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.