Unit 2 Physical Layer

Introduction to Physical Layer

• The physical layer of the OSI model interacts with hardware and signaling mechanisms, defining cabling, wiring, and signal representations.
• It converts frames from the data link layer to electrical pulses for transmission over various media.

Signals

• Data can be analog (e.g., voice) or digital (e.g., file), represented as either digital or analog signals.
• Digital signals consist of discrete voltage pulses, while analog signals are continuous electromagnetic waves.

Transmission Impairment

• Signals deteriorate during transmission due to factors like:
  • Attenuation: Weakening of signals over distance, requiring sufficient strength for interpretation.
  • Dispersion: Spreading and overlapping of signals, influenced by frequency.
  • Delay Distortion: Arrival of signals at different times based on velocity and frequency mismatches.
  • Noise: Random disturbances that distort signals, categorized into:
    • Thermal Noise: Caused by heat agitation in conductors.
    • Intermodulation: Noise from interference between multiple frequencies.
    • Crosstalk: Disturbance from foreign signals affecting media.
    • Impulse Noise: Resulting from irregular disturbances like lightning.

Transmission Media

• Two forms of transmission media:
  • Guided Media: Communication occurs through cables like UTP, coaxial, or fiber optics, guiding the signal directly.
  • Unguided Media: Wireless transmission without physical connection, allowing information to be picked up by anyone within range.

Channel Capacity

• Channel capacity, or data rate, is the speed of information transmission influenced by:
  • Bandwidth: The physical limitation of media.
  • Error-rate: Incorrect receptions due to noise.
  • Encoding: Number of levels used for signaling.

Multiplexing

• Technique for mixing multiple data streams over a single medium using a multiplexer and demultiplexer.

Switching

• Switching enables data transfer between non-directly connected devices through interconnecting devices that receive, store, analyze, and forward data.
• Categories of switching techniques:
  • Circuit Switching: Establishes a physical path for the duration of a call, used in traditional telephone networks.
  • Message Switching: Stores entire messages at switching nodes before forward transmission one hop at a time.
  • Packet Switching: Breaks messages into packets sent independently, allowing for flexible routing and reduced delays.

Data Link Layer

• Transfers data between adjacent network nodes; focuses on local delivery of frames and may include error detection and correction.
• Examples of data link protocols: Ethernet, PPP, HDLC.
• Designed to handle media contention and collision resolution.

Data Link Layer Functions

• Provides services like:
  • Unacknowledged Connectionless Service: Sends frames without acknowledgment, suitable for low-error environments.
  • Acknowledged Connectionless Service: Requires acknowledgment for each frame, enhancing reliability over unreliable channels.
  • Acknowledged Connection-Oriented Service: Establishes a connection, ensuring ordered, reliable delivery of frames.

Framing

• Data link layer organizes bit streams into frames, which include header, payload, and trailer for effective transmission and error-checking.
• Framing methods include:
  • Character Count: Uses a field to specify frame size; less common due to synchronization issues.
  • Flag Bytes with Byte Stuffing: Frames start and end with special flag bytes, using escape bytes to avoid confusion with data content.

Error Detection Mechanisms

• Each frame carries a checksum to verify data integrity, discarding erroneous frames and potentially sending error reports.
• Use of framing methods ensures synchronization and integrity in data transmission.### Byte-Stuffing and Bit-Stuffing
• Byte-stuffing in PPP protocol is limited to 8-bit characters; newer methods are required for variable-length characters.
• Bit-stuffing uses a flag byte (01111110) at both the start and end of data frames.
• A 0 is automatically stuffed into the outgoing stream after five consecutive 1 bits to avoid confusion with the flag byte.
• Receiver removes the stuffed 0, ensuring seamless communication.
• Bit stuffing allows easy identification of frame boundaries, aiding in synchronization even if the receiver loses track.

Physical Layer and Framing Methods

• Framing methods rely on physical medium encoding that includes redundancy for reliable communication.
• Certain LANs encode bits with combinations of high-low and low-high transitions, helping receivers identify data boundaries.
• Many data link protocols combine character counts with other methods for enhanced frame validation using checksums.

Error Control

• Ensures all frames are delivered properly and in order; vital for reliable, connection-oriented services.
• Systems use acknowledgements (positive or negative) for feedback on frame receipt.
• Timers are employed to retransmit frames if acknowledgements are not received, preventing indefinite hanging.
• Sequence numbers on frames enable receivers to manage retransmissions and distinguish original frames from duplicates.

Flow Control

• Critical for managing data flow between fast senders and slower receivers to prevent overwhelm.
• Feedback-based flow control allows receivers to inform senders about their status.
• Rate-based flow control limits transmission speeds directly via protocol mechanisms without receiver feedback.

Error Detection and Correction

• Wireless communications experience higher error rates; redundant information is essential to handle transmission errors.
• Strategies include Error-Correcting Codes (Forward Error Correction) and Error-Detecting Codes.
• Hamming distance determines the capability of a code to detect and correct errors.
• Minimum Hamming distance of d + 1 is necessary to detect d errors; distance of 2d + 1 is required for correcting d errors.

Examples of Error Codes

• Single parity bit codes can detect single errors with a Hamming distance of 2.
• More complex codes can correct errors based on greater distances, ensuring proper identification of original codewords even with multiple errors.
• Designing codes requires understanding the relationship between message bits, check bits, and the total number of code patterns necessary for error correction.

Practical Application of Error Codes

• The balance between redundancy and efficiency influences the design of error detection and correction strategies, optimizing data integrity across various transmission mediums.

This quiz covers the fundamentals of the Physical Layer in the OSI model, focusing on its role in hardware interaction and signaling mechanisms. Learn about the physical connectivity, hardware components, and the functions that define this essential OSI layer. Test your understanding of how the Physical Layer interfaces with the Data-Link Layer.