ch03 Physical layer-1

Chapter 3: Physical Layer

Learning Objectives

  • Discuss the different types of network circuits and media.

  • Understand digital transmission of digital data.

  • Explore analog transmission of digital data.

  • Discuss digital transmission of analog data.

  • Analyze analog and digital modems.

  • Examine multiplexing techniques.

3.1 Introduction

  • Physical Layer Definition: The physical layer encompasses the network hardware, including servers, clients, and circuits.

  • Key Focus: How clients and servers transmit data via circuits.

  • Circuit Types: Include a combination of physical media and special-purpose devices.

  • Types of Circuits:

    • Physical Circuit: The actual wire connecting devices.

    • Logical Circuit: The transmission characteristics between devices.

Data Types

  • Digital Data: Represented in binary (1 or 0).

  • Analog Data: Used for continuous signals.

  • Data Transmission: Can occur in the same form as produced, or can involve conversion.

  • Modem Function: A modem translates between digital and analog data formats.

Benefits of Digital Transmission

  • Error Rates: Fewer errors compared to analog transmission.

  • Transmission Rates: Higher maximum transmission rates.

  • Efficiency: More efficient use of bandwidth.

  • Security: Easier to encrypt, providing improved security.

  • Integration: Simplifies the integration of voice, video, and data on the same circuit.

Parameter Agreement in Transmission

  • Agreement on Symbols: Sender and receiver must use a common set of symbols.

  • Symbol Rate Agreement: Both parties must agree on the rate of symbols transmission.

3.2 Circuits

  • Circuit Configuration: Basic physical layout of the circuit, can be categorized as:

    • Point-to-Point: Dedicated circuits between two points; can be costly.

    • Multipoint: Multiple devices share the same circuit; more efficient but with restrictions on simultaneous use.

Circuit Types

Point-to-Point Circuit

  • Direct connection from one device (computer) to another.

  • Designated as dedicated, potentially leading to high costs.

Multipoint Circuit

  • Involves multiple computers sharing the same circuit (shared circuit).

  • Only one device can use the circuit at a time.

  • Reduces cable requirements and enhances efficiency.

Cable Types and Connections

  • Cross-Over vs. Straight-through:

    • Cross-over cables: Used for similar devices (e.g., switch-to-switch).

    • Straight-through cables: Used for differing devices (e.g., router to PC).

Data Flow Types

  • Simplex Transmission: One-way transmission (e.g., radio).

  • Half-Duplex Transmission: Two-way but one direction at a time (e.g., walkie-talkies).

  • Full-Duplex Transmission: Simultaneous two-way transmission.

Multiplexing Techniques

  • Definition: Breaking a high-speed circuit into multiple lower-speed logical circuits.

  • Types of Multiplexing:

    • Frequency Division Multiplexing (FDM): Divides the circuit horizontally.

    • Time-Division Multiplexing (TDM): Shares a circuit among multiple devices in turns.

    • Statistical Time-Division Multiplexing (STDM): Based on statistical analysis of usage.

    • Wavelength Division Multiplexing (WDM): FDM variant used in fiber optics.

3.3 Communication Media

  • Defined: Media is the physical substance carrying data.

  • Types:

    • Guided Media: Physical medium like twisted-pair wires.

    • Wireless Media: Transmission through the air (e.g., microwave).

Types of Communication Media to Review

  • Twisted-Pair Cable: Minimizes electromagnetic interference, typically used in LANs.

  • Coaxial Cable: Copper core, less prone to interference, not commonly installed today.

  • Fiber-Optic Cable: Uses light pulses in thin glass strands, high capacity and speed.

  • Radio, Microwave, and Satellite: Forms of wireless media.

Factors in Media Selection

  • Considerations include network type, cost, distance, security, error rates, and speed.

3.4 Digital Transmission of Digital Data

  • Binary Data Production: Computers produce binary data.

  • Coding Scheme: A standard for representing messages, consisting of bits.

Coding Techniques

  • Character Representation: Characters as symbols with consistent meanings.

  • Predominant Schemes: ASCII, ISO 8859, and Unicode.

Transmission Methods

Parallel Transmission

  • All bits of a data element transferred simultaneously across multiple connections.

Serial Transmission

  • Data sent bit-by-bit sequentially over one wire, slower compared to parallel.

Digital Transmission Overview

  • Definition: Binary pulses representing data. Requires agreement on symbols and symbol rate.

Signaling Techniques

Unipolar Signaling

  • One voltage level (0V for binary 0, +5V for binary 1).

Bipolar Signaling

  • Alternates between positive and negative voltages.

  • Variants include Nonreturn to Zero (NRZ), Return to Zero (RZ), and Alternate Mark Inversion (AMI).

Ethernet and Data Transmission

  • Technology: Commonly used in LANs, employing digital transmission.

  • Manchester Encoding: A form of signaling with periodic voltage changes to represent bits.

3.5 Analog Transmission of Digital Data

  • POTS: Plain Old Telephone Service for voice communication.

  • Analog Transmission: Signal continually varies in a wave-like pattern.

Sound Wave Characteristics

  • Amplitude: Height of the wave (measured in decibels).

  • Frequency: Length of the wave (measured in hertz).

  • Phase: Direction of wave initiation (measured in degrees).

Modulation Techniques

  • Techniques for shaping sound waves to represent data values include:

    • Amplitude Modulation (AM): Changes wave amplitude, more noise-susceptible.

    • Frequency Modulation (FM): Varies frequency, not amplitude.

    • Phase Modulation (PM): Changes the phase to represent bits.

Bit Rate vs. Baud Rate

  • Bit Rate: Number of bits per second transmitted.

  • Baud Rate: Signaling speed based on signal changes per second.

Circuit Capacity

  • Data Capacity: Fastest rate data can be sent in bits per second.

  • Bandwidth: Difference between highest and lowest frequencies, e.g., standard telephone lines (4,000 Hz).

Modems

  • Modems translate digital to analog signals and vice versa.

  • Throughput Rate: Influenced by the modem's data transmission rate.

3.6 Digital Transmission of Analog Data

  • Analog to Digital Conversion: Requires codecs for data translation.

Quantizing Error and Reduction

  • Quantizing Error: Difference between analog and digital signals.

  • Reduction Techniques: Increase amplitude levels or sample frequently.

Voice Data Transmission via Telephones

  • Common Carrier Networks: Predominantly use digital transmission.

  • Codec Functionality: Converts analog signals to digital.

Instant Messaging and VoIP**

  • ADPCM: Efficiently encodes voice data for lower-speed circuits.

  • VoIP: Transmits conversations over digital networks, allowing for integration with LANs.

3.7 Implications for Cyber Security

  • Physical Security Needs: Protection of physical layer components.

  • Vulnerabilities: Laptops, USB drives, and mobile devices can lead to data theft and malware introduction.

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