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Data Transmission
Data transmission is the process of sending digital data between devices. It is fundamental to modern communication.
This section covers:
  • Data packetization and network transmission
  • Data transmission methods (serial, parallel, simplex, duplex)
  • Error detection and correction
  • Data encryption for security
Packets and Packet Switching
Data transmitted over a network is broken into small, organized units called packets. This method enhances transmission speed, reliability, and flexibility, allowing multiple users to share network resources efficiently.
Packet Structure
  • Header: Contains destination address, packet sequence number, and source address.
  • Payload: The actual data being transmitted.
  • Trailer: Contains error-checking data to ensure integrity upon arrival.
Packet Switching Process
  1. Data is split into individual packets.
  1. Each packet may travel via a different path through the network.
  1. Routers direct each packet along an optimal route.
  1. Packets may arrive out of original sequence.
  1. Upon arrival, packets are reordered using their sequence numbers.
  1. The original data is reconstructed from the reordered packets.
This system ensures network robustness; if one path fails, packets can utilize alternate routes.
Advantages
  • Network resilience: Alternate routes ensure delivery despite path failures.
  • Efficient bandwidth use: Shared network capacity across multiple users.
  • Multiple user access: Allows many users to transmit data simultaneously over the same infrastructure.
Methods of Data Transmission
Transmission Modes:
Simplex:
Data flows in one direction only, from sender to receiver.
  • Unidirectional data flow.
  • No feedback from receiver.
Example: Traditional radio broadcasts.
Advantage: Simple and inexpensive. Disadvantage: Lacks two-way communication.
Half-Duplex:
Data flows in both directions, but not simultaneously.
  • Bidirectional, alternating flow.
  • Devices take turns sending and receiving.
Example: Walkie-talkies.
Advantage: More flexible than simplex. Disadvantage: Delays occur when switching direction.
Full-Duplex:
Data flows in both directions simultaneously.
  • Simultaneous bidirectional flow.
  • Both devices send and receive concurrently.
Example: Telephone conversations.
Advantage: Fastest and most efficient communication. Disadvantage: Requires more complex hardware.
Data Transmission Types:
Serial Transmission:
Data bits are sent one at a time over a single channel.
  • Bits transmit sequentially.
  • Less susceptible to interference over long distances.
Example: USB.
Advantage: Reliable over long distances. Disadvantage: Slower than parallel for short distances.
Parallel Transmission:
Multiple bits are sent simultaneously over multiple channels.
  • Several bits transmit concurrently.
  • Requires multiple wires.
Example: Internal computer buses.
Advantage: Faster data transfer for very short distances. Disadvantage: Prone to signal interference and expensive cabling.
Universal Serial Bus (USB)
How USB Transmits Data
USB uses serial data transmission.
Example speed: USB 3.0 (5 Gbps)
Key Benefits of USB
  • Universal Standard
  • Hot-Swappable
  • Plug and Play
  • Power Delivery
Drawbacks of USB
  • Cable Length Limitations
  • Limited ports may require hubs
  • Multiple connector types
Error Detection Methods
Errors in data transmission occur due to interference, electrical noise, or hardware issues, which can corrupt information. Error detection methods are vital to ensure data integrity by allowing systems to identify when these errors have occurred.
Parity Check
Adds an extra bit to data to make the total number of 1s either odd or even, for basic error detection.
How it works: A parity bit is set (0 or 1) to achieve a pre-determined count (even or odd) of 1s in the data block. If the count at the receiver is incorrect, an error is detected.
Example (Even Parity): Data 1011010, Parity bit 0. Transmitted: 10110100.
Limitation: Detects only odd numbers of bit errors.
Parity Byte and Parity Block Check can detect more errors.
Checksum
A calculated value derived from the data; the sender includes it with data, and the receiver re-calculates to compare.
Process:
  1. Sender calculates sum of data blocks.
  1. Sends sum (checksum) with data.
  1. Receiver recalculates and compares.
Limitation: Different errors can produce the same checksum.
Echo Check
The receiver sends the received data back to the sender for comparison with the original.
Process:
  1. Sender transmits data.
  1. Receiver sends data back (echoes it).
  1. Sender compares original with echo.
Advantage: Very reliable detection. Disadvantage: Doubles transmission time/bandwidth.
Check Digit
An extra digit added to important numbers to detect data entry errors.
Examples:
  • ISBN (International Standard Book Numbers)
  • Barcodes
Purpose: Alerts if a number is mistyped, catching single-digit and transposition errors.
Automatic Repeat Query (ARQ)
ARQ detects errors and ensures data retransmission until it arrives correctly, making transmission reliable even over noisy connections.
The ARQ Process
1
Sender transmits packet
Sender transmits packet and starts timeout timer.
2
Receiver checks for errors
Receiver checks for errors using detection methods.
3
Acknowledgement Response
ACK (no errors): Receiver sends ACK. Sender sends next packet.
NAK (errors): Receiver sends NAK. Sender retransmits packet.
4
Timeout scenario
If no acknowledgement received, sender retransmits packet.
Key Components of ARQ
  • Positive Acknowledgement (ACK)
  • Negative Acknowledgement (NAK)
  • Timeout
Encryption
Encryption protects data from unauthorized access by converting plaintext to ciphertext using keys.
Symmetric Encryption
Uses the SAME key for both encryption and decryption.
How it works:
  1. Sender and receiver agree on a secret key.
  1. Sender encrypts plaintext into ciphertext using the key.
  1. Sender transmits ciphertext.
  1. Receiver decrypts ciphertext back to plaintext using the same key.
Advantages:
  • Fast and efficient
  • Efficient for large data
Disadvantages:
  • Key distribution problem
  • If key intercepted, security lost
Example: AES
Asymmetric Encryption
Uses a PUBLIC key for encryption and a PRIVATE key for decryption.
How it works:
  1. Receiver generates public/private key pair.
  1. Receiver shares public key openly.
  1. Sender encrypts message using receiver's public key.
  1. Sender transmits ciphertext.
  1. Receiver decrypts message using their private key.
Advantages:
  • Solves key distribution problem
  • Secure with strangers
Disadvantages:
  • Slower than symmetric encryption
  • Requires more computational power
Example: RSA
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