Data Transmission

Last updated May 26, 2023 Edit

• Compsci
• Theory
• Networking

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

• 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 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

• 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.

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

• 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

• 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

• 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 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.

• 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 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

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