Error Control Techniques Flashcards
What is Automatic Repeat Request?
Sender sends data in packets. The receiver checks each received packet using techniques such as checksums or CRC’s (cyclic redundancy checks).
If the packet is error-free, the receiver ACKS.
If the packet has errors, the receiver NACKS or does not ACK. The sender will continue sending until the packet is received or a maximum number of retransmission attempts is reached.
Explain Stop-and-Wait ARQ
The sender sends a packet and waits for an ACK, then moves on to the next packet.
If they get a NACK, they retransmit the same packet.
Explain Go-Back-N ARQ
Sender sends a window of packets at a time.
The receiver ACKS and the sender sends the next window of bits.
If the receiver detects errors or out of order packets, it sends a NACK to the sender indicating the last correct bit. The sender resends from that bit onward.
What is Bit Parity?
When a packet of bits is sent, a parity bit is added to the sequence. This bit indicates if all the 1’s including the parity bit add up to an even or odd number.
Even parity indicates that the 1’s add up to an even number.
Odd parity indicates that the 1’s add up to an odd number.
The receiver then checks if this is correct.
Explain Cyclic Redundancy Check
It is a form of checksum which uses a polynomial to define the algorithm to generate the checksum.
What is a Checksum
A number is generated from the data packet based on some algorithm.
The receiver calculates the number from the data and the same algorithm, and compares it to the one sent.
What is Modulo 2?
The sum of two binary digits is calculated WITHOUT carrying over any extra values beyond 1. Essentially it works as an XOR.
Give a broad overview of Hamming Code Error Correction
A checksum which also alows correction, not just detection.
Hamming (7,4) would mean 3 bits are added to a 4 bit data block, resulting in a 7 bit encoded block.
Each parity bit is responsible for checking a specific subset of bits in the encoded block.
The receiver can determine the location of errorrs and correct them - to a certain extent.
