Computer Networks · free preview
Signals and Encoding
Direct links come in many media, copper, fiber, and air, characterized by bandwidth, frequency, and reach, yet the architecture makes them look alike to the layers above, and the first problem on every link is encoding bits into signals, where the naive NRZ scheme suffers baseline wander and clock drift, NRZI fixes runs of ones only, Manchester merges the clock at the cost of half the link's efficiency, and 4B/5B buys back most of that efficiency by recoding data to guarantee transitions.
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?
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