CS144 7-3 Clocks and Clock Recovery

Introduction to Computer Networking, Stanford University
https://lagunita.stanford.edu/courses/Engineering/Networking-SP/SelfPaced/about

Unit 7: Lower Layers 중 Clocks and Clock Recovery 파트 수업 필기 정리

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Clocks

• data is transmitted using a 'clock'

• the receiver needs to know when to sample the arriving data

  

  
  when the clocks are running at different speed

  • how different are these clocks?
    • units of parts per million
    • ± 100 ppm = ±0.01%but usually the clocks are not drifting quickly, so offsets are relatively fixed

 

Asynchronous Communication

• make sure the packet not too long (control p)
• if we know the tolerants between the clocks to reliably decode the packet

 

Synchronous Communication

used in more practical situations

• Encoding the clock with the data
• Recovering the clock
• Getting the data into the receiver's clock "domain"

1. Sender sends 10mega bits per second (using 10MHz clock) through network link
2. in Transmit Clock Domain, Clock Recovery Unit recovers the 'sender's clock'
3. put the data into Elasticity Buffer
4. Receiver(RX) processes it with its own clock

 

How to send clock info encoded?

• make sure there's sufficient transitions in it,

  otherwise the clock recovery unit could think it as half or doubled signal

 

Manchester (En)Coding

• 0 at downwards transition
• 1 at upwards transition
• d.c. balance? → set a threshold voltage level between 0 and 1 transition, so the recovery unit distinguishes transitions better
• But in this way, we need to insert more transitions than we need (doubling the bandwidths in the worst case)

4B5B Encoding

4-bit data = 4 bits of original data
5-bit data = 5 bits of encoded data

• this encoding method guarantees that there are some certain transitions
  • you can always be sure that outgoing data has enough transition in it
• Fewer transition than Manchester coding, but people have developed the circuits so now it's most commonly used

 

 

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Problem Set

Question 1

An infrared remote control uses asynchronous communications to send 64 bits of data using a 1MHz clock. The clocks at the two ends differ by at most 10kHz.

(10kHz is 1% of 1MHz. 1% = 10,000 ppm)

True Statement

If the receiver’s clock is 10kHz faster than the transmitter’s clock, then we will count a bit twice.

Question 2

True statements about Manchester coding?

Answers

• Used to introduce transitions between 0 and 1 to aid clock recovery
• Guarantees a transition during every bit time

Question 3

You friend discovers another form of block coding: 4B/6B, and she asks you to compare it Manchester coding and 4B/5B. Which of the following might you discover in your analysis?

Answers

• The overhead of 4B/6B will be less than Manchester coding but more than 4B/5B.
• 4B/6B will allow for more control codes than 4B/5B and Manchester coding.