Tillförlitlig dataöverföring

Question 1

What are the 3 basic methods for error detection?

Answer 1

• Parity bit (even or odd number of ones) Possibly even checking a different parity maybe with 2 or 4 bits instead.
  Can detect single bit errors but otherwise worst.
• Checksum (check is sum added with codesum is all ones, used with IP, TCP & UDP)
  Divide message in smaller chuncks. Add the chunks together with extra bits being added to the beginning. Generate a codesum. The codesum is also sent to the reciever who checks if the codesum + other sum adds up to all ones.
• CRC - cyclic redundacy check (divide, send quotient together with the remainder.)
  Divide by generator to get codeword. add rest to end. The reciever gets the generator and can check if it works.

Question 2

FEC - forward error correction means?

Answer 2

If something goes wrong and the sender does not recieve the whole packet, an FEC code can be used for the sender to correct the mistake by themselves. For example having a parity bit helps to know how many 0/1 that should arrive each chunk.

Question 3

Why are ACKs needed?

Answer 3

The sender needs to know if the reciever has recieved the message or not.

Question 4

What are the three different ways the ACK procedure (Automatic repeat requests ARQ) can be done?

Answer 4

• Go-Back-N ARQ
  n bits are sent by a sliding window. The reciever discards the frames not expected. If accepted, move window, otherwise resend.
• Stop & wait ARQ
  Sends one frame, waits for ack. Otherwise resend.
• Selective reject (/repeat) ARQ
  Send n frames. Resend specifik frames not accepted.

Question 5

What are the two types of errors?

Answer 5

Single bit error and burst errors.

Question 6

What is the difference between flow control and error control?

Answer 6

Flow control is to make sure the frames are sent at a speed the reciever can recieve them.
Error control is to handle errors induced by the transmittion system to the message.