Data Compression & Transmission Summary

📊 Data Compression

Data compression reduces file size by re-encoding data to use fewer bits. The primary goals are to save storage space and reduce transmission time.

Lossless Compression

Definition: Reduces file size WITHOUT losing any original data

Key Principle: Eliminates statistical redundancy

Perfect Reconstruction: Original data can be fully restored

Compression Ratio: Typically 2:1 to 3:1

Common Methods:

Run-Length Encoding (RLE): Replaces repeated sequences with count-value pairs
Dictionary Coding (LZW): Builds dictionary of frequent patterns
Huffman Coding: Assigns shorter codes to frequent characters

Use Cases: Text files, program code, databases, ZIP files, PNG images

Lossy Compression

Definition: Permanently discards some data to achieve higher compression

Key Principle: Removes perceptually less important information

Approximate Reconstruction: Output is similar but not identical to input

Compression Ratio: 10:1 to 100:1+

Common Methods:

JPEG (Images): Chrominance subsampling, DCT, quantization
MP3 (Audio): Psychoacoustic modeling, auditory masking
Video Compression: Spatial & temporal compression

Use Cases: Photos, music, video streaming, web media

Comparison: Lossless vs Lossy Compression

Aspect	Lossless	Lossy
Data Integrity	Perfect	Imperfect
File Size Reduction	Moderate	Significant
Reversibility	Fully reversible	Not reversible
Quality	No loss	Quality degradation
Best For	Text, code, archives	Media files

📡 Data Transmission

Data transmission involves sending digital data from one device to another over communication channels.

Transmission Methods

Serial Transmission

Bits sent one after another on a single channel

Advantages:

Reliable over long distances
Lower cost (fewer wires)
Less interference
Bits arrive in order

Disadvantages:

Slower than parallel
Requires start/stop bits

Examples: USB, Ethernet, SATA

Parallel Transmission

Multiple bits sent simultaneously on multiple channels

Advantages:

Faster data transfer
Simple programming
Efficient for short distances

Disadvantages:

Expensive (more wires)
Signal skewing issues
Limited cable length

Examples: Internal computer buses, printer ports

Transmission Types (Direction)

Simplex

One-direction only

Example: Keyboard to computer, printer

Half-Duplex

Both directions, but not simultaneously

Example: Walkie-talkies

Full-Duplex

Both directions simultaneously

Example: Telephone, broadband internet

USB (Universal Serial Bus)

Type: Serial, both half/full-duplex

Features:

Automatic device detection
Plug-and-play
Power delivery
Standardized interface

Limitations:

Cable length restricted (~5 meters)
Speed limitations compared to Ethernet

📦 Packet Switching

Packet switching is a method of grouping data that is transmitted over a digital network into packets.

Data Packet Structure

A data packet consists of three main parts:

PAYLOAD

TRAILER

Actual Data

Error Checking

How Packet Switching Works

Data Segmentation: Large files split into packets
Individual Routing: Each packet sent independently
Path Selection: Routers choose optimal paths
Reassembly: Destination device reconstructs original data

Benefits

Efficient line usage
Fault tolerance (rerouting)
Network scalability
High transmission rates

Drawbacks

Packet delay (latency)
Potential packet loss
Reordering overhead
Not ideal for real-time streaming

Process Flow

Original Data → Split into Packets → Individual Routing → Reassembly → Original Data

↓

Different paths possible

↓

Routers direct each packet

🔍 Error Handling & Security

Error Detection Methods

Parity Check

Odd/even bit counting to detect errors

Checksum

Algorithmic value verification

Echo Check

Data sent back for comparison

Automatic Repeat Request (ARQ)

Positive Acknowledgement

ACK for successful reception

Negative Acknowledgement

NACK for error detection

Timeout

Resend if no response

Encryption

Symmetric Encryption

Single key for encryption/decryption

Asymmetric Encryption

Public/private key pairs

💡 Key Takeaways