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.