosstalk: A wire picks up a signal from an adjacent wire.
    • Impulse Noise: Spikes (high energy) from lightning or power surges.

Performance Metrics

  1. Bandwidth: The range of frequencies a medium can pass (Hz) or the maximum speed of bits (bps).
  2. Throughput: The actual measurement of how fast data can flow through a network (usually lower than bandwidth).
  3. Latency (Delay): Time taken for a message to travel from sender to receiver.
    • Latency = Propagation time + Transmission time + Queuing time + Processing time.
  4. Jitter: The variation in packet arrival time (crucial for multimedia).

3. Data Rate Limits

The maximum data rate of a channel depends on the bandwidth available, the signal levels used, and the quality of the channel (noise).

Noiseless Channel: Nyquist Bit Rate

Assume a perfect channel with no noise.

TEXT
BitRate = 2 × Bandwidth × log2(L)

  • Bandwidth: Bandwidth of the channel in Hz.
  • L: Number of signal levels used to represent data.

Noisy Channel: Shannon Capacity

In reality, channels have noise. Shannon determines the theoretical highest data rate.

TEXT
Capacity = Bandwidth × log2(1 + SNR)

  • SNR (Signal-to-Noise Ratio): .
  • Note: If SNR is given in dB, convert it first: .

4. Transmission Media

A. Guided Media (Wired)

Waves are guided along a physical path.

1. Twisted Pair Cable

  • Structure: Two insulated copper wires twisted together to reduce crosstalk and electromagnetic interference (EMI).
  • Types:
    • Unshielded Twisted Pair (UTP): Common in LANs (e.g., Cat5e, Cat6).
    • Shielded Twisted Pair (STP): Metal casing covers wire pairs for better noise protection.
  • Connectors: RJ45.

2. Coaxial Cable

  • Structure: Central conductor, insulating dielectric, outer conductor (shield mesh), and plastic jacket.
  • Characteristics: Higher frequency range than twisted pair.
  • Standards: RG-59 (TV), RG-58 (Thin Ethernet), RG-11 (Thick Ethernet).
  • Connectors: BNC.

3. Fiber Optic Cable

  • Structure: Core (glass/plastic), Cladding (lower refractive index), and Jacket.
  • Physics: Uses Total Internal Reflection to guide light.
  • Modes:
    • Multimode: Multiple beams from a light source move through the core (LED source). Good for shorter distances.
    • Single-mode: Single beam (Laser source). Smaller core, long-distance.
  • Advantages: Immune to EMI, very high bandwidth, low attenuation.

B. Unguided Media (Wireless)

Transport electromagnetic waves without a physical conductor.

1. Radio Waves (3 kHz – 1 GHz)

  • Omnidirectional (send in all directions).
  • Used for AM/FM radio, television.
  • Susceptible to multipath interference.

2. Microwaves (1 GHz – 300 GHz)

  • Unidirectional (requires line-of-sight alignment).
  • Used for cellular phones, satellite networks, wireless LANs.

3. Infrared (300 GHz – 400 THz)

  • Short-range communication.
  • Cannot pass through walls.
  • Used for remote controls, IrDA ports.

C. Cabling Standards (TIA/EIA-568)

For terminating twisted pair cables into RJ45 connectors:

  • T568A: Green-White, Green, Orange-White, Blue, Blue-White, Orange, Brown-White, Brown.
  • T568B: Orange-White, Orange, Green-White, Blue, Blue-White, Green, Brown-White, Brown (Most common).
  • Straight-through: Both ends T568B (PC to Switch).
  • Crossover: One end T568A, one end T568B (PC to PC).

5. Digital-to-Digital Conversion (Line Coding)

Converting digital data (bits) into digital signals (voltage pulses).

Key Characteristics

  • DC Component: Undesirable constant voltage (frequency zero) which cannot pass through transformers.
  • Synchronization: Receiver uses transitions in the signal to synchronize with the sender's clock.

Coding Schemes

1. Unipolar Scheme

  • NRZ (Non-Return to Zero): Uses positive voltage for 1, zero voltage for 0.
    • Drawback: High cost, DC component, no synchronization.

