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错误检测
链路会损坏比特，因此每一帧都携带冗余校验比特；检错的精髓在于用最少的比特换取最强的保证。因特网校验和以反码算术对 16 位字求和，充当廉价的最后一道防线；循环冗余校验把报文视为二进制算术上的多项式，并附加除以标准生成多项式所得的余数，能捕获所有短于生成多项式的突发错误；前向纠错则以始终开启的冗余为代价，在错误频繁或重传缓慢的场合避免重传。
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?