Data Transmission

• Data Packet: files/data is split into small packets for easier control than long steam of data
• Packet part:

  • Header
  • Payload
  • Trailer
• Header:

  • IP address of sender
  • IP address of receiver
  • Sequence number
  • Size
• Payload:

  • Actual data
• Trailer:

  • Method of identifying the end of the packet
  • Error checking to ensure
• Packet switching:

  • Data packets are sent through multiple routers
  • Routers decide which router the packet take and choose the shortest path to reach their destination
  • Reassembled correctly at the destination.
• Simplex: one direction
• Half duplex: both direction, not simultaneously
• Full duplex: both direction, simultaneously
• Serial:

  • Less risk of external interference
  • More reliable (long distance)
  • Won't be skewed
  • Small data
  • Send data (long distance)
  • Less expensive (less hardware require)
• Serial: one bit sent at a time on a single wire
• Parallel:

  • Faster rate
  • Works well (short distance)
  • Can be skewed
  • Suitable method (speed important)
  • Most appropriate (time-sensitive)
  • Expensive (require more hardware)
• Parallel: many bits sent at a time on several wires
• USB: half-duplex and full-duplex data transmission.

  When plugged in to a computer,

  • The computer automatically detects the device.
  • Once recognised, the device driver software is loaded so the computer and the device can communicate effectively.
  • If the device is new, the computer will find the device driver software that matches the device. If not found, the computer will prompt the user to download the device driver software
• Benefits USB:

  • Automatically detected, drivers automatically loaded
  • Only fit one way, prevents incorrect connections
  • Industry standard
  • Support different transmission rate
  • No need external power source
  • Backward compatible
• Drawbacks USB:

  • Maximum cable length (5m)
  • Transmission rate is less than 120 MB/sec
• Parity checking

  • Count the number of 1s in the parity.
  • It can be odd or even parity.
  • Usually the most significant bit (left most) is the parity bit (parity block, a whole byte can be the parity byte)
  • Error is detected when the bits don't add up to the given parity (odd, even)
• Checksum:

  • Sender and receiver agree on a checksum algorithm.
  • Sender uses this algorithm to calculate the checksum value.
  • Sender sends the data packet with the checksum value (stored in the trailer)
  • Receiver re-calculates the checksum value using the agreed algorithm and compares both values.
  • If both values are the same, no error detected. If both values are not the same, error detected.
• Echo check:

  • Sender sends data to receiver.
  • Receiver re-sends a copy of the data back to sender.
  • Sender compares the original and the receiver's copy.
  • If both values are the same, no error detected. If both values are not the same, error detected.
• Check Digits: are calculated from all the other digits in the data (ex-codes). The check digit would be the last digit of the code.
• Automatic Repeat Requests (ARQs)

  • Uses acknowledgements and timeouts to make sure the user received the data
  • The receiver would check the data for any errors
  • If none are found, a positive acknowledgement is sent to the sender. If errors are found, a negative acknowledgement will be sent, and the data will be sent again.
  • The sender uses timeouts to wait for a pre-determined amount for the acknowledgement.
  • If no acknowledgements are received after the timeout, the data will be sent again to the receiver.
• Encryption :a process of turning the data into an unreadable form so it doesn’t make sense to hackers and other attackers
• Plaintext: the original data that is being sent
• Ciphertext: the text produced after encryption
• Symmetric Encryption:

  • It uses an encryption key for the encryption process
  • The same key is used for encrypting and decrypting the data.
• Asymmetric Encryption:

  • Uses a public key and a private key ( public key available to everyone, private key only available to the user)
  • The receiver would have the private key, and they would send the public key to the sender.
  • The sender can encrypt the message with the public key, and the data can be decrypted using the private key.