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信号与编码
直接链路有多种介质——铜缆、光纤和空气，其特征由带宽、频率和传输距离刻画；但体系结构使它们在上层看来并无二致。每条链路上的首要问题是将比特编码为信号：简单的 NRZ 方案会遭受基线漂移和时钟漂移，NRZI 仅能解决连续 1 的问题，曼彻斯特编码以牺牲一半链路效率为代价嵌入时钟，而 4B/5B 通过重新编码数据以保证跳变，从而挽回了大部分效率。
1. A landscape of links
A landscape of links
2. Bits must become signals
Bits must become signals
3. Three encodings, three tradeoffs
Three encodings, three tradeoffs
4. Beyond two levels
Beyond two levels
5. The point of the link abstraction
Why does a network architecture make a Wi-Fi link, a fiber link, and a cellular link look the same to the layers above?
6. Two failures of NRZ
Which pair correctly names the two problems that long runs of identical bits cause under NRZ encoding?
7. Reading NRZI
Under NRZI, the signal makes a transition to encode one bit value and stays put for the other. Which statement is right?
8. Manchester's price
What does Manchester encoding buy, and at what cost?
9. The 4B/5B compromise
How does 4B/5B encoding achieve its guarantee of at most three consecutive zeros on the wire?
10. Symbols beyond bits
A modulation scheme can distinguish 64 different signal patterns per interval. How many bits does each signal carry?
11. Manchester's efficiency
Manchester encoding guarantees a transition in every bit cell by sending two signal transitions per bit. Its efficiency, useful bits per signal transition, is what percent?
12. The recoding scheme
Name the encoding scheme that maps each 4 bits of data into a 5-bit codeword to bound runs of zeros, achieving 80 percent efficiency.
13. The drifted threshold
Under NRZ, the receiver tracks a running average of the signal to distinguish high from low, and long runs of identical bits drag that average until real transitions become hard to detect. What is this failure called?
14. Wavelength from frequency
A wave's speed in the medium divided by its frequency gives what physical quantity?