• Technologies that allow users to transmit data from one point to another

Serial Data Transmission

  • Well suited for long distance communication, but the speed of the transmission is lower as there is only one channel used.
  • There is no need for data synchronisation as bits are sent in one channel.
  • USB is a common example

Parallel data transmission

  • Parallel transmission is faster than serial transmission as multiple channels are used.
  • Used internally within a computer
  • An internal timer synchronises the data on different channels
  • 8-64 bit data buses are used

Asynchronous data transmission

  • In async data transmissions, the data is sent in groups in an agreed bit pattern. Bits are combined with a start and a stop bit.

Synchronous data transmission

  • A synchronous data transmission sends data as a stream of bits.
  • The data also contains timing signals that are generated by an internal clock.

Universal Serial Bus

  • A USB device has four pins

  • Two pins are responsible for connecting wires, which carry the power and ground connection

  • The two other pins connect the wires that transfer data

  • Hence the cable used to connect two USB ports is a four-wire shielded cable.

  • USB devices are detected by a computer due to a small change in voltage from the device being connected.

  • Computer determines the specifications of the USB device

  • Computer loads the appropriate driver to communicate with the device

Error-checking methods

  • When data is transmitted through a channel, it could get corrupted or changed by electric fields or dirty equipment
  • Hence, it is very important to implement error checking method sin computer technology.
  • These methods are used to detect and correct errors:
    • Automatic Repeat Request (ARQ)
    • Parity Checking
    • Majority Voting
    • Checksum
    • Echo check
    • Check digits

Automatic Repeat Request (ARQ)

  • In Automatic Repeat Request, ACKNOWLEDGEMENT and TIMEOUT ensure that error-free messages are received at the destination.
  • ACK is a message sent by the receiver to confirm that they have received the data correctly.
  • TIMEOUT is the time allowed to elapse before an ACK is received
  • If the ACK is not received within the TIMEOUT, the message is automatically resent.

Parity Checking

  • Parity checking uses a parity bit in each byte of data. This bit is allocated before transmission.

  • An even parity has an even number of bits 1s in a byte. An odd parity has an odd number of bit 1s in a byte.

  • Systems may use even or odd parity.

  • The sender and receiver make an agreement prior to the transmission about the type of parity.

  • To find the exact location of the bit changed, a block of data is sent. The parity is checked both horizontally and vertically.


  • A value called a checksum is attached to the end of the block of data. This value can indicate transmission errors.

  • Checksum = the sum of all the digits, if the sum of all digits is less than or equal to 255

  • Else, the following algorithm is used:

    • Divide the sum X by 256
    • Round down to the nearest whole number, Y
    • Multiply Y by 256 and denote this number as Z
    • Checksum = X-Z
  • The sender transmits the block of data with checksum

  • When received at the receiver end, the checksum of the block of data is calculated.

  • If the calculated checksum is equal to the checksum transmitted by sender, then there are no transmission errors

  • If both the checksums are not equal, the sender is requested to resend the data.

Echo check

  • In this the data received is sent back to the sender
  • The sender compares the two sets of data for transmission errors
  • This method is not very reliable because it cannot be decided in which transmission the data was corrupted.
  • An error-free echo check would prove that no data has changed or been corrupted.

Check digits

  • Check digits is another method that is used to detect errors.
  • In this method, an additional digit is added to the end of a string of numbers
  • This bit is calculated using a standard algorithm and is called a check digit