Data Transmission

• Validation is checking an input to see that it is realistic or conforms to a set of rules

  Examples:

  • Range Check: Checks that a value is between two values (age is between 0 and 120)
  • Length Check: Checks to see the length is a given number of characters (password is 8 characters)
  • Type Check: Check if a value is the correct data type (an integer is a whole number)
  • Presence Check: Check if a value is present or not (is “y” present?)
  • Format Check: Check the input matches a format (letter, number ex. A1)
• Validation can only check that the data entered is reasonable (the right format)

  Verification is used to double-check that the data entered in is typed in correct.

  For example, a user setting a new password may be asked to type is twice
• A check digit is a digit added to the end of a number to check that the number is valid.

  This has many uses, like

  • To check the validity of a credit card number
  • To check the barcode number on a book was scanned correctly
• Echo check

  Data sent to receiver, and receiver sends back the same data. Sender compares both sets of data, if they are are different, an error has occurred.
• Check digit

  A one-digit character added to the end of a number, and used to check for errors when data is entered. Used for barcodes, ISBN numbers and credit card numbers.
• Parity bit

  an extra bit added to a byte; used to check transmission accuracy.
• Packet Switching


  a mode of data transmission in which a message is broken into a number of parts that are sent independently, over whatever route is fastest for each packet. they may arrive out of order and reorder in the destination.
• Half Duplex

  Transmission takes place in both directions, but not at the same time.
• Serial Transmission

  Data is transmitted one bit at a time down a single wire.
• Header Contains

  Destination Address

  Originating Address

  Packet Number
• Payload


  The actual data in a data packet.
• Data packets contain

  Header

  Payload

  Trailer
• Trailer

  Signals the end of the packet. Also may contain error detection data such as checksum.
• Checksum

  Data is run through an algorithm that produces a checksum which is sent along with the data. When packet arrives, the receiving computer generates the checksum and compares it with the checksum that was sent. If the checksum doesn't match the file have been corrupted and altered in the transmission.
• Crosstalk

  Signal on a line creates disturbance on another line - this can corrupt data whilst it is being transmitted.
• Skew

  Each wire in a parallel cable has slightly different properties, causing the bits to arrive at different times. Receiver needs to wait for all of the bits to arrive before next lot are sent. Transmission has to be over very short distance to reduce this problem.
• Router

  A type of computer that forwards data across a network. In a packet-switching network, routers are the nodes in the network. They check the destination address on each packet to calculate where to send it to next.
• Parallel Transmisson

  Multiple Bits Transmitted at a Time

  Multiple Wires Used
• Simplex

  Data sent in 1 direction only
• Duplex

  Data sent in both directions at the same time
• Odd parity bit

  Added to make a binary string have an odd number of 1s
• Even Parity Bit

  Added to make a binary string have an even number of 1s
• Parity Byte Error Checking

  Extra byte send with message. Each bit of the parity byte is a parity bit for the bit positions in the message.
• Encryption

  Process of converting readable data into unreadable characters to prevent unauthorized access.
• Plaintext


  The original text - before encryption.
• Ciphertext

  Data that has been encrypted and is therefore unreadable.
• Encryption key

  Used to encrypt and decrypt data.
• Symmetric Encryption

  An encryption method whereby the same key is used to encode and to decode the message
• Asymmetric encryption

  Two keys are used; one key encodes the message (the public key), and the other key decodes the message (the private key). Public key can be widely shared.
• Encryption Strength

  determined by a number of factors including the type of encryption algorithm used and the size of the cryptographic key.
• Why encrypt data?

  So if the data is intercepted it is unreadable.
• ARQ (Automatic Repeat Request)

  Uses acknowledgement and timeout. If the acknowledgement isn't sent back to the sender before the timeout occurs, the data is automatically resent. The receiver can send a "NACK" (negative acknowledgement) if the data arrived but with errors, so that the sender does not need to wait for the timer to run down before re-sending.
• • Information on the internet is broken down into packets and are created by TCP and transmitted over the internet
• Packets are small chunks of information/data 
• TCP stands for Transmission Control Protocol and is used for organising data transmission over networks
• Small chunks of data are easier and quicker to route over the internet than big chunks of data
• Routing involves finding the most optimal path over a network
• What do packets contain?

  • Packets are “chunks” of information. This information is called the “payload”
  • Packets act like postage letters, each one has: 
  • a delivery address (destination IP address)
  • a return address (source IP address),
  • and a message (data payload)
  • Packets are split into three parts:
  • Packet header
  • Payload (the actual data)
  • Trailer
  • The header contains:
  • Source IP
  • Destination IP
  • Packet number
  • Error checker e.g. a checksum or parity bit
  • The trailer contains:
  • Additional error checks
  • End of packet notification
• Packet Switching

  1. Find closest routers to destination device
  2. Send data over different routes to not build network traffic
  3. Message is broken into packets which are sent over different routes (to maximise speed)
  4. Receiver receives data (which is put in the right order using packet numbers)
  5. Routers contain routing tables that track nearby routers (so the data can find the right route to the destination).

  
• Serial Transmission

  • One bit is sent at a time across a single wire