Physical Layer Protocols & Data Encoding

• Physical layer protocols concern transmission of signals over a communication channel.
• It is the first layer in the OSI model.
• It defines the physical characteristics of the network.

Data Encoding

• Data encoding is the process of converting data into a format suitable for transmission.
• Common encoding schemes include:
  • NRZ (Non-Return to Zero)
    • Represents 1s with one voltage level and 0s with another.
    • Simple, but suffers from baseline wander and clock recovery issues for long sequences of the same bit.
  • NRZI (Non-Return to Zero Inverted)
    • A transition represents a 1, and no transition represents a 0.
    • Addresses the issue of long sequences of 1s in NRZ.
  • Manchester Encoding
    • A transition in the middle of the bit period represents a 0 or 1.
    • The direction of the transition indicates the bit value.
    • Solves the clock recovery problem but doubles the signaling rate.
  • 4B/5B Encoding
    • Maps 4 bits of data into a 5-bit code to ensure frequent transitions.
    • Used with NRZI to provide clock synchronization.
  • Block coding schemes
    • Block coding replaces m-bit groups with n-bit groups.

Transmission Media

• Transmission media are the physical pathways for communication signals.
• Guided media: physical cables like twisted pair, coaxial cable and fiber optic cable
• Unguided media: wireless transmission through air or space.

Twisted Pair Cable

• Consists of two insulated wires twisted together to reduce noise and interference.
• Types include:
  • Unshielded Twisted Pair (UTP): Commonly used for Ethernet networks.
  • Shielded Twisted Pair (STP): Includes a foil or braid shield for better noise immunity.
• Categories of UTP cables (Cat5, Cat5e, Cat6, Cat6a, Cat7) support different data rates and frequencies.

Coaxial Cable

• Features a central conductor surrounded by insulation, a shield, and an outer jacket.
• Offers better noise immunity than UTP.
• Used in cable TV and older Ethernet networks.

Fiber Optic Cable

• Transmits data as light pulses through thin glass or plastic fibers.
• Offers high bandwidth, low attenuation, and immunity to electromagnetic interference.
• Types include:
  • Single-mode fiber: Smaller core, longer distances, higher bandwidth.
  • Multimode fiber: Larger core, shorter distances, lower cost.

Wireless Media

• Electromagnetic waves used for transmission.
• Frequency bands are regulated to prevent interference.
• Types of wireless media:
  • Radio waves: Omnidirectional, penetrate walls, used for Wi-Fi and Bluetooth.
  • Microwaves: Higher frequency, line-of-sight, used for satellite communication.
  • Infrared: Short range, used for remote controls.

Modulation Techniques

• Modulation is the process of varying properties of a carrier wave to encode data.
• Types include:
  • Amplitude Modulation (AM): Varying the amplitude of the carrier wave.
  • Frequency Modulation (FM): Varying the frequency of the carrier wave.
  • Phase Modulation (PM): Varying the phase of the carrier wave.
  • Quadrature Amplitude Modulation (QAM): Varying both amplitude and phase.

Multiplexing

• Multiplexing enables multiple signals to share a single communication channel.
• Types include:
  • Frequency Division Multiplexing (FDM): Dividing the bandwidth into frequency channels.
  • Time Division Multiplexing (TDM): Dividing the channel into time slots.
  • Wavelength Division Multiplexing (WDM): Transmitting multiple light wavelengths through a single fiber.

Switching

• Circuit Switching: Establishes a dedicated path between sender and receiver.
• Packet Switching: Data is divided into packets and sent independently.

Circuit Switching

• Establishes a dedicated physical path between sender and receiver before data transmission
• Used in traditional telephone networks
• Phases:
  • Circuit establishment
  • Data transfer
  • Circuit disconnect
• Advantages:
  • Guaranteed bandwidth
  • Low delay
• Disadvantages
  • Inefficient use of bandwidth
  • Setup overhead

Packet Switching

• Data is divided into packets, each containing destination and source addresses
• Packets are routed independently through the network
• Types:
  • Datagram
    • Each packet is routed independently
    • Packets may arrive out of order
  • Virtual Circuit
    • Establishes a logical connection before data transfer
    • Packets follow the same path

Data Rate Limits

• Theoretical maximum bit rate of a channel
• Nyquist Theorem:
  • For a noiseless channel: Maximum data rate = 2 * Bandwidth * log2(Number of signal levels)
• Shannon Capacity:
  • For a noisy channel: Maximum data rate = Bandwidth * log2(1 + Signal-to-Noise Ratio)
• Bandwidth refers to the range of frequencies a channel can transmit.
• Signal-to-Noise Ratio (SNR) represents the strength of the signal relative to the noise.

Digital Subscriber Line (DSL)

• Provides digital communication over existing telephone lines.
• Uses frequency division multiplexing to separate voice and data signals.
• Types include:
  • ADSL (Asymmetric DSL): Higher downstream bandwidth than upstream.
  • SDSL (Symmetric DSL): Equal downstream and upstream bandwidth.
  • VDSL (Very-high-bit-rate DSL): Higher bandwidth than ADSL.

Cable Modems

• Provide Internet access over coaxial cable infrastructure used for cable TV.
• Share bandwidth with other users in the same neighborhood.
• DOCSIS (Data Over Cable Service Interface Specification) is the standard for cable modems.

Wireless Technologies

• Wi-FI (Wireless Fidelity):
  • Based on the IEEE 802.11 standards
  • Operates in the 2.4 GHz and 5 GHz frequency bands
  • Uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
• Bluetooth:
  • Short-range wireless communication
  • Used for connecting devices like headsets, keyboards, and mice
• Cellular Technologies:
  • Uses cell towers to provide wireless coverage over a wide area
  • Generations include 2G, 3G, 4G, and 5G
  • 5G offers higher data rates and lower latency

Network Topologies

• Physical arrangement of nodes and links in a network.
• Common topologies include:
  • Bus Topology:
    • All devices connected to a central cable (the bus)
    • Simple but prone to collisions
  • Star Topology:
    • All devices connected to a central hub or switch
    • Easy to manage and troubleshoot
  • Ring Topology:
    • Devices connected in a closed loop
    • Data travels in one direction
  • Mesh Topology:
    • Each device connected to multiple other devices
    • High redundancy and fault tolerance
    • High cost and complexity

Network Devices

• Hub:
  • Connects devices in a star topology
  • Operates at the physical layer
  • Broadcasts data to all connected devices
• Switch:
  • Connects devices in a star topology
  • Operates at the data link layer
  • Forwards data only to the destination device
• Router:
  • Connects different networks
  • Operates at the network layer
  • Routes data based on IP addresses
• Repeater:
  • Regenerates signals to extend transmission distance
  • Operates at the physical layer

Error Detection

• Techniques used to detect errors introduced during transmission.
• Parity Check:
  • Adds a parity bit to ensure the number of 1s is either even or odd.
  • Simple but can only detect single-bit errors.
• Checksum:
  • Adds the data units and appends the sum to the end of the data.
  • Detects errors with higher probability.
• Cyclic Redundancy Check (CRC):
  • Divides the data by a predetermined divisor and appends the remainder to the end of the data.
  • Powerful error detection technique.