Computer Networks · free preview
Error Detection
Links corrupt bits, so every frame carries redundant check bits, and the craft of error detection is buying the strongest guarantee with the fewest bits, the Internet checksum sums 16-bit words in ones' complement arithmetic and serves as a cheap last line of defense, the cyclic redundancy check treats messages as polynomials over binary arithmetic and appends the remainder of a division by a standard generator polynomial, catching all burst errors shorter than the generator, and forward error correction trades always-on redundancy for avoiding retransmission where errors are common or retransmission is slow.
1. Redundancy with a purpose
Redundancy with a purpose
2. The Internet checksum
The Internet checksum
3. Cyclic redundancy check
Cyclic redundancy check
4. Detect or correct
Detect or correct
5. Why not send two copies
The simplest imaginable error detection sends the message twice and compares. Why is this scheme considered poor?
6. The checksum's blind spot
Which corruption will the Internet checksum fail to detect?
7. What CRC-32 guarantees
Which error class does Ethernet's degree-32 CRC detect with certainty?
8. The CRC transmission algorithm
Which sequence correctly describes how a sender computes the transmitted string for CRC with a degree-k generator?
9. When correction beats detection
Forward error correction spends redundant bits on every frame. When is that the right trade?
10. Ones' complement carry
What distinguishes ones' complement addition from ordinary binary addition in the Internet checksum?
11. The check's word size
The Internet checksum views the message as a sequence of words of a fixed width and sums them. How many bits wide is each word?
12. The divisor's degree
In CRC, the sender and receiver agree in advance on a polynomial that divides the extended message. What is this shared polynomial called?
13. Ethernet's check
Ethernet protects each frame with a cyclic redundancy check of a fixed length that catches every burst up to that many bits. How many check bits does it carry?
14. Correction in advance
What is the name of the error handling strategy that sends enough redundancy for the receiver to repair damage without any retransmission, used for example in 802.11 wireless?
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