Computer Science (2210)
Topic 7 of 11Cambridge O Levels

Data Transmission

Methods and protocols for sending digital data between devices and ensuring its accuracy.

Data transmission is the process of sending digital data from one device or location to another over a communication medium. This is a fundamental concept in computing, underpinning everything from using a USB mouse to browsing the internet.


### Methods of Data Transmission


There are two primary methods for transmitting binary data:


  • Serial Transmission: In serial transmission, data is sent one bit at a time sequentially over a single wire or channel. Think of it like cars travelling down a single-lane road. While this method is slower than parallel transmission, it is highly reliable over long distances because the bits arrive in the correct order and there is less chance of interference between signals. It also requires less complex and cheaper wiring.

    • * Examples: USB (Universal Serial Bus), Ethernet network cables, and data sent over the internet.


  • Parallel Transmission: In parallel transmission, multiple bits of data (often a full byte) are sent simultaneously over multiple wires or channels. Imagine this as a multi-lane highway where eight cars can travel side-by-side. This method is significantly faster than serial transmission for short distances. However, it is expensive due to the need for multiple wires and is susceptible to two main problems:

    • * Crosstalk: Interference between the parallel wires, which can corrupt the data.

    * Skew: The bits travelling on different wires may arrive at the destination at slightly different times, causing data corruption. This problem worsens with distance.

    * Examples: Older printer connections (Centronics port), and internal computer components like buses connecting the CPU to RAM.


    ### Modes of Data Transmission


    This refers to the direction of data flow between two connected devices:


    * Simplex: Data can only be transmitted in one direction. The flow is unidirectional, from sender to receiver. There is no way for the receiver to respond.

    * Example: A television broadcast sends signals to your TV, but your TV cannot send signals back to the broadcast station. A computer keyboard only sends data to the computer.


    * Half-Duplex: Data can be transmitted in both directions, but not at the same time. Devices must take turns sending and receiving.

    * Example: Walkie-talkies. Only one person can speak at a time while the other listens.


    * Full-Duplex: Data can be transmitted in both directions simultaneously. Both connected devices can send and receive data at the same time.

    * Example: A telephone call or a video conference, where both parties can speak and be heard at the same time.


    ### Methods of Error Detection


    During transmission, data can be corrupted by electronic noise or interference. Error detection methods are used to check if the received data is the same as the sent data.


  • Parity Check: A simple method where an extra bit, the parity bit, is added to each byte of data. The sender and receiver agree beforehand to use either even parity or odd parity.

    • * Even Parity: The parity bit is set to 0 or 1 to ensure the total number of 1s in the byte (including the parity bit) is an even number.

    * Odd Parity: The parity bit is set to make the total number of 1s an odd number.

    * Process: The sender calculates and adds the parity bit. The receiver recalculates the parity of the received byte. If it doesn't match the agreed-upon parity (e.g., an odd number of 1s is received when using even parity), an error is detected.

    * Limitation: A parity check can only detect an odd number of bit errors. If an even number of bits are flipped, the parity will still appear correct, and the error will go undetected.


  • Check Digit: This is a form of redundancy check often used to validate identification numbers upon data entry. It is typically the last digit of a number, calculated from the other digits using a specific algorithm (e.g., the Modulo-11 algorithm). When a user enters the number into a system, the computer re-calculates the check digit based on the main digits entered and compares it to the check digit that was typed. If they don't match, the system flags it as an input error.

    • * Examples: The final digit of a barcode number (UPC/EAN) or an ISBN for books.


  • Automatic Repeat reQuest (ARQ): A more robust protocol for error control. It uses acknowledgements and timeouts to ensure reliable data transmission.

    • * Process:

  • The sender transmits a data packet, which includes an error-checking value (like a checksum).
  • The receiver checks the packet for errors.
  • If the data is error-free, the receiver sends a positive acknowledgement (ACK) signal back to the sender.
  • If the data is corrupted, the receiver sends a negative acknowledgement (NAK) signal, requesting re-transmission.
  • The sender also uses a timeout mechanism. If it does not receive an ACK or NAK within a set period, it assumes the packet was lost and automatically re-sends it.

    • * ARQ is highly effective because it not only detects errors but also provides a mechanism for correction by re-transmitting lost or corrupted data.

    Key Points to Remember

    • 1Serial transmission sends data one bit at a time over a single wire, ideal for long distances (e.g., USB).
    • 2Parallel transmission sends multiple bits at once over multiple wires, offering higher speed but only for short distances (e.g., internal computer buses).
    • 3Simplex mode allows data flow in one direction only (e.g., radio broadcast).
    • 4Half-duplex mode allows data flow in both directions, but not simultaneously (e.g., walkie-talkie).
    • 5Full-duplex mode allows simultaneous two-way data flow (e.g., telephone call).
    • 6A Parity Check adds a single bit to a byte to make the total count of '1's either even or odd for basic error detection.
    • 7A Check Digit is a calculated final digit of an ID number used to verify the accuracy of data entry.
    • 8Automatic Repeat reQuest (ARQ) is a protocol that uses acknowledgement (ACK/NAK) signals and timeouts to request the re-transmission of corrupted or lost data packets.

    Pakistan Example

    NADRA CNIC Verification

    In Pakistan, the 13-digit Computerized National Identity Card (CNIC) number issued by NADRA likely incorporates a **check digit** as its final digit. When you enter your CNIC for online services like mobile SIM biometric verification or opening a bank account, the system can perform a mathematical calculation on the first 12 digits to verify the 13th digit. This allows for an instant check against common typing mistakes, reducing errors before the number is even sent for verification against the central database. This transmission to the central server would use **serial transmission** over the internet, protected by robust error-control protocols like **ARQ** to ensure the citizen's sensitive data is transferred accurately and reliably.

    Quick Revision Infographic

    Computer Science — Quick Revision

    Data Transmission

    Key Concepts

    1Serial transmission sends data one bit at a time over a single wire, ideal for long distances (e.g., USB).
    2Parallel transmission sends multiple bits at once over multiple wires, offering higher speed but only for short distances (e.g., internal computer buses).
    3Simplex mode allows data flow in one direction only (e.g., radio broadcast).
    4Half-duplex mode allows data flow in both directions, but not simultaneously (e.g., walkie-talkie).
    5Full-duplex mode allows simultaneous two-way data flow (e.g., telephone call).
    6A Parity Check adds a single bit to a byte to make the total count of '1's either even or odd for basic error detection.
    Pakistan Example

    NADRA CNIC Verification

    In Pakistan, the 13-digit Computerized National Identity Card (CNIC) number issued by NADRA likely incorporates a **check digit** as its final digit. When you enter your CNIC for online services like mobile SIM biometric verification or opening a bank account, the system can perform a mathematical calculation on the first 12 digits to verify the 13th digit. This allows for an instant check against common typing mistakes, reducing errors before the number is even sent for verification against the central database. This transmission to the central server would use **serial transmission** over the internet, protected by robust error-control protocols like **ARQ** to ensure the citizen's sensitive data is transferred accurately and reliably.

    SeekhoAsaan.com — Free RevisionData Transmission Infographic

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