data transmission (2)

• data packets


  broken down 'chucks' of data that allow it to be transmitted faster from the sending device to the receiving device
• structure of data packet


  • header
  • payload
  • trailer
• header

  • sender's ip address
  • receivers ip address
  • packet number
• payload


  • data / part of message
• trailer


  • bits to show end of packet
• it takes the data multiple packets to reconstruct the original message, and the packets must be rearranged in the same order they left the senders computer
• packet switching


  a method to transmit data efficiently from the sending to the receiving computer.

  the data packets are each sent via a different route to their destination and may arrive out of order
• factors determining the route a packet takes

  • route availability
  • congestion
  • error detected - so resent
• advantages of packet switching


  • packets take different routes to get to their destination - ensures the message is sent no matter the route
  • transmission errors (corrupt, missing packets) can be detected when they occur and packets can be resent
• disadvantage of packet switching


  • data can b dropped if it has moved around the network for a certain period of time
  • each packet has a time to live (TTL), the max amount of time a packet can stay on a network before it is devliered
  • measured in hops
• transmission methods


  how data is transmitted from the sender to the receiver
• serial transmission

  a single wire is used to connect the sending device and receiving device. bits are transmitted one at a time
• advantages of serial transmission


  more reliable over long distances, likely hood of data being lost is slim

  data packets arrive in order
• disadvantage of serial transmission

  relatively slow

  not suitable for transmitting large amounts of data over short distances
• parallel transmission


  multiple wires are used to connect the sending device and the receiving device.

  several bits are sent simultaneously down the wires (channels), each bit travelling on a separate wire
• advantages of parallel transmission


  • it is fast
• disadvantages of parallel transmission


  • bits arrive out of order
  • it is expensive to implement over longer distances
• types of data transmission


  it depends of the devices sharing data and the purpose of the data transmission

  • simplex data tranmission
  • half-duplex data transmission
  • full duplex data transmission
• simplex data transmission


  only one device can transmit data. data moves in only one direction.

  from sending device to receiving device
• half-duplex data transmission


  both the sending device and the receiving device can both send data and receive data, but only one at a time. data can move in both directions but only in one direction at a time
• full-duplex data transmission


  both the receiving device and sending device can send data and receive data at the same time. data can be transmitted in both directions simultaneously
• USB


  stands for Universal Serial Bus

  an interface that a computer uses for communication with external devices, it is standard for connecting devices to their parents or host controllers.

  it uses serial transmission.

  as they only travel in one direction, they have different connectors at each end and cannot accidentally be connected incorrectly
• advantages of USB


  it is universal - it is a standard that is now widely used in most common devices

  it can transfer power as well as data, even at the same time
• disadvantage of USB


  there are multiple different connectors so if you lose your devices cable you need to make sure to replace it with the correct version
• USB device


  eg. flash drive

  plugs directly into a USB port
• USB cable


  used to connect a separate device into a USB port
• check digit


  an extra digit added to a number that is calculated from the existing digits in the number. it is used to check for error during data entry.

  they can easily detect incorrect digits, transposition errors, extra digits or omitted digits in a number
• examples of check digits


  • international standard book number (ISBN)
  • product codes
  • barcodes
  • vehicle identification numbers (VIN)
• checksum


  a type of error detection method.

  data is sent in blocks together with calculated value known as the checksum.

  the receiving computer will perform the same calculation on the data to regenerate the checksum and then compare the two checksum values.

  if there is a mismatch then an error has occurred and the receiving computer will request the sending computer to resend the data. this process is repeated until the correct data is received
• rules of checksum


  rule 1: if the sum of the bytes in the block is less than or equal to 255 then this value is taken as the checksum

  rule 2: if the sum of the bytes in the block is greater than 255 the following steps take place:

  • divide the sum of the bytes in the block by 256 to get the quotient
  • convert the quotient to an integer to give the integer value - always round down
  • multiply the integer value by 256 to get the product
  • subtract the product from the sum of the bytes to get the checksum

  rule 3: the result of either rule 1 or rule 2 is the checksum of the block
• echo check


  a method of error detection

  the receiving computer sends back an exact copy of the data the sending computer

  the sending computer then compares the data it originally sent to the data it has received back

  if a match between the two data sets, then the transmission took place without error

  if there is an error the sending computer resends the data, as the process starts again
• disadvantage of echo check


  • difficult to tell whether the data was corrupted in transit to the destination or on its way back
  • it is slow, as the complete data has to be sent twice
• ARQ


  automatic repeat query.

  if the receiving computer detects an error it can request the sending computer to resend the data packet.

  when the data is received at the destination computer the computer sends a short message (Acknowledgement - ACK)

  this indicated whether the transmission was successful or not

  a positive ACK suggests that transmission was completed without error

  a negative ACK suggests that transmission had some errors so the sending computer needs to resend the data

  this process is repeated until a positive ACK or a defined fixed time has passed (time out)
• encryption


  technique of scrambling data (plain text) to make it unreadable to a person who doesn't have the key / algorithm to decrypt the data (into cipher text)
• symmetric encryption


  the key used for encrypting a message is the same one used for decrypting the message.

  the receiver will receive the key and message separately.
• asymmetric encryption


  one key is used for encryption and the other key is used for decryption.

  a public key is to encrypt, it is visible to all the computers that want to exchange an encrypted message

  a private key is to decrypt, it is only known to the computer where it resides