2. Polar Schemes (Uses Positive and Negative Voltages)

  • NRZ-L (Level): Level of voltage determines value (e.g., +V for 0, -V for 1).
  • NRZ-I (Invert): Change or lack of change determines value. Invert voltage on '1', no change on '0'.
  • RZ (Return to Zero): Uses three values (+, -, 0). Signal returns to 0 in the middle of each bit. Good sync, but requires high bandwidth.
  • Biphase (Manchester):
    • Combines RZ and NRZ-L.
    • Transition happens at the middle of the interval.
    • Low-to-High represents 1 (or 0 depending on convention), High-to-Low represents 0.
    • Used in Ethernet.
  • Differential Manchester: Transition at the middle is for sync; presence/absence of transition at the start determines the bit.

3. Bipolar Schemes (AMI)

  • AMI (Alternate Mark Inversion): 0 is zero voltage. 1 is alternating positive and negative voltages.
  • Benefits: No DC component.

6. Analog-to-Digital Conversion (Digitization)

Converting analog signals (like voice) into digital data (bits). The primary technique is PCM (Pulse Code Modulation).

PCM Steps

  1. Sampling (PAM - Pulse Amplitude Modulation):

    • Measuring the amplitude of the signal at equal intervals.
    • Nyquist Theorem: Sampling rate must be at least 2 times the highest frequency () of the signal.
    • .

  2. Quantization:

    • Assigning the sampled values to specific discrete levels (rounding off).
    • Introduces Quantization Error (noise).

  3. Encoding:

    • Converting the quantized values into streams of bits.

Delta Modulation (DM)

  • Simplified PCM. Records only the change from the previous sample (+delta or -delta) rather than absolute value. 1 bit per sample.

7. Digital-to-Analog Conversion (Modulation)

Converting digital data (bits) into an analog signal (carrier wave) to send over telephone lines or wireless links (Modems).

Techniques

  1. ASK (Amplitude Shift Keying):

    • Frequency and Phase remain constant.
    • Amplitude changes to represent 1 or 0.
    • Highly susceptible to noise.

  2. FSK (Frequency Shift Keying):

    • Amplitude and Phase remain constant.
    • Frequency changes ( for bit 1, for bit 0).

  3. PSK (Phase Shift Keying):

    • Amplitude and Frequency remain constant.
    • Phase changes (e.g., 0 degrees for 0, 180 degrees for 1).
    • More robust against noise than ASK/FSK.
    • QPSK (Quadrature PSK): Uses 4 phases to encode 2 bits at a time.

  4. QAM (Quadrature Amplitude Modulation):

    • Combines ASK and PSK.
    • Variations in both Amplitude and Phase.
    • Highly efficient data usage (e.g., 16-QAM, 64-QAM).

8. Analog-to-Analog Conversion

Modulating an analog signal (like voice) onto a high-frequency analog carrier signal (like radio).

  1. AM (Amplitude Modulation): Carrier amplitude varies with the amplitude of the modulating signal.
  2. FM (Frequency Modulation): Carrier frequency varies with the amplitude of the modulating signal. (Better noise immunity than AM).
  3. PM (Phase Modulation): Carrier phase varies with the amplitude of the modulating signal.

9. Multiplexing

A technique used to combine and transmit multiple signals over a single medium simultaneously to utilize bandwidth efficiently.

1. FDM (Frequency Division Multiplexing)

  • Type: Analog technique.
  • Mechanism: Signals are modulated onto different carrier frequencies.
  • Guard Bands: Strips of unused frequency separate channels to prevent overlapping.
  • Example: AM/FM Radio broadcasting, Cable TV.

2. WDM (Wavelength Division Multiplexing)

  • Type: Analog technique (for Fiber Optics).
  • Mechanism: Conceptually same as FDM, but uses light wavelengths (colors).
  • Prism: Uses a prism/diffraction grating to combine and split light beams.

3. TDM (Time Division Multiplexing)

  • Type: Digital technique.
  • Mechanism: The timeline is divided into slots. Each user gets the entire bandwidth for a specific fraction of time.

Variations of TDM:

  • Synchronous TDM:
    • Time slots are pre-assigned to sources.
    • If a source has no data, the slot is sent empty (wasted capacity).
  • Statistical TDM:
    • Dynamic allocation. Slots are allocated only to sources that have data to send.
    • More efficient but requires overhead (addressing info in slots) to identify the owner of the data.