Data Packets

• Data in a file is broken into small units called packets

• Each packet contains:

  • Packet Header

    • Destination address (IP Address)

    • packet number

    • Originators address

  • Payload (Actual Data)

  • Trailer/Footer

    • Indicates the end of the packet

    • Errors (from any error detection systems) 

Packet Switching

• Data is broken/split/divided into packets

•  Each packet (could) take a different route

•  A router controls the route/path a packet takes (selecting the shortest/fastest available route/path)

•  Packets may arrive out of order

•  Once the last packet has arrived, packets are reordered

•  If a packet is missing/corrupted, it is requested again

Methods of data transmission

• Serial 

  • Advantages: One bit at a time, single wire, less interference, cheaper

  • Disadvantages: slower

• Parallel

  • Advantages: Multiple bits at the same time, multiple wires/channels, faster

  • Disadvantages: data can be skewed, interference, expensive

• Simplex (transmission one direction only, E.g. Keyboard to computer)

• Half-duplex(transmission in both directions, but only one direction at a time)

• Full-duplex (transmission in both directions simultaneously)

Universal Serial Bus (USB)

• Uses Serial Transmission

• USB Interface: USB Port//USB cable //USB Connection//USB Device

• Advantages

  • automatically detected and configured

  • only fits one way

  • universal standard

  • supports multiple data transmission speeds

  • backward compatible

• Disadvantages

  • limited cable length and transmission speed

Error Checking Methods

• • An error may occur when data is transmitted due to Interference // crosstalk (2023)

Parity Check

• • Odd or Even Check

  • Each byte has:

    • 7 bits

    • 1 extra bit (Parity Bit), the first or last bit of the byte

  • Process

  • Parity Check is set (Even/Odd)

  • The number of 1's are counted

  • A parity bit is added to each byte/7 bits before transmission

  • ... to make the sum of the bits/1 or 0 s in each byte odd or even

  • After transmission, if the number is odd/even no error is detected (as chosen in part-I)

  • After transmission, if the number is not even/odd an error is detected (as chosen in part-I)

Checksum

• • uses calculated value to check for errors

  • Process

    • value calculated from the data to be transmitted (before transmission)

    • Value appended with the data to be transmitted

    • Receiving device calculates

      • Match (No Error)

      • Do not Match (Error)

    • E.g. Modulous 11

Echo Check

• • Comparison of data sent and received

  • Process

    • Sending device transmits the data

    • Receiving device transmits it back to sender

    • Sending device compares the  data

      • Match (No Error)

      • Do not Match (Error)

Automatic Repeat Request (ARQ)

• Negative or Positive Acknowledgement

• Time-out

• Process

  • Data packet transmitted

  • Checked for errors

  • Positive Acknowledgement (data error-free)

    • next packet transmitted

  • Negative /No Acknowledgement or timeout occurs

    • resend data packets (until Positive Acknowledgement)

Check Digit

• Errors due to data entry (manual or automatic)

  • Previously calculated data (E.g. Barcode or ISBN Number)

  • Data entered

  • Check Digit recalculated

  • matches with stored data (data entered correctly)

  • does not match with stored data (data entered incorrectly)

Encryption

• • Plain Text to Cipher text

  • If the data is intercepted, it can not be understood [2024]

  • Encryption Key to scramble the data (making it meaningless for the hacker)

  • Methods

    • Symmetric (Shared key, Same key to Encrypt and Decrypt)

      • Plain text encrypted into Cipher text using an Encryption Key

      • Cipher text and Encryption key sent to receiving devices

      • Same key is used to decrypt cipher text to plain text

    • Asymmetric (different keys to Encrypt and Decrypt)

      • Plain text encrypted into Cipher text using a Public Key (Encryption algorithm)

      • Cipher text sent to receiving device

      • cipher text can not be decrypted to plain text using a public key but using a private key