Types and Methods of Data Transmission (Copy)

Data Packets

• Sending data over long distances
  • Requires breaking data in data packets
  • Also called datagrams
• Usually very small
  • 64 KiB
  • Easier to control over long continuous data stream of data
  • Each packet can be sent through a different route to the same destination
  • If one transmission route too busy, the other one can be used.
• Reassembling the data at the destination can be a problem
• Packet Structure
  • It has a packet header, payload and trailer.
  • Header
    • IP address of the sender and receiver
    • Sequence number of the packet to allow proper reassembly
    • Size of the packet in bytes
  • Payload
    • Actual data of the packet
  • Trailer
    • Method that identifies the end of the packet
      • Important to separate one data from another when sending and receiving
    • Error checking method
      • Including cyclic redundancy checks (CRCs)
        • Sending computer adds all the 1-bits in the payload
        • Store this as the hex value before the payload is sent
        • After arriving the receiving computer recalculates the number of 1-bits in the payload
        • The recalculated value is checked against the original value of the payload
        • No transmission errors if both values match
          • If not, packet needs to be resent.
        • Packet switching
          • Data transmission method where a message is broken into multiple packets
            • Each packet sent independently
            • At the destination, they are reassembled to make the correct order
              • This information is present in the header
            • Nodes
              • At each transmission stage
              • Contain a router
              • Router determines which route a packet needs to take to ensure reaching the destination
              • Destination IP address is used in this process.
            • Benefits of packet switching
              • No need to tie one communication line
              • Re-routing can overcome failed, busy or faulty lines
              • Easier to expand package usage
              • High data transmission rate possible
            • Drawbacks of packet switching
              • Packet can be lost
                • Needs resending
              • The method is more prone to errors compared to real-time-streaming
              • Delay at destination because packets require re-ordering
              • Packets may get lost due to bouncing from router to router, and never reach a destination.
                • Network will halt if the lost packets increase much
                • A system clog occurs
                • This issue can be solved by hopping
                  • A hop number is added to the header of each packet
                  • Every time a packet leaves a router, the hop number is reduced by 1.
                  • Each packet has a maximum hop number
                  • If the hop number reaches 0 and the packet has still not reached the destination, it will be deleted at the next router
                  • The missing packet is flagged by received
                  • Re-send request made to sender.